Steric Effects of Open d Shells

In a nutshell, the d electrons repel the bonding electrons. They get in the way of the bonds and, to a greater or lesser degree, frustrate the attraction between metal and ligands. In essence, the proposed minimal overlap of d orbitals with the ligands, but significant repulsive interaction with the bonds, is equivalent to a focus upon the two-electron operator rather than the one-electron operator that is, upon repulsions rather than overlap. 

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A minor success is also seen in complexes of d and d" ions, in which the distorted octahedral geometries observed may be rationalized (and indeed predicted) in terms of the Jahn-Teller effect, and ultimately in terms of the steric activity of the open d shell. This is a common feature in copper(n) chemistry, and you will... 

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