1. science
2. physics

2321
J. Phys. Chem. 1991, 95, 2327-2339
Interactlons of NO and CO with Pd and Pt Atoms
Gregory W. Smith and Emily A. Carter*
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1569
(Received: July 2, 1990)
We report ab initio generalized valence bond and Correlation-consistent configuration interaction studies of CO and NO
interacting with Pd and Pt atoms. We find dramatically different bonding mechanisms for the two ligands, which are easily
understood in terms of changes in the electronic structure of the metal and the ligand. CO bonds to both Pd and Pt by a
u donor/* back-bonding mechanism, yielding linear geometries. Our calculations predict that the ground (‘E+)state of
PdCO is bound by 27 kcal/mol, while the ground (l2+)state of PtCO is bound by only 18.5 kcal/mol. By contrast, PdNO
and PtNO are both bent, with the dominant bonding involving a covalent u bond between a singly occupied metal do orbital
20 kcal/mol],
and the singly occupied NO 27r* orbital. While the ground (2A’) state of PtNO is strongly bound [D,(Pt-NO)
NO binds very weakly to Pd [D,(Pd-NO) I4 kcal/mol]. Linear excited states (2Z+and
of PtNO and PdNO are predicted
to be only weakly bound or unbound. However, corresponding linear cationic states (IZ+and ’n) are strongly bound, but
the cationic bent (IA’) states are still the ground states of PtNO+ and PdNO’. These stark contrasts, in which NO binds
strongly to Pt but weakly to Pd while CO binds much more strongly to Pd, are due to the preference for closed-shell species
to bind strongly to other closed-shell species (e.g., CO to Pd) and for radicals to bind strongly to other radicals (e.g., NO
to Pt).
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I. Introduction
A great deal of interest exists in the interaction of nitric oxide
and carbon monoxide with transition metals, in part because of
the crucial role these metals play in automobile exhaust catalysis
and because of a fundamental desire to understand the nature of
the bonding between various transition metals and different types
of ligands such as NO and CO.I-)O NO and CO adsorption has
( I ) Bradshaw, A. M.; Hoffmann, F. M. Surf.Sci. 1978, 72, 513.
(2) Ertl, G.; Koch, J. Adsorption-Desorption Phenomena; Ricca, F., Ed.;
Academic Press: New York, 1972; p 345.
(3) Tracy, J. C.; Palmberg, P. W. J . Chem. Phys. 1969, 51, 4852.
(4) Steininger, H.; Lehwald, S.;Ibach, H. Surf.Sci. 1982, 123, 264.
( 5 ) Hoge, D.; Tushaus, M.; Schweizer, E.; Bradshaw, A. M. Chem. Phys.
Lett. 1988, 151, 230.
(6) Mieher, W. D.; Whitman, L. J.; Ho, W. J . Chem. Phys. 1989, 91,
3228.
(7) Winicur, D. H.; Hurst, J.; Becker, C. A.; Wharton, L. Surf.Sci. 1981,
109, 263.
(8) Conrad, H.; Ertl, G.;Kuppers, J.; Latta, E. E. Sur/. Sci. 1977,65,235.
(9) Jorgenscn, S.W.; Canning, N. D. S.;Madix, R. J. Surf.Sci. 1987,179,
322.
(IO) Nyberg, C.; Uvdal, P. Surf. Sci. 1988, 204, 517.
(11) Hayden, B. E. Surf.Sci. 1983, 131, 419.
(12) Bartram, M. E.; Koel. B. E.; Carter, E. A. Sur/. Sci. 1989,219,467.
(13) (a) Comrie. C. M.: Weinberg. W. H.: Lambert. R. M. Surf. Sci.
1976,5< 619. (b) Pirug, G:; Bonze], fi. P.; Hopter, H.;Ibach, H. J . b e m .
Phys. 1979, 71, 593.
(14) Gland, J. L.; Sexton, B. A. Surf. Sci. 1980, 94, 355.
(15) (a) Gorte, R. J.; Schmidt, L. D.; Gland, J. L. Surf.Sci. 1981, 109,
367. (b) Gorte, R. J.: Gland, J. L. Surf Sci. 1981, 102. 348.
(16) Kundig, E. P.; McIntosh, D.; Moskovits, M.; Ozin, G.A. J . Am.
Chem. Soc. 1973, 95, 7234.
(17) Darling. J. H.: 0ade.n. J. S.J . Chem. Soc..Dalton Trans. 1973. 1079.
(18) Whyman, R. J. 8rganomet. Chem. 1973,63,467.
(19) Misono, A.; Uchida, Y.;Hidai, M.; Kudo, K. J . Organomef. Chem.
1969, 20, P7.
(20) Bradford, A. M.; Douglas, G.;Manoj1ovic’-Muir,L.; Muir, K. W.;
Puddephatt. R. J. Organometallics 1990, 9, 409.
(21) Jack, T. R.; May, C. J.; Powell, J. J. Am. Chem. Soc. 1977,99,4707.
(22) Fischer, E. 0.;Shustcr-Woldan, H. Z . Nafurforsch. 1964, 766.
(23) Blomberg, M. R. A.; Brandemark, U.; Johansson, J.; Siegbahn. P. E.
M.; Wennerberg, J. J . Chem. Phys. 1988, 88, 4324.
(24) Pacchioni, G.;Koutecky, J. J . Phys. Chem. 1987, 91, 2658.
(25) Blomberg, M. R. A.; Lebrilla, C. B.; Siegbahn, P. E. M. Chem. Phys.
Leu. 1988, I50, 522.
(26) Gavezzotti, A.; Tantardini, G.F.; Simonetta, M. Chem. Phys. Leu.
1986, 129, 577.
(27) Rohlfing, C. M.; Hay, P. J. J . Chem. Phys. 1985,83,4641.
(28) Basch, H.; Cohen, D. J . Am. Chem. Soc. 1983, 105, 3856.
been studied extensively on well-defined single-crystal surfaces
of Pd and Pt,l-I5 in complexes isolated in argon matrice~,’~.’’
and
on clusters.18-22High-resolution electron energy loss spectroscopy
(HREELS) and infrared reflection-absorption spectroscopy
(IRAS) have been used to determine vibrational frequencies of
adsorbed NO and CO on single-crystal surfaces of Pd and
Pt,1,495,”2Js while temperature-programmed desorption (TPD)
and low-energy molecular beam scattering (LEMS) experiments
have extracted the binding energies of NO and CO on Pd and
Pt.2J36-9J2-’5 Often these properties are measured as a function
of adsorbate coverage. At lower coverages (e < 0.5 ML) (ML
= monolayer)), more than one surface metal atom is available
to each adsorbed molecule, possibly allowing the formation of
bridge-bonded species on the surface. Since our calculations
involve only one metal atom, direct comparisons are made only
to atop (terminally bonded) surface species.
Many experiments have demonstrated that CO binds in both
linear atop and bridging geometries on Pd and Pt surfaces.’-’ The
interaction of CO and Pd and Pt appears to be insensitive to crystal
facial structure but is extremely sensitive to surface coverage.
Bridging carbonyls are bound by 34-36 kcal/mol to Pd, while atop
carbonyls are bound by only 22-23 kcal/mol, since the atop C O S
only appear at high coverages where repulsive lateral interactions
reduce their heat of ad~orption.~,~
The C-0 vibrational frequency
is strongly coverage and site dependent, with bridging CO’s exhibiting we
1820 cm-’, whereas atop CO’s have o, 2100
cm-I.’ While CO first adsorbs in bridge sites on Pd, CO initially
prefers atop sites on Pt (w,(Pt-C)
470 cm-l and o,(C-O)
2100 ~ m - l ) .Bridging
~
C O appears above Bco = 0.17 ML with
w,(Pt-C)
380-470 cm-l and w,(C-0)
1855 cm-l.e In
contrast to Pd, both bridging and atop CO’s on Pt are bound by
30-35 kcal/mol, for Bco 5 0 . 5 ML.4f47
Several experiments examining NO chemisorption on Pd and
Pt surfaces have demonstrated the geometric versatility of NO
as an adsorbate, with bridging, linear atop, and bent atop geometries observed.*-I5 At low coverages, NO exists in bridging sites
on both Pd8-Io and Pt11-’5surfaces. At higher coverages, linear
atop NO is observed on Pt( 11I), while coadsorption of oxygen
atoms produces bent atop NO.1ZHREELS data for high coverages of NO on Pd( 100) may be due to the presence of bent atop
NO.IO Another study suggests that Pd( 100) precovered with S
atoms may cause bent atop bonding of NO to the s ~ r f a c e . ~
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(29) Basch, H. Chem. Phys. Lett. 1985, 116, 58.
(30) Bauschlicher, C. W., Jr.; Bagus, P. J . Chem. Phys. 1984, 80, 944.
0022-3654191 12095-2327%02.50/0 0 1991 American Chemical Society
2328 The Journal of Physical Chemistry, Vol. 95, No. 6,1991
Smith and Carter
Experimental binding energies for N O on Pd and Pt vary
dramatically with coverage. Complementary vibrational studies
indicate that these shifts in the binding energies are due to changes
in the adsorption geometry. Typically, bridging NO binds strongly
to both Pd and Pt at low ONO: D,(Pd-NO)
31-32 k ~ a l / m o l ~ * ~
25-36 kcal/m01.'~-'~Atop NO appears at
and D,(Pt-NO)
higher coverages and is more weakly bound, with binding energies
between 14 and 24 kcal/mol for Pd and Pt8*9J2-15
N-O vibrational
frequencies for bridging N O on Pd and Pt are lower (we
1500-1600 cm-l)el'3'3b9'4*15b
than for atop linear or bent NO on
Pd and Pt (we 1680-1 790 ~ m - ~ ) . ~ ~ ~ 9 ~ ~ ~ , ~ ~ 7 ~ ~ ~
Many complexes of CO and NO with Pd and Pt have been
Figure 1. Qualitative depiction of bonding between N O and one metal
isolated and characterized with IR and X-ray diffraction, including
atom: (a) linear *Z+MNO; (b) bent 2A' MNO; (c) 211 MNO. The
PdCO and PtCO in low-temperature matrices [w,(C-O)
2050
valence orbitals shown are discussed in detail in the text.
c ~ - ' ] . ' ~ JTerminal
~
carbonyls in Pt(C0)2(PPh3)2 and Pd(CO)(PPh3), exhibit C-O stretching frequencies of 1996 and 1954
fashion amounts to allowing NO 5u donation to the metal, concm-l and 1955 cm-I,l9 respectively, while triply bridging COS
comitant with metal du back-bonding to the 2u* orbitals of NO,
in trinuclear platinum complexes have Pt-C bond lengths of -2.0
leading to a linear 211 state (Figure IC). Since the 2u* level is
A and low C-O vibrational frequencies (- 1765-1827 cm-1).20
partially occupied for NO, we expect repulsive effects to inhibit
Finally, Pd and Pt complexes with terminal N O ligands exhibit
du-pu back-bonding, hence weakening this bond relative to
N-O stretching frequencies of 1740-1 790 cm-1.21,22
metal-CO interactions and thus disfavoring the 211state of MNO.
Several theoretical studies have been carried out to predict the
The rest of this work is outlined as follows. Section I1 presents
electronic structure of transition metal-NO and -CO
details of the ab initio calculations, section I11 presents results
Siegbahn and c o - ~ o r k e r sstudied
~ ~ the 'Z+ ground state of NiCO
and discussion for metal-CO and metal-NO interactions, and
by using coupled pair functional (CPF), modified CPF (MCPF),
section IV summarizes our findings.
and multireference contracted configuration interaction (MRCCI)
methods. Pd-CO bonding has been studied by Pacchioni and
11. Calculational Details
Koutecky2' by employing a nonrelativistic pseudopotential for Pd
All electrons on C, N, and 0 were treated explicitly. The
and the multireference doubly excited configuration interaction
Dunning32valence double-{contractions of the C, N, and 0 (9sSp)
(MRD-CI) method. Siegbahn and co-workers have also used the
Gaussian primitive bases of Huzinaga" were used, with one set
CPF approach to study PdCO and Pd2C0.2s
of d-polarization functions added (s(C) = 0.64, s(N) = 0.76, s(0)
Gavezzotti et alez6performed Hartree-Fock (HF) calculations
= 0.95). Relativistic effective core potentials (RECPs) and the
on PtCO using a relativistic pseudopotential for Pt. Rohfling and
Pd and Pt basis sets of Hay and Wadt31awere used to represent
Hay27performed unrestricted Hartree-Fock (UHF) with Molthe metal valence orbitals, with the Pd (3s3p4d) and Pt (3s3p3d)
ler-Plesset second-order perturbation theory (MP2) calculations
primitive functions contracted to (3s2p2d). Although all of the
on Ni, Pd, and Pt carbonyls, employing the same relativistic
calculations reported here utilized these 10-electron RECPs
effective core potentials (RECPs) for Pd and Pt that we use in
(RECP(10), where the 10 valence electrons are treated explicitly),
the current
Basch and Cohen2* performed small
we did carry out a test of the accuracy of this potential with regard
multiconfiguration self-consistent field (MCSCF) calculations,
to the prediction of binding energy and bond length. In particular,
followed by small valence level CI calculations on PtCO, using
we examined the effect of explicitly including the outermost core
an RECP for Pt and ECPs for C and 0. In later work, Basch
electrons in the calculation. Other authors have shown31cthat
carried out H F gradient calculations to predict the vibrational
such core polarization effects are quite small for Pt but may be
modes of PtCO and PtN0+.29
considerable for Pd. Thus, we examined how the Pd-CO bond
The only other theoretical study of N O binding to a metal was
length and binding energy varied for two RECPs: the one used
performed by Bauschlicher and Bagu~,~O
who used CASSCF/
throughout our study, RECP( lo), and the Hay-Wadt 18-electron
CISD calculations to find that the ground state of NiNO is the
effective core potential,31bRECP( 18), that includes only up
linear *E+state, with the bent 2A' state more weakly bound. They
through the n = 3 shell of the Pd core (Le., the 4s and 4p electrons
suggested that the more diffuse valence s and d orbitals of Pd and
are treated explicitly, in addition to the valence electrons). We
Pt may reduce the strength of the metal-NO u bond, which might
contracted the Hay-Wadt (5sSp4d) primitive basis to (5s3p2d),
stabilize the bent states of PdNO and PtNO. Our calculations
in order to have similar flexibility in both basis sets. We find that
provide strong support for this suggestion (vide infra).
