Inorganic Chemistry Sixth Edition Chapter 7- PART B Chapter 20 d Metal complexes: electronic structure and properties Modified By Dr. Cheng-Yu Lai PHYSICAL CHEMISTRY: MATTER, AND CHANGE 2E| PETER ATKINS| JULIO DE PAULA | RONALD FRIEDMAN INORGANIC CHEMISTRYQUANTA, 6E| SHRIVER| WELLER| OVERTON | ROURKE | ARMSTRONG ©2014 W. H. H. FREEMAN FREEMAN AND D COMPANY ©2014 W. COMPANY Figure 20.1 (a) The six ML vectors of an octahedral complex [ML6]n can be defined to lie along the x, y and z axes. (b) The five d orbitals; the dz2 and dx2−y2 atomic orbitals point directly along the axes, but the dxy, dyz and dxz atomic orbitals point between them. (c) The formation of a dz2 orbital from a linear combination of dx2−y2 and dz2−x2 orbitals. The orbitals have been generated using the program Orbital Viewer [David Manthey, www.orbitals.com/orb/index.html]. Basis for Bonding Theories Models for the bonding in transition metal complexes must be consistent with observed behavior. Specific data used include stability (or formation) constants, magnetic susceptibility, and the electronic (UV/Vis) spectra of the complexes. Bonding Approaches Valence Bond theory provides the hybridization for octahedral complexes. For the first row transition metals, the hybridization can be: d2sp3 (using the 3d, 4s and 4p orbitals), or sp3d2 (using the 4s, 4p and 4d orbitals). The valence bond approach isn’t used because it fails to explain the electronic spectra and magnetic moments of most complexes. Crystal Field Theory is an electrostatic model. Coulomb Interactions –attraction between metal ion and ligand electrons –repulsion between metal electrons and ligand electrons deciquanta means 10 quantum of energy. d orbitals splitting d Orbital Splitting In some texts and articles, the gap in the d orbitals is assigned a value of 10Dq. The upper (eg) set goes up by 6Dq, and the lower set (t2g) goes down by 4Dq. The actual size of the gap varies with the metal and the ligands. __ __ e g dz2 dx2-y2 __ __ __ __ __ 0.6∆o Spherical field 0.4∆o __ dxy __ __ t 2g dxz dyz Octahedral field ∆o Ligands, viewed as point charges, at the corners of an octahedron affect the various d orbitals differently. Strong ligand d Orbital Splitting The colors exhibited by most transition metal complexes arises from the splitting of the d orbitals. As electrons transition from the lower t2g set to the eg set, light in the visible range is absorbed. The Spectrochemical Series Based on measurements for a given metal ion, the following series has been developed: I-<Br-<S2-<Cl-<NO3-<N3-<F-<OH-<C2O42-<H2O <NCS-<CH3CN<pyridine<NH3<en<bipy<phen <NO2-<PPh3<CN-<CO + Pairing Energy Ligand Field Stabilization Energies for Octahedral Complexes Octahedral crystal field stabilization energies (CFSE) for dn configurations; pairing energy, P, terms are included where appropriate (see text). High- and lowspin octahedral complexes are shown only where the distinction is appropriate. Table 20.3 High Spin Low Spin Due to small splitting (-3/5 ΔT for an e orbital and +2/5 ΔT for a t2 orbital). Dt = 4/9Do Crystal field splitting diagrams for octahedral (left-hand side) and tetrahedral (right-hand side) fields. The splittings are referred to a common barycentre. See also Fig. 20.2. Figure 20.8 Why do d8 metal compounds often form square planar compounds z Thought experiment: Make a square planar compound by removing two ligands from an octahedral compound L L L M L y L L dx2-y2 dz2 x L L M L L dx2-y2 dxy dxy dxz,dyz dz2 dxz,dyz Octahedral Square Planar dx2-y2 dx2-y2 dz2 dxy dxz,dyz dxy dxy dxz,dyz dx2-y2 dz2 H2O H2O Ni 2 OH2 Cl OH2 H2O Octahedral Coordination number =6 Ni(II) d8 S = 1 dxz,dyz Tetrahedral Octahedral OH2 dz2 Cl Square Planar 2- N N Cl C C Ni 2- Ni Cl Tetrahedral (CN=4) C N C N Square Planar (CN=4) Ni(II) d8 S =1 Ni(II) d8 S = 0 Crystal field splitting diagrams for some common fields referred to a common barycentre. Splittings are given with respect to oct. Figure 20.11 Ligand Field Theory Crystal Field Theory completely ignores the nature of the ligand, treating ligands as point charges and does not take into account the overlap ligands and metal- atom orbitals. As a result, it cannot explain the spectrochemical series. Sigma bond approach only . Ligand Field Theory uses a molecular orbital approach. Initially, the ligands can be viewed as having a hybrid orbital or a p orbital pointing toward the metal to make σ bonds. Metal orbital Symmetry label Degeneracy S a1g 1 px, py, pz t1u 3 dxy, dyz, dzx t2g 3 Ligand Field Theory Consider the group orbitals of all six ligands in octahedral geometry. Oh E Γσ 6 8C3 6C2 6C4 0 0 2 3C2 i 6S4 8S6 3σh 6σd 2 (=C4 ) 2 0 This reduces to A1g + Eg + T1u 0 0 4 2 Ligand Field Theory The A1g group orbitals have the same symmetry as an s orbital on the central metal. Ligand Field Theory The T1u group orbitals have the same symmetry as the p orbitals on the central metal. (T representations are triply degenerate.) Ligand Field Theory The Eg group orbitals have the same symmetry as the dz2 and dx2-y2 orbitals on the central metal. (E representations are doubly degenerate.) Ligand