Bond Lengths in Octahedral Complexes

Consider the repulsive effects of the d electrons in a series of MLe complexes as the d configuration of the central metal varies across the transition-metal series. All d electron density will repel the bonding electron density. The effects on the 2g eleetron density will be relatively small, however, as these orbitals largely lie inbetween the bonding regions. On the other hand, electron density directly frustrates the bonding. 

Two further consequences of the steric activity of open d shells are also important. One, which might seem somewhat circular but does not, in fact, involve any double counting , is that longer bonds are accompanied by smaller ligand-fields, that is, by 

Transition MetalChemistry.M. Gerloch, E. C. Constable Copyright 1994 VCH Verlagsgesellschaft mbH, Weinheim ISBN 3-527-29218-7 

Planar coordinated systems, you will recall from Chapter 1, formed a major group of exceptions to the otherwise very successful geometry modelling of Kepert. That model explicitly neglected any steric role for the non bonding electrons, however. Let us now recognize and incorporate the steric activity of the d shell in systems. 

consider an octahedral nickel(ii) complex. The strong-field ground configuration is 2g g- The repulsive interaction between the filled 2g subshell and the six octahedrally disposed bonds is cubically isotropic. That is to say, interactions between the t2g electrons and the bonding electrons are the same with respect to x, y and z directions. The same is true of the interactions between the six ligands and the exactly half-full gg subset. So, while the d electrons in octahedrally coordinated nickel(ii) complexes will repel all bonding electrons, no differentiation between bonds is to be expected. Octahedral d coordination, per se, is stable in this regard. 

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