the
Pd-CO bond distance is lengthened by 0.07 A and the binding
Our ab initio theoretical study focuses on both quantitative and
energy is correspondingly lowered by 6.4 kcal/mol (H 10%) when
qualitative aspects of CO and N O interacting with Pd and Pt
the RECP( 18) potential is employed. Thus, while core polarization
atoms. We expect to find quite different behavior for Pd versus
is somewhat significant for Pd, these effects do not change our
Pt and CO versus NO, since Pd and Pt have different ground
overall conclusions for Pd or Pt, based on the RECP( 10) potential
electronic states (Pd is d'O while Pt is s'd9) and NO is a radical
(vide infra).
while CO is closed shell. Indeed, qualitatively we find that whereas
The geometries of states were optimized by utilizing analytic
MCO (M = Pd, Pt) forms only linear, low-spin ground states,
gradients
of GVB-PP wave functions (generalized valence bond
the presence of the 2u electron in NO allows it to either covalently
with perfect singlet-pairing restriction^).^^ The first-order wave
bond or form a CO-like bond to transition-metal atoms. Two
possibilities exist for covalently bound MNO states (Figure 1):
(32) Dunning, T. H., Jr. J . Chem. Phys. 1970,53, 2823.
(i) a linear 22+state, where the NO 2u* electron is spin paired
(33) Huzinaga, S. J . Chem. Phys. 1%5,42, 1293.
with a metal d r electron to form a covalent u bond and where
(34) (a) The details of the generalized valence bond method may be found
the N 2s (NO Sa) pair may form a u-donor bond via donation
in: Hunt, W. J.; Dunning, T. H., Jr.; Goddard, W. A., 111 Chem. Phys. Lcrr.
into an empty metal u orbital (Figure la) or (ii) a bent 2A' state,
1969, 3, 606. Goddard, W. A., 111; Dunning, T. H., Jr.; Hunt, W. J. Chem.
where the NO 2r* electron is spin paired with a metal u electron
Phys. Lerr. 1%9,4,231. Hunt, W. J.; Goddard, W. A,, 111; Dunning, T. H..
Jr. Chem. Phys. Lett. 1970.6, 147. Hunt, W. J.; Hay, P. J.; Goddard, W.
to form an M-N covalent u bond but where little or no N O 5u
A., 111, J. Chem. Phys. 1972, 57,738. Bobrowicz, F. W.; Goddard, W. A.,
donation occurs (Figure Ib). Restricting NO to bond in a CO-like
111 In Methods ofEIecrronic Srrucrure Theory; Schaefer, H. F., Ed.;Plenum:
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(31) (a) Hay, P. J.; Wadt, W. R. J . Chem. Phys. 1985,82,270. (b) Hay,
P. J.; Wadt, W. R. J . Chem. Phys. 1985,82,299. (c) Rohlfing, C. M.; Hay,
P. J.; Martin, R. L. J . Chem. Phys. 1986, 85, 1447.
New York, 1977; pp 79-127. (b) Yaffe, L. G.; Goddard, W. A., 111 Phys.
Reu. A 1976, 13, 1682. (c) Dupuis, M.; King, H. F. J . Chem. Phys. 1978,
68, 3998. Rappi, A. K.; Upton, T. H. Orgammerallics 1984,3, 1440. Upton,
T. H.; Rap*, A. K. J . Am. Chem. Soc. 1985,107, 1206. Schlegel, H. B. J .
Compur. Chem. 1982.3, 214.
Interactions of N O and CO with Pd and Pt Atoms
TABLE I: Atomic State Splittings at the Hartree-Fock Level for Pd,
Pa+,Pt, a d PtCu
state
TE~
AEc
AE(RNHF)" AE(expt)'
3 $0
0.0
0.0
IS Pd (d")
-29.10660
'D Pd (s'd9)
-29.1 1 1 38
0.0
2.3
21.9
11.5
33.5
ID Pd (s'd9)
-29.09308
'F Pd (s2d8)
-29.02477
54.3
50.5
77.9
'D Pd+ (d9)
-28.877 35
0.0
4.1
4F Pd+ (s'd')
-28.785 96
57.3
77.7
'F Pd' (~'d')
-28.53280
216.2
'D Pt (s'd9)
-26.238 92
0.0
0.0
0.9
IS Pt (dIo)
-26.199 95
24.5
20.7
17.5
'F Pt (s2d8)
-26.21264
16.5
9.2
21.2
'DPt (s'd9)
-26.21538
14.8
38.6
'D Pt+ (d9)
-25.95933
0.0
9.6
4F Pt+ (s'd8)
-25.946 62
8 .o
27.1
'F Pt+ (S*d')
-25.798 64
100.8
91.5
Experimental values are averaged over J states. bHartree-Fock
total energy in hartrees. CRelativeenergy at the Hartree-Fock level
using a (3sZp2d) basis set and an RECP,'In in kcal/mol. "Relativistic
numerical Hartree-Fock
in kcal/mol.
Reference 37,
kcal/mol.
functions for M-NO and MNO+ were at the GVB(4/8)PP level,
where a GVB(nI2n)PP wave function involves n GVB electron
pairs each described by two natural orbital^.'^.^ The GVB pairs
for the 2Af and 2Z+states of MNO and the 'A' and IZ+states
of MNO+ were the N-O u and u bonds, the M-N bond, and the
N O 5u orbital (derived from the N 2s), while the GVB pairs for
the 211state of M N O and the 311 state of MNO+ were the N-O
u and u bonds for both states, both metal d u orbitals for the
neutral complex, and the N O 5u for the cationic complex. Thus,
311 MNO+ was treated at the GVB(3/6)PP level, in order to
maintain the same number of active orbitals for all states of NO
bound to M and M+.
The first-order wave function for the I
Z' state of MCO was
a GVB(6/12)PP wave function, where the C-O u and u bonds,
both metal d r orbitals, and a metal du orbital were correlated.
A GVB(S/lO)PP wave function was employed for the 3Z+ and
open-shell IZ+states, both of which involved correlating the C-O
u and u bonds and both d r orbitals on the metal atom. Utilizing
a GVB(S/IO)PP wave function for the triplet and open-shell singlet
states allows the same number (12) of active orbitals as for the
GVB(6/12)PP wave function for the I
Z' state of MCO. Higher
levels of configuration interaction (CI) were then added to the
resulting wave f u n c t i o n ~ . ~ ~ J ~
111. Results and Discussion
A. Control Calculations. To check the accuracy of the RECPs,
relative energies for low-lying states of Pt and Pd within the
(3s2p2d) basis set were compared with experiment and with
relativistic numerical Hartree-Fock (RNHF) calculations (Table
I). H F theory within the RECP/valence basis set description
(HF/RECP) and R N H F theory both describe the Pd 3D-'S
splitting poorly. R N H F theory predicts the correct ground state
(IS)but with a 3D-lS splitting of only 2.3 kcal/mol, whereas the
experimental3' J-averaged 3D-1S splitting is 21.9 kcal/mol.
HF/RECP predicts the 3D state to be 3.0 kcal/mol lower than
IS. Clearly, inclusion of extensive electron correlation is extremely
important for reproducing the
splitting in Pd; we have
chosen to employ the experimental state splitting in order to obtain
reliable energetics (vide infra). The only remaining concern over
the use of this RECP is whether an overstabilized s1d9state of
(35) Carter, E. A,; Goddard, W. A,, 111 J . Chem. Phys. 1988,88, 3132.
( 3 6 ) The following abbreviations are used: HF = HartreeFock. S,,, =
single excitations from all valence orbitals to the virtual space; SD = single
and double excitations from the orbital indicated to the virtual space; RCI
= all single and double excitationsallowed by symmetry within the indicated
GVB pairs such that each GVB pair always contains two electrons;GVBCI
= a full CI in the GVB orbital space.
(37) Moore, C. E. Atomic Energy k w f sAs Derived From rhe Analyses
of OpticalSpccrra;US.Government Printing Office: Washington D.C. 1971;
VOI. 111. pp 38-43. 181-185.
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2329
(3n)
TABLE II: Equilibrium Properties for NO
and CO ('E+)at tbe
CVB-PPLevel
molecule
method
TE, hartrees Re, A we,cm-'
pa
NO
GVB(2/4)PP -129.32453 1.159 1808.6
NO
GVB(3/6)PP -129.32958 1.157
1813.4
-0.017
NO
expt
1.151b 1904.2b *0.153e
CO
GVB(3/6)PP -112.81856 1.134 2273.5
-0,191
CO
expt
1.12gb 2169.8b -0.112"
Magnitude of dipole moment vector in debye. A positive sign indicates the negative end of the dipole points toward the 0 atom.
*Reference 38. cReference 40. dReference 39.
Pd causes problems in the description of PdCO or PdNO. Since
we find 3D and 'S Pd to be nearly degenerate, there is no large
preference for either state, leading to little bias in the nature of
the wave function. In other words, CO and N O can bind readily
to dIo and s1d9Pd, respectively, with no inherent qualitative biases.
HF/RECP and R N H F both describe the Pt J-averaged 'SJD
= 20.7
splitting fairly well: AEWp = 24.5 kcal/mol and URN,,
kcal/mol compared with the e ~ p e r i m e n t aJ-averaged
l~~
splitting
of 16.6 kcal/mol. We have utilized both higher level CI predictions and experimental values for this splitting (vide infra).
We have also examined the ID states of Pd and Pt a t the HF
level and including electron correlation. The ID state of Pd is
correctly predicted to be the second excited state, although the
splitting is far too low (1 1.5 versus 33.5 kcal/mol experimentally; Table I). GVBCI( 3/6) calculation^^^ yield an improved
value of 19.3 kcal/mol. The ID-IS splitting for Pt is poorly
described at the H F level (-9.7 versus 21.1 kcal/mol experimentally). However, GVBCI( 3/6) calculation^^^ a t least obtain
the correct ordering of states, with a ID-'S splitting of 5.1
kcal/mol. As discussed in the next section, even the artifically
low '&IS promotional energy does not allow strong metal-ligand
interactions to take place, and the larger, correct ID-% splitting
only makes bonding to the ID state even more unlikely.
Although the 3F-1Ssplittings for Pd and Pt are not predicted
quantitatively correctly by these methods (e.g., AERECP(Pd)=
51.3 kcal/mol, AEExp(Pd) = 77.9 kcal/mol; AERECp(Pt) = -8.0
kcal/mol, "(Pt)
= 3.7 kcal/mol), none of the MNO or MCO
states studied dissociate to SFmetal atoms; thus this deficiency
should not have a serious effect on the results of our study.
Electronic-state splittings for Pd+ and Pt+ at the HF/RECP
level are in reasonable agreement with experiment, considering
the lack of inclusion of electron correlation. The HF/RECP
4F(s'd8)-2D splitting for Pd+ is 57.3 kcal/mol, compared with the
e ~ p e r i m e n t a lJ-averaged
~~
splitting of 73.6 kcal/mol. The
HF/RECP level 4F(s'd8)-2D splitting for Pt+ is 8.0 kcal/mol
compared with the experimental3' J-averaged splitting of 17.5
kcal/mol. The Pt+ 4F(s2d7)-2Dsplitting at the H F RECP level
is 100.8 kcal/mol, compared with the experimentall' J-averaged
splitting of 81.9 kcal/mol. Since none of the MNO or MCO states
studied dissociate to the 4F states of Pd+ or Pt+, quantitative
predictions of these splittings are not necessary, though the
qualitative descriptions are reasonable.
To check the accuracy of all-electron ab initio calculations on
the ligands N O and CO, bond lengths and vibrational frequencies
from GVB(3/6)-PP calculations were compared to experiment
(Table 11). The predicted bond lengths are 0.006 A Ion er than
while
those observed experimentally (ReGVB(NO)= 1.157
%EXP(NO)= 1.151 A:* R$vB(CO) = 1.134 A, while &Exp(CO)
= 1.128 Aj8). The predicted N-0 and C-0 vibrational frequencies are both within 5% of the experimental values38
(ueGVB(NO)= 1813 cm-l and u,EXP(NO) = 1904 cm-'; weGVB(CO) = 2274 cm-' and u,EXP(CO)= 2170 cm-I), which are quite
typic1 errors for such valence DZP basis sets. These values allow
us to calibrate our vibrational frequencies and geometries for N O
and CO bound to metal atoms (vide infra).