Field Theory Since the ligands don’t have a combination with t2g symmetry, the dxy, dyz and dxy orbitals on the metal will be nonbonding when considering σ bonding. Ligand Field Theory – σ bond only The molecular orbital diagram is consistent with the crystal field approach. Note that the t2g set of orbitals is non-bonding, and the eg set of orbitals is antibonding. Ligand Field Theory The electrons from the ligands (12 electrons from 6 ligands in octahedral complexes) will fill the lower bonding orbitals. { 9 Bonding orbitals – 18 electrons capacity Ligand Field Theory { The electrons from the 4s and 3d orbitals of the metal (in the first transition row) will occupy the middle portion of the diagram. σ bond only ; t2g non- bonding Nature of the Ligands- Considering π Bonding Crystal field theory and ligand field theory differ in that LFT considers the nature of the ligands. Thus far, we have only viewed the ligands as electron pairs used for making σ bonds with the metal. Many ligands can also form π bonds with the metal. Group theory greatly simplifies the construction of molecular orbital diagrams. Considering π Bonding To obtain Γred for π bonding, a set of cartesian coordinates is established for each of the ligands. The direction of the σ bonds is arbitrarily set as the y axis (or the py orbitals). The px and pz orbitals are used in π bonding. x x z z z y y z y x y x z y x y z x Oh E 8C3 6C2 6C4 Γπ 12 0 0 6 Px + 6 Pz = 12 vectors 0 Considering π Bonding Consider only the px and pz orbitals on each of the ligands to obtain Γπ. 3C2 i 6S4 8S6 3σh 6σd 2 (=C4 ) -4 0 0 0 0 0 Considering π Bonding Oh E 8C3 6C2 6C4 Γπ 12 0 0 0 3C2 i 6S4 8S6 3σh 6σd 2 (=C4 ) -4 0 0 0 0 0 This reduces to T1g + T2g + T1u + T2u. The T2g set has the same symmetry as the dxy, dyz and dxz orbitals on the metal. The T1u set has the same symmetry as the px, py and pz orbitals on the metal. Considering π Bonding π reduces to: T1g + T2g + T1u + T2u. • The T1g and T2u group orbitals for the ligands don’t match the symmetry of any of the metal orbitals. • The T1u set has the same symmetry as the px, py and pz orbitals on the metal. These orbitals are used primarily to make the σ bonds to the ligands. • The T2g set has the same symmetry as the dxy, dyz and dxz orbitals on the metal. π Bonding The main source of π bonding is between the dxy, dyz and dxz orbitals on the metal and the d, p or π* orbitals on the ligand. π Bonding The ligand may have empty d or π* orbitals and serve as a π acceptor ligand, or full p or d orbitals and serve as a π donor ligand. π Bonding The empty π antibonding orbital on CO can accept electron density from a filled d orbital on the metal. CO is a pi acceptor ligand. filled d orbital empty π* orbital π Donor Ligands (LM) All ligands are σ donors. Ligands with filled p or d orbitals may also serve as pi donor ligands. Examples of π donor ligands are I-, Cl-, and S2-. The filled p or d orbitals on these ions interact with the t2g set of orbitals (dxy, dyz and dxz) on the metal to form bonding and antibonding molecular orbitals. π Donor Ligands (LM) The bonding orbitals, which are lower in energy, are primarily filled with electrons from the ligand, the and antibonding molecular orbitals are primarily occupied by electrons from the metal. Key points: π-Donor ligands decrease O whereas π-acceptor ligands increase O; the spectrochemical series is largely a consequence of the effects of π bonding when such bonding is feasible. σ Bonding vs. Bonding π Donor Ligands (LM) The size of ∆o decreases, since it is now between an antibonding t2g orbital and the eg* orbital. This is confirmed by the spectrochemical series. Weak field ligands are also pi donor ligands. π Acceptor Ligands (ML) Ligands such as CN, N2 and CO have empty π antibonding orbitals of the proper symmetry and energy to interact with filled d orbitals on the metal. π Acceptor Ligands (ML) The metal uses the t2g set of orbitals (dxy, dyz and dxz) to engage in pi bonding with the ligand. The π* orbitals on the ligand are usually higher in energy than the d orbitals on the metal. π Acceptor Ligands (ML) The metal uses the t2g set of orbitals (dxy, dyz and dxz) to engage in pi bonding with the ligand. The π* orbitals on the ligand are usually higher in energy than the d orbitals on the metal. π Acceptor Ligands (ML) The interaction causes the energy of the t2g bonding orbitals to drop slightly, thus increasing the size of ∆o. Summary 1. All ligands are σ donors. In general, ligands that engage solely in σ bonding are in the middle of the spectrochemical series. Some very strong σ donors, such as CH3- and H- are found high in the series. 2. Ligands with filled p or d orbitals can also serve as π donors. This results in a smaller value of ∆o. Summary 3. Ligands with empty p, d or π* orbitals can also serve as π acceptors. This results in a larger value of ∆o. — increasing ∆O → π donor weak π donor no π effects π acceptor π donor weak π donor no π effects π acceptor I, Br, Cl, F H2O PR3, CO NH I-<Br-<Cl-<F-<H2O<NH3<PPh3<CO π donor< weak π donor<σ only< π acceptor
© Copyright 2024