Octahedral complexes, valence bond

The ligands may be viewed as populating the valence orbitals by dative bonding, in which case the EFG results from non-equivalent donation to the various p or d orbitals. The extent to which ea.ch ligand populates a particular orbital depends on the donor capacity of the ligand, and also on the p (or d) character of the hybrid orbital on the acceptor (Clark, Maddock Platt, 1972). In an octahedral complex, the bonds have one-half p character and one-third d character. A ligand of donor power [Pg.53]

Although the complexity increases rapidly there is no reason that Walsh diagrams cannot be constructed for XY3 pyramidal, XY4 tetrahedral, XYS octahedral, and other molecules. In fact, they have been prepared, but their applications will not be described here. Insofar as these diagrams are amenable to quantitative interpretation, the predictions are in accord with what we know from experimental evidence and valence bond methods. 

The valence bond picture for six-coordinate octahedral complexes involves dispi hybridization of the metal (Fig. I i.lc. d). The specific d orbitals that meet the symmetry requirements for the metal-ligand o bonds are the four-coordinate d complexes discussed above, the presence of unpaired electrons in some octahedral compounds renders the valence level ( — l)J orbitals unavailable for bonding. This is true, for instance, for paramagnetic [CoFJ3- (Fig. I I.lc). In these cases, the VR model invokes participation of -level dorbitals in the hybridization. 

Valence bond theory describes the bonding in complexes in terms of two-electron, coordinate covalent bonds resulting from the overlap of filled ligand orbitals with vacant metal hybrid orbitals that point in the direction of the ligands sp (linear), sp3 (tetrahedral), dsp2 (square planar), and d2sp3 or sp3d2 (octahedral). 

The valence bond model constructs hybrid orbitals which contain various fractions of the character of the pure component orbitals. These hybrid orbitals are constructed such that they possess the correct spatial characteristics for the formation of bonds. The bonding is treated in terms of localised two-electron two-centre interactions between atoms. As applied to first-row transition metals, the valence bond approach considers that the 45, 4p and 3d orbitals are all available for bonding. To obtain an octahedral complex, two 3d, the 45 and the three 4p metal orbitals are mixed to give six spatially-equivalent directed cfisp3 hybrid orbitals, which are oriented with electron density along the principal Cartesian axes (Fig. 1-9). 

The sequence of energy levels obtained from a simple molecular orbital analysis of an octahedral complex is presented in Fig. 1-12. The central portion of this diagram, with the t2g and e levels, closely resembles that derived from the crystal field model, although some differences are now apparent. The t2g level is now seen to be non-bonding, whilst the antibonding nature of the e levels (with respect to the metal-ligand interaction) is stressed. If the calculations can be performed to a sufficiently high level that the numerical results can be believed, they provide a complete description of the molecule. Such a description does not possess the benefit of the simplicity of the valence bond model. 

Hybridization in an octahedral complex in the valence bond model of coordination ompound the formation of bonds to an octahedral transition metal ion would involve six d sp h /hrid orbitals. In the approach used here the diff( ring syininetry properties of the s, thii p, p and p.. and the d, and d orbitals used in bonding are explicitly taken into account. However, the 3 all of the p and two ol the d metal valence shell orbitals are otill used in o-bond tormation. 

A complete study of the molecular orbitals for an octahedral complex sue as [Cr(CN)6] or [Co(NH3)6] " " would require linear combinations of all the valence atomic orbitals of the metal and of the ligands. An approximation isl to take the metal valence a.o.s (nine a.o.s for a metal of the first transition series (five 3d orbitals, one 4s and three 4p orbitals)) together with six a.o.s from the ligands, one for each atom directly bonded to the metal atom. Ini general, these six a.o.s are quasi-localized molecular orbitals (see Chapter 8), which point from the ligand to the metal and have essentially non-bonding character ... 

For the octahedral complexes of transition metals studied in Chapter 11, important transitions are those of energy A (the ligand field splitting parameter) between the non-bonding tzg orbitals and the anti-bonding Cg orbitals. The simplest example is provided by the hexaquotitanium(in) ion, [Ti(H20)6] , whose fundamental valence configuration is (t2g) The absorption band at 493 nm, responsible for the purple colour of the complex, is assigned to the t2g e transition (Fig. 12.9). 

We apply molecular orbital theory to octahedral coordination complexes just as we did for the simple metal carbonyl in Section 8.2. We begin by constructing the cr MOs from the valence d, s, and p orbitals of the central metal atom and the six ligand orbitals that point along the metal-ligand bond directions in an octahedral complex. In the case of the Cr complexes we use as examples, the relevant... 

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