Dipole moments were also calculated for N O and C O at their
equilibrium geometries at the GVB-PP level. The predicted value
1,
(38) Huber,K.; Herzberg, G. Constants of Diatomic Molecules; Van
Nostrand Reinhold Co.: New York, 1979.
Smith and Carter
2330 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
TABLE III: Calculated Properties of MCO States (M = Pd and Pt)
PdCO
lz+
OS" Iz+
TEb
AE'
D,( M-CO)"
RJMC-O)'
RCW-CO)'
e,(M-C-O)g
w,(MC-O)*
we(M-CO)*
we( M - C - O bend)*
Pi
-141.968 65
0.0
27.2
1.14
1.96
180.0
2253
428
56 1
1.30
Ptco
lz+
3z+
-1 39.074 24
0.0
15.4 (18.5)"
1.13
1.99
180.0
1976
600
56 1
1.12
-1 41.935 6 1
20.7
-1 41.91 6 65
32.6
e
1.13
4.15
180.0
1889
22
556
-0.79
e
1.13
4.54
180.0
1886
28
555
-0.55
OS" Iz+
3z+
-1 39.062 39
7.4
1.O
1.13
2.99
180.0
1914
348
558
-1.68
-1 39.037 40
23.1
e
1.13
4.05
180.0
1891
32
556
-0.56
#OS = open-shell singlet; correlates with ID Pd and Pt. bTotal energy at the GVB(6/12)-PP level for the 'Z+ state and at the GVB(S/IO)-PP
level for the open shell IZ+and 3Z+states (in hartrees). 'GVB-PP relative energy (in kcal/mol). "Adiabatic Pd-CO dissociation energy in kcal/mol
at the GVBC1(6/12) level (see text). Value in parentheses is our best estimate for the true De. 'Unbound with respect to ground-state fragments.
/Equilibrium bond length in angstroms. g Equilibrium bond angle in degrees. * Vibrational frequency in cm-I. 'Magnitude of dipole moment vector
in debye. A positive sign indicates the negative end of the dipole points toward the 0 atom.
Pd-C-0
TABLE IV: Electron Distribution in MCO for M = Pd and Pto
state
IC+ PdCO
OSe
PdCO
3Z+PdCO
Iz+PtCO
os, lz+P t c o
3z+P t c o
free CO
u
r
donationb back-bonding'
0.03
0.01
0.01
0.4 1
0.01
0.05
0.17
0.00
0.00
0.20
0.00
0.0 1
M"
Cd
od
9.82
10.02
10.00
10.23
10.02
10.06
6.12
5.93
5.94
5.71
5.93
5.91
5.95
8.06
8.05
8.0'6
8.06
8.05
8.03
8.05
a Electron populations are calculated from Mulliken populations
summed over both natural orbitals of the GVB pairs. bTotal electron
population donated to M from the CO 5a orbital. 'Total electron
population donated to C and 0 from M d r orbitals. "Total electron
distribution. Includes only valence orbitals on M. 'OS = open-shell
singlet; correlates with 'DM.
of 4 . 1 9 1 D for CO compares well with an experimental value39
of -0.1 12 D (where the minus sign indicates C-0' polarity). It
is interesting to note that the polarity of CO is correctly predicted
from the theoretical dipole moment, but Mulliken population
analysis (Table IV) indicates the opposite polarity. In fact, if the
Mulliken population data were taken to indicate a charge of +0.05
e on C and -0.05 e on 0, the dipole moment would be +0.27 D.
Thus, theoretical dipole moments are a better measure of charge
transfer than Mulliken population analyses. For NO, we find a
dipole moment of -0.017 D compared with an experimental4 value
of magnitude 0.153 D. Although the sign of the dipole of NO
has not been determined experimentally, our result is in qualitative
agreement with ab initio calculations of Walch and Goddard41
and Langhoff et al.42that predict values of -0.10 and -0.17 D,
respectively.
B. Metal-CO Interactions. PdCO: We find the ground state
of PdCO to be the I F state, in agreement with previous theoretical
s t ~ d i e s . ~ ~ *The
* ~ *IZ+
* ~ state of PdCO is formed through a t~
donor/?r back-bonding mechanism. Pd has a 4dI0valence electron
configuration in this state, avoiding repulsive Pd 5s-CO 5a interactions and allowing d?r back-donation. The other two PdCO
states that we examined are a 3Z+ state that correlates with 3D
Pd and an open-shell
state that correlates with ID Pd at infinite
Pd-C separation. These two states have the Pd 5s orbital partially
occupied, resulting in a repulsive interaction with the CO group.
The first excited state is found to be the 3Z+ state (T, = 20.7
kcal/mol), and the second excited state is found to be the open
shell
state (T' = 32.6 kcal/mol, Table 111). Both excited states
(39) Rosenblum, B.; Nethercot, A. H.Jr.; Townes, C. H.Phys. Rev. 1958,
109,400.
(40) Neumann, R. M. Asfrophys. J. 1970, 161, 779.
(41) Walch, S. P.; Goddard, W.A., I11 Chem. Phys. Leu. 1975.33, 18.
(42) Langhoff, S. R.; Bauschlicher, C. W., Jr.; Partridge, H.J. Chem.
Phys. 1988.89.4909.
(43) Entries for the parent molecules and fragments have the following
form: calculation/total energy in hartrees (number of configurations/number
of spin eigenfunctions).
I
I
I
1
ONE
ONE
I
I
I
I
I
I
I
.- _ -.
I
Figure 2. GVB(6/12)PP bonding orbitals for PdCO: (a) CO St7 donor
bond; (b) one of two identical Pd d r back-bonds; (c) CO covalent u bond;
(d) one of two identical CO r bonds; (e) Pd d s lone pair. Contours range
from -0.5 to 0.5 au at intervals of 0.04 au.
have purely repulsive Pd-C interactions with respect to their
diabatic asymptotes (ID or 3D Pd and CO) and thus are also
unbound with respect to ground-state IS Pd (by 21.9 and 33.5
kcal/mol, Table I).
Since all electrons in CO are paired, no covalent bonding to
the metal is possible. A linear geometry is preferred for PdCO,
in order to maximize overlap of the orbitals involved in t~ donation/?r back-bonding. Our calculations predict that the IZ+state
of PdCO has a short Pd-CO bond length of 1.96 A (Table 111).
By contrast, the open-shell I F and 321+ states are predicted to
have very long Pd-CO bond lengths, 4.1 5 and 4.54 A, respectively.
All three states have C - O bond lengths of 1.13 A. Our predicted Pd-CO distance of 1.96 A is in good agreement with
previous theoretical predictions that range from 1.87 to 2.1 1 A
(Table VI). The value of 2.1 1 A is derived from a nonrelativistic
treatment of Pd," which points out artifacts that may result from
the lack of inclusion of relativistic orbital contractions. Our value
-
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2331
Interactions of N O and C O with Pd and Pt Atoms
TABLE V: l2+MCO Bond Energies for M = Pd and PP
HF
GVB(6/12)PP
RCI(6/ 12)
GVBCI(6/12)
-142.04571 (729/3012)
-142.05970 (18973/57 168)
-141.96865 (64/64)
IZtPdCO
-141.89592 ( ] / I )
-139.14408 (729/3012)
-139.16002 (18973/57 168)
-139.00368 (1/1)
-139.07424 (64/64)
'E+ P t c o
-112.87644 (27/37)
-1 12.87765 (45/55)
-112.75859 (1/1)
-112.81856 (8/8)
12' Cob
-29.1 1796 (8/8)
-29.138 50 (27/37)
-29.13869 (45/55)
-29.10660 ( I / ] )
IS Pdb
1s Ptb
-26.19995 (1/1)
-26.21 1 25 (8/8)
-26.219 46 (27/37)
-26.226 37 (45/55)
20.2
19.3
27.2
D,(Pd-CO)c
19.3
35.1
28.3
27.9
D,d'lb(Pt-CO)d
30.2
D,ld"b(Pt-CO)'
8.6
8.2
10.5
15.4 (18.5)'
'Reference 36. Aside the total energy in hartrees are the number of spatial configurations/spin eigenfunctions for the given wave function (in
parentheses). *Correspondingwave functions are HF, GVB(3/6)PP, RCI(3/6), and GVBCI(3/6). CTheadiabatic bond dissociation energy (0,)
is
Z
' CO and IS Pd, in kcal/mol. dThe bond dissociation energy DtUb is the energy to dissociate to ! E + CO and IS Pt, in
the energy to dissociate to I
kcal/mol. #The adiabatic bond energy D,ldiaballows Pt to relax in a spin-forbidden transition to its )D ground state (kcal/mol). The Pt IS-)D
splitting (19.7 kcal/mol) was calculated by comparing the total energy of a IS Pt RCI( 1/2) wave function from which single and double excitations
were allowed from the correlated d pair (RCI(I/2)*SDd.pi, to the total energy of a 3D Pt HF wave function from which all single and double
excitations were allowed from the open shell s and d orbitals (HF*SD,,,,d). /Best estimate for the true De, using the experimental IS3D splitting
(16.6 kcal/mol).
TABLE VI: ComDarison of Theoretical and Exwrimental Prowrties for l2+PdCO and PtCO
we( M C - O
D,(M-CO)"
PdCO
this work
MP2'
MRDCIX
MRCC112*
CPF20h
expt
PtCO
this work
MP2'
SCF"
SCF"
cIP
expt
R,(MC-O)b R,(M-CO)b
27.2
37.4
7.8
28
33
2 2'
1.14
1.185
1.16
15.4 (18.5)
37.4
27.0
14.8
42.7
31 f 10
1.13
1.184
1.96
1.882
2.1 1
1.87
1.91
w,(MC-O)e
w,(MCO)e
bend)c
Pd
428
561
1.30
1.86
1976
600
56 1
1.12
1.79
2157
527
550
2104,' 2073: 1996,' 1954'
468'
2253
2050i 2045/ 2096,k 2092,' 1955"
1.159
1.99
1.977
1.91
1.754
1.707
'Adiabatic M-CO dissociation energy in kcal/mol. Value in parentheses is best estimate. Equilibrium bond length in angstroms. Vibrational
frequency in cm-I. dMagnitude of dipole moment vector in debye. A positive sign indicates the negative end of the dipole points toward the 0 atom.
]Matrix isolation.16J7'Atop CO adsorbed
'Reference 27. /UHF value. #Reference 24. hReference25. 'Binding energy for atop CO on Pd(l1
on Pd(100).' 'Atop CO adsorbed on Pd(llI).l "Pd(CO)(PPh3),.I9 "Reference 26. OReference 29. PReference 28. PBinding energy of CO on
Atop-bonded CO on Pt(l1
JAtop-bonded CO on Pt( 11
'Pt(C0)2(PPh3)2.'8
Pt( 1 1
of 1.96 A agrees with other relativistic calculations to within -0.08
A, and all methods predict the same WC-O bond length to within
-0.05 A.
The
state exhibits r back-donation from Pd to C O via
delocalization of the Pd d r orbitals toward the C atom, with
concomitant o donation from C to Pd via delocalization of the
C 2s (the CO 50) orbital toward Pd (Figure 2). Donation by
the CO Sa and Pd d r bonding orbitals is minimal in the open-shell
I P and 'E+ states, due to the large, diffuse Pd 5s orbital that
induces o repulsions between Pd and CO.
Mulliken population analysis indicates that -0.2 electron is
transferred from Pd to CO in the
state (Table IV), due
primarily to r back-bonding from Pd to C (0.17 electron). The
other two states (open-shell
and '2') have electron distributions [Pd (-IO), C (-5.9), and 0 (-8,l)I similar to the
separated fragments Pd and CO, as expected with such long Pd-C
equilibrium separations. The transfer of charge from Pd to C in
the IZ+state results in a positive dipole moment (Pd+-CO-) of
1.30 D (Table III), in reasonable agreement with a U H F value
of 1.86 D.27 In this case, both measures of charge transfer (dipole
moment and Mulliken populations) are consistent with each other.
The lack of charge transfer in the open-shell lZ+and 'Z+ states
results in small, negative dipole moments (Pd%O+) of -0.79 and
-0.55 D, respectively, in the same direction as for free CO.
The Pd-CO interaction in the I P state is very strong. Table
V depicts the increase in bond strength with increasing inclusion
of electron correlation. The best level of calculation, a
GVBCI(6/12) wave function, involves a full CI within the G V B
valence space of a GVB(6/12)-PP wave function and predicts a
Pd-CO bond energy of 27 kcal/mol. This is in excellent agreement with previous relativistic MRCCIl2 calculations of Siegbahn
and c o - w ~ r k e r sthat
~ ~ predict a bond energy of 28 kcal/mol.
Siegbahn and c o - w ~ r k e r salso
~ ~ carried out relativistic CPF
calculations using a larger basis set (1 ls8p4d3f for Pd and 5s4pld
for C and 0) and correlating 20 electrons to obtain a best De(Pd-CO) of 33 kcal/mol (Table VI). UHF/MP2 calculations
of Rohlfing and Hay2' give De = 37 kcal/mol, in reasonable
agreement with Siegbahn's result. However, as observed by
Siegbahn and co-~orkers?~
this agreement is probably fortuitous,
since the MP2 De for NiCO is 58 kcal/mol, compared to the best
theoretical value of 33 kcal/m~l.~'Pacchioni and Koutecky% used
a nonrelativistic pseudopotential for Pd and the MRD-CI method
to calculate a nonrelativistic De of 8 kcal/mol, again indicating
the importance of including relativistic effects for a proper description of the bonding.
The heat of adsorption for atopbonded CO on Pd( 100) is -22
kcal/mol at Oco = 0.5 ML,' which'is lower than our predicted
value of 27 kcal/mol. However, the measured surface binding
energy is no doubt lower than our value due to coverage-dependent
effects (i.e., repulsive lateral interactions between neighboring
adsorbed CO molecules).
The
state of PdCO has a predicted w,(Pd-CO) of 428 cm-'
that is much larger than those for the open-shell IZ+and 'Z+ states
(22 and 28 cm-I, respectively), as expected from the relative
Pd-CO bond strengths of these states (Table 111). We predict
a C-O vibrational frequency of 2253 cm-I, downshifted by only
20 cm-' from our vibrational frequency in free CO. A C-O
vibrational frequency of 2045-2050 cm-' was observed for PdCO
isolated in a matrix,16J7 while the o,(C-0) observed for Pd(CO)(PPh,), is 1955 cm-I.I9 C-O vibrational frequencies of 2096
and 2092 cm-* have been measured2' for C O adsorbed in linear
atop sites on Pd( 100) and Pd( 11 l), respectively. Since the ex-
2332 The Journal of Physical Chemistry, Vol. 95, No. 6,1991
perimental vibrational frequency for free CO is 2170 cm-I, we
see that the shifts seen experimentally due to CO interacting with
Pd range from 74 to 215 cm-I, significantly larger than our
calculated shift of 20 cm-I. The discrepancy between the predicted
and experimental shifts in w,(C-0) is probably due to basis set
truncation.
PtCO: We find that the ground state of PtCO is the lZ+state
Cjust as in PdCO), with a u donor/* acceptor bond, in agreement
with previous theoretical studies.2b29 We again examined two
other states of RCO an open-shell IZ+state that correlates with
ID Pt and 'Z+ state that correlates with 3D Pt at infinite Pt-C
separations. Just as for PdCO, the first excited state was found
to be the 'Z+ state (T,= 7.4 kcal/mol), with the open-shell lZ+
state lying higher in energy (T,= 23.1 kcal/mol). The preference
for Pt to be 'D leads to a much smaller 'X+-'2+ state splitting
for PtCO than for PdCO. In actuality, the 'Z+-IZ+ splitting may
be a bit larger than 7.4 kcal/mol, since we overestimate the
stability of the 'D state of Pt (Table I). Since we also underestimate the 'D-'D splitting in Pt by -24 kcal/mol, we expect
that the true open-shell IZ+state lies much higher than 23.1
kcal/mol above the ground state. Indeed, we find that the repulsions between the partially occupied Pt 6s and CO 5u lead to
an extremely weakly bound 3Z+state and a dissociative open-shell
IZ+state (Table 111). Thus, Pt must be promoted from its jD
ground state to the IS excited state (- 17 kcal/mol higher) in order
to avoid u repulsions and form a strong bond to CO.
Relativistic contraction of the valence orbitals of Pt results in
similar orbital extents for both Pd and Pt, leading to geometries
for the IZ+states of PtCO and PdCO that are very similar (Table
111). Our predicted Pt-C bond length (1 -99 A) agrees well with
previous theory of Rohlfing and Hay (1.977 A)27and Gavezzotti
et al. (1.91 A)26(Table VI). The values obtained by Basch and
CohenZ8for the equilibrium Pt-C bond length are somewhat
shorter (1.71-1.75 A). This is most likely due to the difference
in the ECPs used for Pt. The C-0 equilibrium bond length is
comparable (within -0.05 A) for all methods. A much greater
discrepancy is found for the 3Z+state, where we find R,(Pt-C)
= 2.99 A, while Basch and Cohen2* found R,(Pt-C) = 1.82 A.
Qualitatively, however, the same trend is observed in both calculations, with '2? PtCO exhibiting a longer bond length than
I
Z
' PtCO. The open-shell lZ+state of PtCO is predicted to have
a long Pt-CO bond and a low Pt-CO vibrational frequency,
indicating that the open-shell IZ+state has a very weak Pt-CO
interaction.
The bonding orbitals of 'Z+PtCO are qualitatively the same
as those for IZ+PdCO (Le., delocalization of metal d r orbitals
and the CO 5u orbital to form a u donorln back-bond) and
therefore are not depicted here. Long Pt-C bond lengths of 2.99
and 4.05 A for the 3Z+and open-shell 'X+ states, respectively,
yield little delocalization of the Pt d* orbitals toward C and little
CO 5u donation to the metal, just as in the open-shell IZ+and
jZ+ states of PdCO.
The propensity for Pt to accept u electrons from CO leads to
a net transfer of 0.23 electron from C to Pt in the IX+ state (Table
IV), opposite to the direction of charge transfer for PdCO.
However, the overall dipole moment for the
state of PtCO
is positive (Pd+-CO-) and very similar to p(PdCO), which indicates d* back-bonding must dominate. In this case, the Mulliken
population and dipole moment data contradict each other, in the
same manner as was found for free CO. The predicted value of
1.12 D for p(PtC0) is in reasonable agreement with U H F calculations of Rohlfing and Hay that predict a dipole moment of
1.75 D (Table VI),*' but our value should be more accurate as
it is derived from an MCSCF wave function. Little charge is
transferred between Pt and CO in the open-shell lZ+and 3Z+
states. Again, the electron populations are what would be expected
[valence Pt (-lo), C (-5.9), and 0 (-8.1)] for Pt and CO
fragments (Table IV), and the open-shell I F and 3Z+ states
exhibit negative dipole moments (Pt--CO+) of -0.56 and -1.68
D, respectively, in the same direction as the value for free CO.
The predicted adiabatic Pt-CO bond energy of 15.4 kcal/mol
(Table V) is much less than that for Pd-CO (0,
= 27 kcal/mol),
Smith and Carter
due to the lS-3D promotional cost for Pt and Pd does not incur,
Since we obtain a ISAD splitting that is too high by 3.1 kcal/mol,
our best estimate of the true Pt-CO bond energy is 18.5 kcal/mol.
Notice that the intrinsic (diabatic) Pt-CO bond strength at our
best level of calculation (Table V) is 35.1 kcal/mol, considerably
higher than the intrinsic Pd-CO bond strength of 27.2 kcal/mol.
Thus, the predicted Pt-CO bond energy of 18 kcal/mol is -8
kcal/mol higher than one might have expected if we had assumed
the Pt-CO and Pd-CO bond strengths to differ only by the 'S-'D
splitting in Pt.
Since Pt-Pt bonding involves primarily the 6s electrons," adsorbates on a Pt surface do not feel strong u-repulsive effects due
to free 6s electrons. Thus a C O molecule feels primarily an
attractive d9 configuration on a Pt surface or cluster, and hence
the Pt-CO bond is expected to be stronger on a surface or cluster
of Pt atoms. Indeed, the heat of adsorption of CO on Pt( 11 1)
is 3 1 f 1 k ~ a l / m o l .Although
~
this value may correspond to the
binding energy for bridging CO, the atop CO heat of adsorption
is very similar. Temperature-programmed EELS6 has shown that
the energy difference between atop and bridge sites is less than
1 kcal/mol. The dominant effect that weakens the metal atomCO bond relative to the surface is the energy required to promote
Pt from its 3D ground state to the state required to form the IZ+
state of PtCO (i.e., dIo Pt is needed for maximal ?r back-bonding
and u donation). This promotional energy is clearly lessened by
the presence of other Pt atoms in Pt metal, such that C O binds
strongly to an atop site (in an analogous way with the lZ+state
of PtCO).
Previous theoretical calculations of the properties of lZ+PtCO
have been carried out using SCF, MP2, and CI methods.2b29
UHF/MP2 calculations of Rohlfing and Hayz7yield an adiabatic
De of 37 kcal/mol (Table VI); however, this method has been
shown to overestimate other metal-CO bond energie~.~'Gavezzotti et a1.26found D,(Pt-CO) = 27 kcal/mol at the SCF level
using a minimal basis set (Table VI); basis set superposition errors
may be responsible for this large value. An SCF level calculation
by B a ~ c hplaces
~ ~ D,(Pt-CO) a t 15 kcal/mol, while a small,
energy-selected CI carried out by Basch and Cohen28puts 0,(Pt-CO) = 43 kcal/mol. There seems to be little agreement
between the various calculations, showing the sensitivity of the
result to the ECP, basis sets, and level of CI. However, our
prediction of D,(Pt-CO) = 18.5 kcal/mol arises from the most
highly correlated wave function used to date and thus is the most
reliable value available.
We find a Pt-CO bond energy of only 1 kcal/mol for 3Z+PtCO
(Table 111). In fact, the 'Z+ state may be unbound, since basis
set superposition effects may be at least 1 kcal/mol. Our result
is in severe disagreement with Basch and Cohen,Z8who found a
D,(Pt-CO) of 19 kcal/mol for this state. However, the trend in
bond energies is the same in both studies: a strongly bound lZ+
state and a weakly bound 'Z' state are predicted.
Although the predicted Pd-CO bond energy (27 kcal/mol) is
larger than the predicted Pt-CO bond energy (18.5 kcal/mol),
the Pd-C and Pt-C vibrational frequencies do not follow this trend
w,(Pd-C) = 428 cm-l while w,(Pt-C) = 600 cm-I. The larger
relativistic contraction of the Pt 6s orbital results in more donation
from the C O 5u pair than for Pd. Significant donation of both
u electrons from CO to Pt (0.41) versus Pd (0.03) and K electrons
from Pt to CO (0.20, Table IV) yields a larger intrinsic or diabatic
(promotionless) bond energy for Pt-CO (35 kcal/mol) than for
Pd-CO (27 kcal/mol). The stronger intrinsic bond between CO
and Pt is indicative of a stronger interaction near its equilibrium
configuration, which will dictate trends in harmonic vibrational
frequencies. Thus we expect and observe a larger w,(Pt-C) than
w,(Pd-C).
Our predicted C-O vibrational frequency for the I F state of
PtCO (1976 cm-I) is in good agreement with w,(C-0) for Pt(C0)2(PPh3)2(1996 and 1954 cm-'),I8 with the infrared spectrum16 exhibited by PtCO isolated in an Ar matrix Iw,(C-O) =
2052 cm-I], and the HREELS spectrum4 of C O adsorbed on
(44) Wang, H.; Carter, E. A., to be submitted.
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2333
Interactions of N O and C O with Pd and Pt Atoms
TABLE VII: Calculated Properties of MNO States (M = Pd and Pt)
PdNO
T E"
AE~
De(M-NO)'
Re(MN-O)'
Re(M-NO)'
B,(M-N-O)'
we(MN-O)g
we( M-NO)g
we(M-N-0 bend)r
Ph
2A'
2n
%+
2A'
-158.448 59
0.0 (17.9)
4.1
1.17
1.90
113.3
1686
292
671
-0.98
-158.44221
4.0 (0.0)
4.8
1.16
2.19
180.0
1866
99
524
0.33
-158.435 59
8.2 (26.1)
. ,
-155.576 28
d
1.16
4.82
180.0
1816
14
521
-0.16
0.0
20.4
1.17
2.16
112.9
1775
226
433
-0.39
PtNO
2II
22+
-155.559 84
-155.53438
26.3
d
1.16
2.39
180.0
1858
120
523
-0.76
10.3
d
1.16
4.40
180.0
1820
203
521
-0.1s
@Totalenergy at the GVB(4/8)-PP level in hartrees. bRelative energy in kcal/mol at the GVB(4/8)-PP level. Values in parentheses are corrected
for the error in the 3D-'S Pd splitting (see text). 'Adiabatic Pd-NO bond dissociation energy in kcal/mol. dunbound with respect to ground-state
fragments at all levels of CI attempted. 'Equilibrium bond length in angstroms. /Equilibrium bond angle in degrees. #Vibrational frequency in
cm-I. *Magnitude of dipole moment vector in debye. A positive sign indicates the negative end of the dipole points toward the oxygen atom.
Pt(ll1) [w,(C-0) = 2100 cm-I]. S C F calculations by B a ~ c h ~ ~ Another possibility for metal-NO bonding is a combination
of a covalent a bond and u-donor bond (Figure la). This 2Z+
on PtCO predicted w,(C-O) = 2157 cm-I, higher than any of the
experimental observations (Table VI). An HREELS loss peak
at 470 cm-l was assigned to the Pt-C stretch for a high coverage,
linear atop state: a bit lower than our predicted value of 600 cm-'
and Basch's prediction of 527 cm-l (Table IV).29 This may again
be due to repulsive lateral interactions between adsorbed C O
molecules.
In summary, we have found that C O will bind strongly to Pd
and Pt only if the metal valence s orbital is empty. This will allow
significant CO 5u donation, which will be accompanied by T
back-bonding from occupied metal d* orbitals to the CO 2 ~ *
orbitals. These bonding considerations favor Pd, which has a dIo
ground state, over ground state, sld9 Pt, since the partially filled
Pt 6s orbital will result in u repulsions. The intrinsic ]E+Pt-CO
bond is found to be 8 kcal/mol stronger than in IE+ Pd-CO, due
to greater C O 5u donation and Pt d r back-bonding. However,
the IS-)D promotional cost of 17 kcal/mol required for Pt to
form a strong bond to C O results in a smaller adiabatic bond
energy for IZ+Pt-CO.
On surfaces, adsorbate-adsorbate interactions and metal-metal
interactions lead to observed CO-surface binding energies that
are different from those for atomic M-CO bonding. Atop-bonded
C O on Pd(100) is observed to have a binding energy of -22
kcal/mol at 0,- = 0.5 ML,) while we predict a Pd-CO binding
energy of 27 kcal/mol for isolated atop-bonded CO. Thus, lateral
interactions between neighboring C O molecules appear to be
repulsive by - 5 kcal/mol. Atop C o o n Pt(ll1) is bound by -30
kcal/mol for Bc0 I 0.5 ML,6v7 while we estimate a D,(Pt-CO)
of 18.5 kcal/mol. It may be that surface Pt atoms do not require
electronic promotion to IS-like states, due to strong metal-metal
interactions involving the 6s electrons.u
Our calculations on PdCO and PtCO offer an array of new
predictions as well as opportunities to compare with previous
theory. Analytic gradients of GVB-PP wave functions have been
used for the first time to fully optimize the geometries and predict
harmonic vibrational frequencies for three states of PdCO and
PtCO. Post-HF (GVB-PP) dipole moments for all three states
of PdCO and PtCO have also been predicted for the first time.
Our prediction of the dissociation energy for 'E+PdCO confirms
previous relativistic all-electron calculations of Siegbahn and
c o - w o r k e r ~ indicating
,~~
the accuracy of our method, while our
R-CO bond energy of 18.5 kcal/mol represents the most accurate
value predicted to date. Finally, we also suggest that the open-shell
IZ+and
states of PdCO and PtCO are probably all dissociative
states.
C. Metal-NO Interactions. There are several ways for transition metals to form bonds to NO. To form a strong covalent
metal-NO u bond, a singly occupied metal u orbital is essential
(e.g., we will require 'D Pt or Pd). Since the unpaired electron
on N O is in a 27r* orbital, this (2A') covalently bound state will
be bent in order to maximize orbital overlap within the u bond
(Figure lb). With no strong repulsive interactions present in this
2A' state, we expect it to be strongly bound.
-
state requires a singly occupied metal d a orbital and an empty
metal u orbital for donation from the N 2s lone pair. Maximal
u- and a-bond overlap is obtained with a linear geometry. Neither
low-lying state of Pt or Pd (ISor 'D) satisfies both criteria for
strong bonding in the 2Z+state: IS will allow a-donation to be
effective with no covalent *-bonding, while )D will allow *-bond
formation, but now u donation is inhibited by the lack of an empty
u orbital. Thus, we expect the 2X+ state to be weakly bound.
It is also possible that N O may bond to a metal in a u donor 17
acceptor fashion similar to MCO bonding (Figure IC). This ll
state requires an empty metal u orbital for donation from the N
2s and doubly occupied metal d a orbitals for a back-bonding into
the partially occupied N O 2a* (i.e., here we require IS Pt or Pd).
A linear geometry is preferred for *IIM N O as for
MCO,
again to maximize r back-bonding and u donation. Therefore,
different electronic states of Pd and Pt will lead to different
preferred geometries: while dIo (IS)Pd or Pt will favor a linear
configuration of MNO, s'd9 ()D) Pd or Pt should exhibit both
linear and bent structures of MNO.
PdNO Pd has a IS (dlO) ground state, with its first excited
)D (s'd9) state 21.9 kcal/mol higher in energy,)' which suggests
that zII PdNO should be preferred over the 2Af and 2Z+states
that both require promotion to the 'D excited state. As mentioned
in section III.A, the HF method has difficulty predicting the
correct ground state for Pd. By increasing the level of electron
correlation in our calculations, we were able to predict the C O K ~ C ~
ground state for Pd using the same basis set. Unfortunately, the
splitting predicted at this level [)D (HF*SD(open-shell s, d))-'S
(RCI(1/2)*SD(d lone pair))]36 is only 1 kcal/mol. Since the
discrepancy between experiment and theory is so large, we have
used the experimental state splitting wherever this value is needed.
All three states examined lie close in energy a t the GVB(4/
8)-PP level: the bent 2A' state is lowest, the linear 211 state is the
first excited state ( T , = 4.0 kcal/mol), and the linear 2Z+ state
is the second excited state ( T , = 8.2 kcal/mol, Table VII).
However, the 2Afand 2Z+states should have been higher in energy
by the 'D-IS promotional cost of 21.9 kcal/mol. Thus, a better
estimate for the state splittings would place zII as the ground state,
with the ZAfand 2E+states 17.9 and 26.1 kcal/mol higher, respectively.
All states of PdNO examined had similar N - O equilibrium
bond lengths (2A' R,(N-O) = 1.17 A, 211and
R,(N-O) =
1.16 A; Table VII). The Pd-N equilibrium bond lengths reflect
the intrinsic or diabatic (promotionless bond strengths of the
different states (2A' R,(Pd-N) = 1.90 , 211 R,(Pd-N) = 2.19
A and 2Z+R,(Pd-N) = 4.82 A). As expected, the 211 and 2Z+
states of PdNO are linear, while the ?A' state is bent (0, 1 1 3 O ) .
Qualitative features of the bonding in the three low-lying states
examined are quite different from one another. The 2A' state
exhibits a covalent bond comprised of the N O 2a* orbital
(localized mostly on N ) and a Pd du orbital, with some delocalization of the N 2s orbital toward Pd. This bond is strong
I
-
A
-
2334 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
Smith and Carter
TABLE VIII: Electron Distributions in MNO for M = Pd and Pt
within M-N bondu
state
M
N
0
u donationb
2A' PdNO
1 .oo
0.85
0.15
2Z+ PdNO
1.oo
0.79
0.2 1
211 PdNO
0.03
2A' PtNO
0.97
0.87
0.16
2Z+ PtNO
1 .oo
0.79
0.21
211 PtNO
0.06
'A' PdNO+
1.05
0.69
0.26
'Z+PdNO+
1.06
0.68
0.26
,II PdNO+
0.08
'A' PtNO+
I .06
0.67
0.27
'Z+PtNO+
1.05
0.69
0.26
,II PtNO+
0.12
211 (NO)d
0.79
0.21
0.06
0.09
0.02
0.02
total
N
7.01
6.94
6.95
6.95
6.95
6.94
6.27
7.00
6.98
6.89
6.93
6.94
6.94
M
r back-bondinp
9.94
10.01
9.97
10.02
9.97
10.01
9.12
9.09
9.07
9.20
9.16
9.12
0
8.04
8.06
8.07
8.04
8.07
8.06
7.91
7.9 1
7.95
7.90
7.9 1
7.94
8.06
OTable IV,footnote a. bTotal electron population donated to in 4
0 5 u - c A ~ dmolecular orbital. Total electron population donated to 4 and
0 from M dr-derived orbitals. dOccupaiion of 2r* orbital on NO.
enough to polarize the diffuse, singly occupied metal valence s
orbital out of the way of the Pd-N bond. Figure 3 depicts the
bonding orbitals for 2A' PtNO, which are qualitatively the same
as those for 2A' PdNO. Notice that the NO 2 r * character is
retained in the metal-N bond pair (Figure 3a) and that the N
2s orbital is slightly distorted toward the metal center for additional
bonding (Figure 3b). The NO u and r bonds remain unperturbed
in the MNO complex (Figure 3c,d). Figure 3e depicts the metal
valence s orbital that shuns the region near the metal-N bond.
The 211 state exhibits r back-donation from Pd to NO via
delocalization of the Pd d r orbitals toward the N atom, with
concomitant u donation from N to Pd via delocalization of the
N 2s orbital toward Pd. The bonding orbitals of this state are
qualitatively the same as those for '2' PdCO (Figure 2) and thus
are not shown. We find that donation by the N 2s orbital and
Pd dr-NO 2 r * overlap are minimal in the 22+state, due to the
diffuse Pd 5s orbital causing u repulsions between Pd and NO,
leading to a purely repulsive (uninteresting) interaction.
The valence electron distribution for 2A' PdNO shows transfer
of only 0.06 electron from Pd to NO (Table VIII), indicating a
truly covalent interaction. Formation of the Pd-N bond results
in polarization of the NO 2 r * orbital toward the N atom (0.85
electron on N versus 0.79 electron on N in free NO; Table VIII),
in order to increase the overlap (SwN= 0.53) in the Pd-N bond.
This polarization of the NO 2r* orbital, along with delocalization
of the N 2s orbital toward Pd, leads to a negative dipole moment
of -0.98 D for the 2A' state (Table VII). The electron distribution
in the linear 211state shows little u donation (0.03 electron) and
very little r back-bonding (0.06electron, Table VIII), suggesting
only weak interactions for this state, even though it has a reasonable Pd-N equilibrium bond length. The positive dipole
moment of 0.33 D for the 211state is consistent with the Mulliken
populations, which indicate that d r back-bonding dominates the
charge transfer. Lack of charge transfer (or any strong interaction,
for that matter) in the 22+state results in a very small, negative
dipole moment of -0.16 D, in the same direction as found for free
NO.
We find that NO binds weakly to Pd atom no matter which
bonding mechanism or state is examined. Although a large intrinsic bond energy of 26.0 kcal/mol is predicted for the covalently
bound 2A' state at our best level of theory (Table IX), the atomic
promotional cost of 21.9 kcal/mol reduces the adiabatic bond
energy to only 4.1 kcal/mol. The u donor/r acceptor bond in
the 211state of PdNO is predicted to be only 4.8 kcal/mol strong
at our highest level of CI (Table X), even though NO is bonding
to the ground state of Pd. Indeed, test calculations (vide supra)
indicate that core polarization effects will reduce these bond
energies by at least 6 kcal/mol, leading to the prediction that *A'
and 211PdNO are probably unbound with respect to ground-state
Pd and NO.
Both linear states (211 and 2Z+) fail to form strong bonds to
NO because of repulsions between the N 2s and Pd 5s and 4da
electrons. These repulsions are largest for the diffuse Pd 5s,
-.o w l
I
I
I
I
I
t
ONEl
I
I
I
I
1
1
Figure 3. GVB(4/8)PP bonding orbitals for ZA' PtNO: (a) covalent
Pt-N bond; (b) N 2s lone pair; (c) NO u bond; (d) NO r bond; (e) singly
occupied Pt 6s orbital. Contours range from -0.5 to 0.5 au at intervals
of 0.04 au except for the Pt 6s orbital (0.01 au).
leading to a dissociative 22+state. Furthermore, the greater
electronegativity of N versus C leads to poor u donation (0.03
electron), and the partially occupied NO 2 r * orbital results in
poor r back-bonding (0.09 electron) for 211 PdNO versus '2+
PdCO (Tables IV and VIII), leading to a much weaker donor/
acceptor bond for PdNO.
Comparison of our predictions to NO adsorbed on Pd surfaces
is complicated by coverage-dependent effects. TPD results of
Conrad et a1.* and Jorgensen et al.9 indicate that NO. is bound
to Pd( 1 1 1) and Pd( 100) by 17 kcal/mol at high coverages, -24
kcal/mol at intermediate coverages on Pd(100), and -32
kcal/mol at low coverages. The NOsurface bond energy most
relevant for our purposes is not 32 kcal/mol, which corresponds
to bridging NO, nor 17 kcal/mol, which corresponds to high
-
The Journal of Physical Chemistry. Vol. 95, No. 6, 1991 2335
Interactions of N O and CO with Pd and Pt Atoms
TABLE IX: 'A' MNO Bond Energies for M = Pd and Pto
2A' PdNO
HF/-158.37008 (1/1)
GVB(4/8)PP/ -1 58.448 59
(16/16)
RCI(4/8)/ -158.48098
(811354)
GVBC1(4/81/ -158.489 39
(603j2iooj
RCISDdI -1 58.496 46
(345oj14356)
RCIS'/-I 58.589 72
(5379/32962)
RCISDSh/ -158.591 75
(8340/45130)
RCISDSGVB'/ -158.596 75
(8745/46425)
TE, hartrecs
2AfPtNO
HF/-155.46976 ( 1 / 1 )
GVB(4/8)PP/ -155.576 28
(16/16)
RCI(4/8)/ -155.610 10
(811354)
GVBCI(4/8)/ -155.61887
(603/2100)
RCISDd/ -155.623 41
(3450/ 14356)
RCIg/-155.707 05
(5379/32962)
RCISDSh/ -155.70887
(8340/45130)
RCISDSGVB'I -155.714 13
(8745/46425)
211NO
HFp29.267 01 (1 /1)
GVB(3/6)PP/ -129.329 58 (8/8)
D,d'lb(Pd-NO)b D,'d(Pd-NO)*
-6.7'
C
8.9'
C
D,(Pt-NO)*
-22.7"'
4.9"'
10.2'
C
8.2"'
7.8'
C
9.w
RCISP/ -129.366 57 (63/144)
11.6'
C
1 1.2"'
RCISc/-129.436 96 (327/ 1211)
24.5'
2.6
18.9"
R C W / -129.43696 (327/1211)
25.7'
3.8
20.0"
RCISGVB'I -129.441 61 (365/1287)
26.0'
4.1
20.4"
RCI(3/6)/ -129.35809 (27/76)
GVBC1(3/6)/ -129.365 60 (77/188)
'References 36 and 43. bThe diabatic bond dissociation energy DFb(Pd-N0) and D,(Pt-NO) are the bond dissociation energies to 211NO and )D M
(kcal/mol). The adiabatic bond energy D,"(Pd-NO) allows Pd to relax in a spin-forbidden transition to its 'S ground state (kcal/mol). The experimental Pd
splitting was used.)* tunbound with respect to ground-state fragments. dRCISD = RCI(M-N u, NO u, NOT, 3/6)*SD(M-N a) + RCI(4/8).
) RCI(3/6). 'RCIS = RCI(M-N u, NO u, NO A, 3/6)*SVaI+ RCI(4/8). 8RCIS RCI(N0 u, NO r,
#RCISP = RCI(N0 u, NO r, 2/4)*S(N 2 p ~ +
2/4)*SV,1+ RCI(3/6). 'RCISDS
RCI(M-N a, NO U, NO A, 3/6)*[SD(M-N U) + SVaJ+ RCI(4/8). 'RCISDSGVB = RCI(M-N U, NO U, NO r,
3/6)*[SD(M-N a) + Sval]+ GVBC1(4/8). 'RCISGVB = RCI(N0 u, NO K , 2/4)*s,,, + GVBCI(3/6). 'Dissociates diabatically to HF )D Pd (total energy
= -29.1 11 38 hartrees). 'Dissociates diabatically to HF*SVaI)D Pd (total energy = -29.1 13 37 hartrees). "'Dissociates to HF )D Pt (total energy = -26.23892
hartrees). "Dissociates to HF*SVaI'D Pt (total energy = -26.24004 hartrees).
TABLE X 'I PdNO Bond Energies'
~~
211 PdNO
HF/-158.379 10 (1/1)
GVB(4/8)PP/ -1 58.44221 (16/16)
RC1(4/8)/ -158.47340 (81/354)
GVBC1(4/8)/ -158.476 52 (331/1076)
RCISC/ -158.564 19 (3048/20398)
RCISW/ -158.51446 (9856/54589)
RCISDGVB'/ -158.515 16 (10060/55217)
TE, hartrees
211 N O
HF/-I 29.267 01 (1 / 1)
GVB(2/4)PP/ -129.324 53 (4/4)
RCI(2/4)/ -129.34942 (9/17)
GVBCI(2/4)/ -129.34990 (11/19)
RCISd/-l 29.403 29 (122/323)
R C I S P / -129.37924 (261/648)
RCISDGVB'/ -129.37964 (263/650)
~
IS Pd
HF/-29.106 60 ( 1 / I )
GVB(2/4)PP/ -29.1 14 13 (4/4)
RCI(2/4)/ -29.121 16 (9/10)
GVBCI(2/4)/ -29.121 23 (11/12)
RCISc/ -29.15736 (168/297)
RCISDh/ -29.12793 (253/308)
RCISDk/ -29.12793 (253/308)
D,(Pd-NO)*
3.4
2.2
1.7
3.4
2.2
4.6
4.8
'References 36 and 43. bThe bond dissociation energy 0,is the dissociation energy to 211N O and IS Pd in kcal/mol. 'RCIS = RCI(Pd d,, Pd
d,,, N O r, 3/6)*S,,1 + RCI(4/8). dRCIS = R C I ( N 0 r, 1/2)*S,,, + RCI(2/4). 'RCIS = RCI(d,,, d,,, 2/4)*SvaI. 'RCISD = RCI(d,,, dyz,N O
x , 3/6)*[SD(d,,) + SD(d,,) + SD(N 2s)] + RCI(4/8). ERCISD = R C I ( N 0 A, 1/2)*SD(N 2s)
RCI(2/4). *RCISD = RCI(d,, d,,, 2/4)*[SD(dJ + SD(d,,)]. 'RCISDGVB = RCI(d,, d,,, N O A, 3/6)*[SD(d,,)
SD(d z ) + SD(N 2s)l
GVBCI(4/8). 'RCISD = R C I ( N 0 r,
1/2)*SD(N 2s) GVBCI(2/4). 'RCISD = RCI(d,, d,,, 2/4)*[SD(d,) + SD(d,,)f.
+
coverages where lateral interactions perturb the bond strength.
Rather, atop-bonded N O at lower coverages is the state closest
to our complex. This state of N O on Pd(100) is bound by 24
kcal/mol, which is similar to the intrinsic bond strength of the
bent 2A' PdNO (26 kcal/mol). However, HREELS data9J0are
inconclusive as to whether atop N O on Pd( 100) is linear, bent,
or both, so it may be that -24-26 kcal/mol is the intrinsic bond
strength for both bent and linear atop N O on Pd. Indeed, the
bond strengths of bent and atop N O are the same (19 f 2
kcal/mol) for N O on Pt(l1 l).12
The dominant effect that weakens the metal atom-NO bond
relative to the surface is the energy required to promote Pd from
its dIo ground state to the s1d9state required for forming the bent
2Afstate of PdNO. This promotional energy is clearly lessened
in Pd metal, such that NO might bind strongly to a bent atop site
(in an analogous way to 2A' PdNO). In particular, the electronic
configuration of Pd in the bulk has substantial 5s 0ccupation,4~
which provides one explanation of why N O binds more strongly
to the bulk metal (no promotional cost is incurred, since some Pd
atoms are s'd9 already). Another possibility is that linear atop
sites are stabilized on the surface by delocalization of partially
occupied valence s orbitals via metal-metal bonding,44 such that
u repulsions between the metal and N O are reduced and then N O
can bind strongly to a linear atop site (in an analogous way to
zZ+PdNO).
The Pd-N stretching frequencies (Table VII) reflect the intrinsic bond strengths of the three states (2A' w,(Pd-N) = 292
cm-I, 211 o,(Pd-N) = 99 cm-', and 2Z+ w,(Pd-N) = 14 cm-I).
(45) Louie, S. G. Phys. Rev. Len. 1978, 40, 1525.
+
+
-
+
The N - O stretching frequency for ZA' PdNO (w,(N-O) = 1686
cm-l) is 150 cm-I lower than that for 211 PdNO (wJN-0) =
1866 cm-I) and 2Z+ PdNO (w,(N-0) = 1816 cm-I), which are
close to the free N O frequency (18 13 cm-' theoretically and 1904
cm-I experimentally; Table 11). The lowering of o,(N-O) in the
2A' state indicates that the N-0 bond has been converted to a
double bond by virtue of interaction with the Pd atom. Jorgensen
et aL9 and Nyberg and Uvdal'O observed N - 0 stretching frequencies for atop-bonded N O on Pd(100) in the 1678-1750-cm-I
range, close to the predicted values for the (zA') bent state and
the (2Z+) linear state of PdNO, again suggesting that both bent
and linear atop N O may coexist on certain Pd surfaces. HREELS
losses corresponding to Pd-N stretches are observed in the 202331-cm-l range,1° in excellent agreement with our predicted Pd-N
stretching frequency of 292 cm-I. Since we predict a Pd-N-0
bend at 671 cm-I, a bending mode on Pd should be observed -600
cm-I, similar to that seen on Pt( 11 1) (vide infra). Nyberg and
UvdalIo do not see such a mode for N O on Pd( 100); however, we
suggest that bent N O may form on Pd( 11 1) precovered with
oxygen, as in the analogous experiments on Pt( 111).l2 Indeed,
HREELS studies for N O adsorption on sulfur-precovered Pd(100)
suggest that bent N O is formed under such condition^.^
Linear and bent N O transition-metal complexes are quite
common. N - 0 stretching frequencies21*22for the complexes
(C5Ph5)PdN0, (C&-t~lyl)~)PdNO,(C5Ph3Et2)PdN0,and
(C5H5)PdN0range from 1755 to 1789 cm-I, also in reasonable
agreement (within -5%) of our predicted frequencies for the bent
(2A') and linear (2Z+) states.
PtNO Pt has a 3Dground state, which suggests that linear
2For bent 2A' PtNO should be preferred over linear 211 PtNO.
Indeed, we find the 2A' state of PtNO to be the ground state, where
2336 The Journal of Physical Chemistry, Vol. 95, No. 6. 1991
Smith and Carter
TABLE XI. Calculated Properties of MNO+ States for M = Pd and Pt
PdNO+
TE"
AE~
De(M-NO)'
Re(MN-O)d
R,(M-NO)d
B,(M-N-O)'
w,(MN-OY
we(^-^^)'
w,(M-N-O bend)'
Pt
1 A'
3n
-158.247 39
0.0
38.8
1.14
1.99
118.4
1866
722
329
-1.65
-158.227 57
12.4
14.6
1.14
2.28
180.0
21 I9
22 1
523
-1.65
PtNO+
IZ+
-158.22669
13.0
20.7
1.14
2.15
180.0
1943
223
524
-2.12
1A'
3l-I
lZ+
-155.329 13
0.0
31.0
1.14
2.20
117.1
1865
632
317
-1.93
-155.309 17
12.5
11.2
1.15
2.43
180.0
2101
199
523
-1.74
-155.307 82
13.4
15.4
1.14
2.33
180.0
201 3
189
523
-2.18
"Total energy in hartrees at the GVB(4/8)-PP level for the singlet states and at the GVB(3/6)-PP level for the triplet state. bRelative energy in
kcal/mol at the same levels as in a. CThebond dissociation energy ( D e ) is the energy to dissociate to 211 N O and 2D Pd+ or Pt+ in kcal/mol.
dEquilibrium bond length in angstroms. eEquilibrium bond angle in degrees. YVibrational frequency in cm-I. #Magnitude of dipole moment vector
in debye (with the metal ion at the origin). A positive sign indicates the negative end of the dipole points toward the oxygen atom.
a strong Pt-N u bond is formed between a du orbital on Pt and
the N O 2 ~ orbital.
*
In the 2Z+ state, a A bond tries to form
between a Pt d r orbital and the N O 2 ~ orbital.
*
However, this
bond is apparently not strong enough to compensate for repulsions
caused by the singly occupied 6s orbital on Pt, resulting in a
dissociative state. The 211 state again would be formed through
a u donor/* back-bonding mechanism, but we find that the ISAD
promotional cost cancels any intrinsic bonding for 211 PtNO. In
terms of total energies at the GVB(4/8)-PP level, the first excited
state is the 2Z+state (T,= 10.3 kcal/mol) and the second excited
state is the 211state (T,= 26.3 kcal/mol, Table VII). The ordering
of these states tracks their ability to form bonds to the ground
state of Pt. Since we make an error of only 5.6 kcal/mol in the
S 3 Dsplitting in Pt at the GVB-PP level ( e v e = 22.2 kcal/mol
versus AEEXP= 16.6 kcal/mol), we expect the electronic state
spectrum for PtNO to be close to these predicted values, with
perhaps the T, for 211 PtNO reduced to 20.7 kcal/mol.
The 2A' state of PtNO has an equilibrium bond angle of 112.9',
essentially the same as that predicted for the 2A' state of PdNO
(Table VII). The equilibrium Pt-N bond length is predicted to
be slightly longer [R,(Pt-N) = 2.16 A] than that calculated for
2A' PdNO [R,(Pd-N) = 1.90 A]. Pt 6s-N 2s repulsions lead to
a very long Pt-N bond length [R,(Pt-N) = 4.40 A] for the 2Z+
state, with essentially no overlap between the Pt d r orbital and
the NO 2a* orbital. Although our calculations predict a reasonable equilibrium bond length for the 211 state [R,(Pt-N) =
2.39 A], the predicted Pt-NO vibrational frequency is quite low
[w,(Pt-N) = 120 cm-I; Table VII], indicative of a very weak Pt-N
bond.
The qualitative features of the bonds formed between Pt and
N O are essentially identical with the PdNO states. The bonding
orbitals of 2A' PtNO are shown in Figure 3, where Figure 3a
depicts the covalent bond between the N O 2u* orbital and a Pt
du orbital, Figure 3b shows some delocalization of the N 2s lone
pair toward Pt, Figure 3c,d shows the rather unperturbed N O u
and A bonds, and Figure 3e shows the Pt 6s singly occupied orbital.
The bonding orbitals of 211 PtNO are qualitatively the same as
those for 211 PdNO and 'Z+PdCO and therefore are not shown.
Finally, just as for 2Z+PdNO, u donation by the N 2s orbital and
Pt dr-NO 27r* overlap are minimal in the 2Z+state, due to the
diffuse Pt 6s orbital causing u repulsions between Pt and NO.
The electron distributions for the three states of PtNO examined
show little net charge transfer between Pt and NO. The electron
populations are what would be expected [valence Pt ( lo), N
(-6.91, and 0(-8.1)] for Pt and N O fragments (Table VIII).
The lack of significant charge transfer for the 2A', 211, and 22+
states results in small negative dipole moments (Pt--NO+) of
-0.39, -0.76, and -0.1 5 D, respectively, in the same direction as
for free N O (Table VII).
Of three likely candidates for the ground state, only the bent
2A' state is predicted to be bound. Table IX shows that our highest
level of CI, RCISDGVB, predicts a strong Pt-N bond of 20.4
kcal/mol, consistent with the high orbital overlap (SPN= 0.47)
in the Pt-N bond. Note that the orbital overlaps track the intrinsic
-
covalent metal-N bond strength, with the higher overlap in the
Pd-N bond (SwN= 0.53) giving rise to a stronger intrinsic bond
strength of 26.1 kcal/mol, even though the adiabatic Pd-N bond
strength is so small (vide supra).
Although only the bent NO state is stable for Pt atom, we
expect that a linear, atop-bonded, 2Z+-like state should compete
with the bent state on a Pt surface, since Pt-Pt bonding interactions will polarize the valence s-band orbitals away from Pt-NO
bonds." TPD ~ t u d i e s ' ~ reveal
J ~ J ~ N O desorption activation energies of 14, 19 f 2, and 25 kcal/mol, for the high-coverage linear
atop state,14the bent surface state,'* and the bridging N O ~ t a t e , ~ ~ , ' ~
respectively. The value of 19 f 2 kcal/mol for the bent NO
binding energy is in excellent agreement with the predicted Pt-N
bond energy in 2A' PtNO.
Our predicted vibrational frequency of 1775 cm-' for the N-O
stretch of the 2A' state is in close agreement with N-O vibrational
frequencies from HREELS observed by Pirug et
and Gland
and c e w o r k e r ~ ' ~forJ ~N~O adsorbed on several singlecrystal faces
of Pt. For high surface coverages, the losses appear at 1790
(Pt(100)),13b1760 (Pt(l10)),'5band 171Ocm-' (Pt(11l)).I4 These
workers did not identify any low-frequency bending modes, and
thus the above frequencies may be associated with linear atop
species. Bartram et a1.12 studied the effect of coadsorbed oxygen
atoms on the bonding of N O to Pt( 111). When the Pt( 11 1)
surface was precovered with 0.75 ML of O(ad), they observed
a new low-frequency mode assigned to adsorbed bent NO. The
vibrational frequencies from HREELS for this state are 1775
(N-O stretching mode), 265 cm-' (Pt-N stretching mode), and
510 cm-' (Pt-N-O bending mode), in excellent agreement with
our predicted values of 1775,226, and 433 cm-l, respectively, for
the bent 2A' state of PtNO.
PdNoC and PtNoC: We have seen that occupation of the metal
valence s orbital inhibits bonding in the linear states of MNO.
To stabilize these linear states, then, we must ionize the metal
center. We have therefore examined the interaction of N O with
Pd+ and Pt+,since these ions in their ground 2D states have only
d valence electrons. We find that the ground states of PdNO+
and PtNO+ are still bent, as in the neutral complexes. These bent
'A' states from even stronger covalent bonds between the metal
du orbital and the N O 2n* orbital than the neutrals do. The other
two states studied were the linear 'Z+ state that bonds via a
covalent A bond and u-donor bond and the linear 311 state that
bonds via a u donor/* back-bonding mechanism. Both lZ+and
states are strongly bound, in contrast to the corresponding
neutral complexes. The 311 and IZ+states for both PdNO+ and
PtNO+ are nearly degenerate excited states about 13 kcal/mol
above the bent 'A' ground states (Table XI).
The equilibrium bond angles for 'A' PdNO+ and PtNO+ are
-5'
larger than those for 2A' PdNO and PtNO in order to
increase N 2s donation to the metal ion (Tables VI1 and XI). The
311 and
states have linear geometries in order to maximize
T overlap, just as for 211 and 2Z+ MNO. All three states for
PdNO+ and PtNO+ have essentially the same predicted N - O
equilibrium bond length of -1.14 A (Table XI). The metal-N
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2337
Interactions of NO and CO with Pd and Pt Atoms
TABLE MI: 'Af MNO+ Bond Enemies for M = Pd+ and Pt+"
TE, hartrees
'Af PdNO+
'A' PtNO+
HF/-I 58.103 65 (1 / I )
HF/-155.185 84 (1 / I )
GVB(4/8)PP/ -1 58.247 39 ( I 6/ 16)
GVB(4/8)PP/ -155.329 13 (l6/16)
RC1(4/8)/ -158.277 19 (81/150)
RCI(4/8)/ -155.35890 (81/150)
GVBCI(4/8)/ -158.285 36 (603/924) GVBCI(4/8)/ -155.367 62 (603/924)
RCISD'/ -158.286 61 (3459/6188)
RCISDc/ -155.368 26 (3459/6188)
RCIS'/ -1 58.377 38 (4776/ 12230)
RCIS'/ -155.453 01 (4776/12230)
RCISDSg/ -1 58.378 61 (7746/17504) RCISDSg/ -1 55.454 02 (7746/17504)
211 NO
HF/-129.267 01 (1 / I )
GVB(3/6)PP/ -129.329 58 (8/8)
RCI(3/6)/ -1 29.358 09 (27/76)
GVBCI(3/6)/ -129.365 60 (77/188)
RCISPd/ -129.366 57 (63/144)
-129.43696 (327/1211)
-129.43696 (327/1211)
De(Pd+-NO)'
-25.5'
25.4'
26.2''
26.6'
26.8'
38.0'
38.8'
De(Pt+-NO)'
-25.41
25.2)
26.01
26.81
26.51
30.3k
3 1.Ok
OReferenceS 36 and 43. 'Table XI, footnote c. 'RCISD = RCI(N0 u, NO 'R, M-N, 3/6)*SD(M-N) + RCI(4/8). dRCISP = RCI(N0 u, NO r ,
2/4)*S(N 2pr) + RC1(3/6). 'RCIS = RCI(N0 U, NO r , M-N, 3/6)*S,I + RCI(4/8). fRCIS =Z RCI(N0 u, NO r , 2/4)*S,1+ RCI(3/6). A'RCISDS
= RCI(N0 u, NO 'R, M-N, 3/6)*[SVaI+ SD(M-N)] + RCI(4/8). 'Dissociates to HF 2D Pd+ (total energy = -28.87735 hartrees). 'Dissociates to HF*&
2D Pd+ (total energy = -28.879 83 hartrees). 'Dissociates to H F 2D Pt+ (total energy = -25.959 33 hartrees). Dissociates to HF*SVaI2D Pt+ (total energy
= -25.967 70 hartrees).
Pd*
Pd"N\o
ONE1
I
I
I
I
*
.--.
'.
ONE
I
I
1
I
I
I
I
I
I
.-- -
-N-0
ONE
ONE
.
ONE
-.
I
I
ONE
I
I
I
I
I
I
I
1
I
I
I
Figure 4. GVB(4/8)PP bonding orbitals for 'A' PdNO':
(a) covalent
Pd-N bond; (b) N 2s lone pair; (c) N O u bond; (d) N O a bond. Contours range from -0.5 to 0.5 au at intervals at 0.04 au.
equilibrium bond lengths reflect the relative bond strengths (vide
infra) of the three states (for Pd, 'A' R,(Pd-N) = 1.99 A, lZ+
R,(Pd-N) = 2.15 A, and 311 R,(Pd-N) = 2.28 A; for Pt, 'A'
Re(Pt-N) = 2.20 A,
R,(Pt-N) = 2.33 A, and 311R,(Pt-N)
= 2.43 A). H F calculations by B a ~ c hon~ l~Z+PtNO' predicted
equilibrium Pt-N and N - 0 bond lengths of 1.678 and 1.13 A,
respectively. While our N - O bond lengths are in agreement, the
large discrepancy in the Pt-N bond length is probably an artifact
of Basch's HF wave function, which often predicts artifically short
bond lengths.
The 'A' states of PdNO+ and PtNO+ form covalent u bonds
between the metal du orbital and the 2n* orbital of NO, with
significant delocalization of the N 2s orbital toward the metal (as
depicted for Pd+ in Figure 4). The
states form covalent a
bonds between a metal dn orbital and the NO 2n* orbital (Figure
5b), concomitant with u donation from NO to the metal via
delocalization of the N 2s orbital into the empty metal s orbital
(Figure sa). A small increase in u charge transfer is observed
in the cationic complexes (-0.05 electron, Table VIII) relative
to the neutral MNO complexes, due to more effective donation
by the N 2s orbital and due to increased electrophilicity of M+.
The 311states bond through a u donor/n back-bonding mechanism,
similar to that for 'Z+ PdCO. However, the positively charged
character of the metal and the partially occupied nature of the
NO 2n* orbital reduces the extent of delocalization of the metal
Figure 5. GVB(4/8)PP bonding orbitals for 'Z+ PdNO+: (a) N O 5u
donor bond; (b) Pd-N covalent u bond; (c) N O u bond; (d) N O u bond.
Contours range from -0.5 to 0.5 au at intervals of 0.04 au.
MCO or 211 MNO. By
dn orbitals for 311M+NO relative to
contrast, the high electronegativity of the metal ion enhances u
donation to the metal for 311 M+NO.
The valence electron distributions in all of the states of MNO+
show a net charge transfer from NO to M+ (for Pd+, 'A' 0.12
electron, lZ+0.09 electron, 311 0.07 electron; for Pt+, 'A' 0.20
electron, IZ+0.16 electron, and 3110.12 electron; Table VIII),
in line with the electronegativity arguments discussed above. The
substantial net donation of charge from NO to M+ in all of the
states leads to large negative dipole moments (for Pd+, 'A' -1.65
D, '2' -2.12 D, and 311-1.65 D; for Pt+,'A' -1.93 D,IZ+-2.1 8
D, and 311-1.74 D; Table XI).
All three of the cationic states of PdNO+ have stronger Pd-N
bonds than the corresponding neutral states. The 'A' state has
the strongest Pd-N bond (De(Pd-N) = 38.8 kcal/mol), with the
predicted bond dissociation energy increasing dramatically with
increasing electron correlation (Table XII). The lZ+and 311states
are now also strongly bound, with predicted binding energies of
20.7 and 14.6 kcal/mol, respectively. Notice that the largest '2'
and 311 Pd+-N bond dissociation energies are obtained for the
RCI*SVaIwave function (Tables XI11 and XIV), indicating the
importance of orbital shape changes (single excitations) in the
description of donor/acceptor bonds.
All three states of PtNO+ are also strongly bound, due to the
absence of repulsions by the metal valence s electron and the
increased electrophilicity of Pt+. The bent 'A' state has the
2338 The Journal of Physical Chemistry, Vol. 95, No. 6,1991
TABLE XIII: 311 MNO+ Bond Enemies for M = Pd+ and Pt+"
TE, hartrees
'II PdNO+
'II PtNO+
HF/-158.16126 (1/1)
GVB(3/6)PP/ -158.227 57 (8/8)
RC1(3/6)/ -158.25565 (27/126)
GVBCI(3/6)/ -158.261 77 (77/300)
GVBSDC/ -158.28440 (4767/12106)
RCIS'/ -158.351 73 (2922/29956)
RCISD'J -158.29291 (6240/42629)
HF/-155.242 69 (1/1)
GVB(3/6)PP/ -155.309 17 (8/8)
RC1(3/6)/ -155.33746 (27/126)
GVBCI(3/6)/ -155.34368 (77/300)
GVBSDc/ -155.366 13 (4767/12106)
RCIS'/ -155.433 77 (2922/29956)
RCISD'/ -155.37465 (6240/42629)
Smith and Carter
'II NO
De(Pd+-NO)b
D.(Pt+-NO)*
HF/-129.267 01 (1/ 1)
GVB(3/6)PP/ -129.329 58 (8/8)
RCI(3/6)/ -129.35809 (27/76)
GVBCI(3/6)/ -129.36560 (77/188)
GVB36SDd/ -129.37967 (359/652)
RCIS'/ -129.448 71 (843/4430)
RCI24SDs/ -129.38774 (732/2691)
10.6h
13.0h
12.7'
1 1.8'
11.3'
14.4
11.5'
10.3'
12.7'
12.6'
11.8'
10.9'
10.9"
11.2'
+
+
"References 36 and 43. bTable XI, footnote c. 'GVBSD = GVBPP(N0 u, NO r,N 2s, 3/6)*[SD(M dxz) SD(M dyz)+ SD(N 2s)l RCI(N0 c, NO
,
N 2 ~3/6).
,
dGVB36SD = GVBPP(N0 U, NO T , N ZS, 3/6)*SD(N 2s) + RCI(N0 U, NO T , N 2 ~3/6).
,
'RCIS RCI(N0 U, NO T , N 2 ~3/6)*Sv,~.
fRCISD = RCI(N0 u, NOT, 2/4)*[SD(M dxz)+ SD(M d,) + SD(N 2s)] + RCI(N0 u, NO r, N 2s, 3/6). rRCI24SD RCI(N0 c, NO x , 2/4)*SD(N
2s) + RCI(N0 u, NO r, N 2s, 3/6). "able XII, footnote h. 'Dissociates to HFSD = HF*[SD(M dxz)+ SD(M dyz)] *D Pd+ (total energy = -28.88679
hartrees). /Table XII, footnote i. "Table XII, footnote j. 'Dissociates to HFSD 'D Pt+ (total energy = -25.969 14 hartrees; see footnote i . "'Table XII,
footnote k.
T,
MNO+ Bond Energies for M = Pd+ and Pt+O
TE, hartrees
'Z+ PdNO+
I Z+ PtNO+
HF/-155.149 70 (1/ 1)
HF/-158.066 65 ( I / 1)
GVB(4/8)PP/ -155.30782 (16/16)
GVB(4/8)PP/ -158.22669 (16/16)
RC1(4/8)/ -155.337 37 (81/150)
RC1(4/8)/ -158.255 83 (81/150)
GVBCI(4/8)/ -158.26260 (331/492) GVBCI(4/8)/ -155.344 17 (331/492)
RCIS'/ -158.34985 (2763/7116)
RCISC/ -155.429 19 (2763/7116)
RCISD'/ -158.291 18 (7815/18101)
RCISDC/ -155.37294 (7815/18101)
RCISDSg/ -158.36425 (9975/23731) RCISDSt/ -155.443 31 (9975/23731)
TABLE XIV
'II NO
HF/-129.267 01 (1/ 1 )
GVB(3/6)PP/ -129.329 58 (8/8)
RCI(3/6)/ -129.35809 (27/76)
GVBCI(3/6)/ -129.36560 (77/188)
RCISd/ -129.43696 (327/1211)
RCISD'/ -129.39592 (663/2379)
RCISDS'/ -129.45225 (963/3514)
De(Pd+-NO)b De(Pt+-NO)b
-48.8'
-48.1'
12.4'
11.9'
12.8'
12.5'
12.3'
12.1'
20.7'
15.4'
11.2'
11.1'
20.2)
14.7'
"References 36 and 43. bTable XI, footnote c. 'RCIS = RCI(N0 u, NO r, M-N, 3/6)*S,,, + RC1(4/8). dRCIS = RCI(N0 u, NO r, 2/4)*s,1 +
] RCI(4/8). fRCISD RCI(N0 U, NO T , 2/4)*[SD(N 2s) + S(N 2 p ) ]
RC1(3/6). 'RCISD RCI(N0 U, NO A, M-N, 3/6)*[SD(M-N) + SD(N 2 ~ ) +
RC1(3/6). 'RCISDS RCI(N0 U , NO r,M-N, 3/6)*[SD(M-N) + SD(N 2s) + S,,J + RC1(4/8). 'RCISDS = RCI(N0 U, NO r, 2/4)*[SD(N 2s)
S,,,]+ RCI(3/6). 'Table XII, footnote h. /Table XII, footnote i. 'Table XII, footnote j. 'Table XII, footnote k.
+
+
strongest Pt-N bond, with D,(Pt-N) = 31.0 kcal/mol. Notice
that both the 'A' and IZ+states of PtNO+ are unbound at the
HV level (Tables XI1 and XIV); this calls into question the utility
of the H F calculations by B a ~ c on
h ~I~Z+PtNO'. As is usual
for covalent bonds, the largest bond dissociation energy is obtained
for the most correlated wave function (RCISDS calculation; Table
XII). The lZ+and jII states are bound by -15 and -11
kcal/mol, respectively. Little variation (-2.5 kcal/mol) exists
in the Pt-N bond dissociation energy of 311 Pt+NO as a function
of electron correlation (Table XIII), consistent with the ion-diole
nature of the bonding that is reasonably well described even at
the SCF level. The largest IZ+PtNO+ bond dissociation energy
is obtained for the RCI*SVaIwave function, again showing how
crucial single excitations are for a proper description of the bonding
in this particular state (Table XIV).
The 'A' state M-NO vibrational frequencies (w,(Pd-N) = 722
cm-' and w,(Pt-N) = 632 cm-I) are much larger than for the other
two states (IZ+we(Pd-N) = 223 cm-' and jII w,(Pd-N) = 221
cm-I; IE+ w,(Pt-N) = 189 cm-' and 311 w,(Pt-N) = 199 cm-l;
Table XI). The M-N vibrational frequencies correlate with the
general trend in bond energies shown in Table XI [Le., 'A' De(M-N) > jII, IZ+D,(M-N)]. BaschZ9found Pt-N and N-0
vibrational frequencies for IZ+PtNO+ of 533 and 2167 cm-I,
much larger than our values of 189 and 2013 cm-I. Again, this
is probably an artifact of the HF method, which tends to predict
narrow potential wells. The N - 0 vibrational frequencies are
90-250 cm-I higher than for states of neutral MNO. This, along
with shorter equilibrium N-O bond lengths for the cationic states,
suggests that the N-O bonds, in addition to the M-N bonds, are
stronger in the cationic complexes. This is consistent with the
observed decrease in occupation of the N O 2 r * orbital for the
cations, leading to stronger N - 0 bonds.
IV. Summary
We have carried out extensive ab initio GVB/CI calculations
to ascertain both qualitative and quantitative features and contrasts
in the interaction of CO and NO with Pd and Pt atoms. We have
shown that CO binds strongly to Pd (0, 27 kcal/mol) but more
weakly to Pt (De 18 kcal/mol), where the differences in bond
strengths can be understood as a combination of two effects that
-
-
-
work in opposite directions: electronic promotional costs of 17
kcal/mol weaken the Pt-CO bond, while more effective CO u
donation to the Pt 6s orbital acts to strengthen the Pt-CO bond
by 8 kcal/mol. Since the large promotional energy associated
with exciting 'D Pt to 'S Pt is essential for bond formation to a
closed-shell ligand such as CO, while ground-state Pd is already
in the bonding IS state, the Pt-ligand bond is necessarily weaker
than the Pd-ligand bond (because differential increases in intrinsic
bond strengths are small compared to promotional energies).
Our calculations of MNO and MNO+ are to our knowledge
the first systematic ab initio studies of these systems. We have
shown that N O bound to a single Pd or Pt metal atom or ion
always prefers a bent structure, confirming the suggestion that
linear states might be disfavored for PdNO and PtNO, in contrast
to NiNOe30 The linear states are higher in energy because of
promotional costs (e.g., for 2Z+ PdNO and 211 PtNO) and repulsive interactions with the valence s and d orbitals on the metal
(present for all linear states). N O is predicted to bind either
weakly or not at all to Pd, since the (preferred) bent state requires
a promotional cost of 22 kcal/mol, which essentially wipes out
the intrinsic bond strength of 26 kcal/mol. By contrast, we find
that N O binds strongly to Pt, because it can bind to the jD ground
state of Pt. N O binds stronger still to the metal cations, for three
reasons: (i) it binds to the 2Dground states of both metal ions
(Le., no promotional costs are incurred); (ii) strong ion-dipole
interactions are present; (iii) no repulsions exist between the N O
5u and metal valence s electrons, since the valence s orbital is
empty.
Thus, we have shown that Pd and Pt have completely opposite
affinities toward binding CO and NO, with Pd preferring CO and
Pt preferring NO. This follows a general trend in which
closed-shell ligands prefer low-spin metal centers, while open-shell
ligands prefer open-shell metals.
Finally, it is important to emphasize that different electronic
states of metals necessarily lead to different molecular structures.
For example, while s1d9Pd and Pt can form either bent or linear
N O complexes, dIo Pd and Pt form only linear complexes. Thus,
consideration of the local electronic state of the metal and the
ligand provides a powerful tool for predicting the relative stabilities
of metal-ligand interactions.
J . Phys. Chem. 1991, 95, 2339-2344
Acknowledgment. Support of this work was provided by the
Office of Naval Research (Grant No. NO001 4-89-5-1 492) and
the donors of the Petroleum Research Fund, administered by the
American Chemical Society. E.A.C. also f3ratefUllY aChOWledges
a National Science Foundation Presidential Young Investigator
Award and a Camille and Henry Dreyfus Distinguished New
2339
Faculty Award.
Registry No. Pd, 7440-05-3; F't, 7440-06-4; CO, 630-08-0; NO,
10102-43-9; pd(co), 41 772-86-5; pt(cO), 498 19-49-0; Pd(NO),
132297-45-1; pt(NO), 132297-46-2; Pd(NO)+, 132297-47-3; F't(NO)+,
97223-72-8.
Ab I nltlo Molecular Orbital Study of Boron, Aluminum, Gallium p-Hydrido-Bridged
Hexahydrides
Charles W. Bock,*
Department of Chemistry, Philadelphia College of Textiles and Sciences, Philadelphia, Pennsylvania 191 44,
and American Research Institute, Marcus Hook, Pennsylvania 19061
Mendel Trachtman, Cindy Murphy, Bob Muschert,
Department of Chemistry, Philadelphia College of Textiles and Sciences, Philadelphia, Pennsylvania I91 44
and Gilbert J. Mains
Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: July 2, 1990)
The structures of H2X(p-H),YHZ,X, Y = B,Al, and Ga, have been optimized at the MPZ(FULL) level by using the 6-31G**
basis set for boron and aluminum and, for gallium, by using a Huzinaga's split-valence basis set augmented by polarization
functions. The structures containing boron all reflected the higher Lewis acidity of boron and tended toward a tetrahedral
arrangement of the hydrogen about this atom. Ga and A1 structures were almost identical, supporting the idea that the
3dI0 orbitals are ineffective in shielding the nuclear charge in Ga. The dissociation energies, i.e., into XH3 and YH3, were
remarkably similar when zero point energy and correlation effects were taken into account. Harmonic vibrational frequencies
were computed for all of the hexahydrides and, where possible, compared with experiment. Since the spectra of the mixed
hydrides are very rich, it should be possible to study the exchange reactions between diborane, digallane, and dialane (when
it is synthesized).
Introduction
Diborane, H2B(p-H)2BH2ris the prototype structure for phydrido bridging in electron-deficient systems. Consequently, the
structural and electronic properties of this novel dimer have been
studied extensively, both experimentally and computationally.'
In contrast, dialane, H2AI(p-H)2AIH2,and digallane, H2Ga(pH)2GaH2,have proven more difficult to study experimentally.
A12H6+was detected vis mass spectrometry as early as 1964,
whereas Ga2H,+ could not be detected under similar laboratory
conditions.2 Digallane, in fact, eluded experimental preparation
until recently when Downs, Goode, and Pulham3 prepared it by
using LiGaH, to reduce Ga2CI2H4(dimeric monochlorogallane)
and concluded they had produced a hydrogen-bridged diboranelike structure from its vibrational spectrum. Ab initio molecular
orbital studies4 have been reported for both dialane and digallane,
both finding a double-hydrogen-bridged structure to be the global
minimum. Although no experimental frequencies are available
for dialane, the agreement between the experimental frequencies
and those computed from ab initio calculations for digallane is
excellent and lends credence to the predicted dialane structure.
Lammertsma, et al. found the dimerization energies to decrease
in the order BH3, AIH3, and GaH3, although the optimizations
were carried out at different levels, e.g., dialane at' the MP2( 1 ) Liebman, J. F., Greenberg. A., Williams, R. E., Eds. Advances in
Boron and Boranes; VCH Publishers: New York, 1988.
(2) (a) Breisacher, P.;Siegel, B. J . Am. Chem. Soc. 1964,86, 5053. (b)
Breisacher, P.; Siegel, B. J . Am. Chem. Soc. 1965, 87, 4255.
(3) Downs, A.J.; Goode. M. J.; Pulham, C. R. J . Am. Chem. Soc. 1989,
I l l , 1936.
(4) Lammertsma, K.; Leszczynski, J. J . Phys. Chem. 1990, 94, 2806.
0022-3654/91/2095-2339$02.50/0
(FULL)/6-31Go* and digallane at the RHF/3-21G* level.
On the other hand, little information is currently available on
the structures, frequencies, and stabilization energies of the mixed
hexahydrides, GaBH,, GaAlH,, and AlBH6. In fact, gallaborane
is the only mixed dimer that has been synthesized and characterized. Pulham et al.5 studied the vibrational spectrum and
provided strong circumstantial evidence for a H2Ga(p-H)2BH2
structure with C, symmetry and this bridged structure was also
supported by electron-scattering experiments. Barone et a1.,6 using
a b initio pseudopotentials, determined that the bidendtate
structures of AIBH, and GaBH, are more stable than the very
symmetric tridentate structures by about 10 kcal/mol. Furthermore, the two bidentate structures appeared to be best described as (BH4)-XH2+,(X = Al, Ga). No calculations were
reported for AIGaH, in either the bidentate or tridentate form.
In this paper we shall compare the structures, frequencies, and
binding energies for the bidentate forms of all six hexahydrides
at similar levels of computation. The computed frequencies should
help in the experimental identification of the mixed hexahydrides
which have to date escaped synthesis. It will be especially interesting to determine the extent to which gallalane is describable
as (AIH4)-GaH2+or (GaH4)-AlHZ+.
Computational Methods
Ab initio calculations were performed using the GAUSSIAN86
and GAUSSIAN 88
of programs on the CRAY Y(5) Pulham, C. R.; Brain, P. T.; Downs, A. J.; Rankin, D. W. H.; Robertson, H. E. J . Chem. Soc., Chem. Commun. 1990, 177.
(6) Barone, V.; Minichino, C.; Lelj, F.; Nino, R. J . Compur. Chem. 1988,
9, 518.
0 1991 American Chemical Society