D-orbital contraction

Since n bonding is believed to be more important in low oxidation states, as d orbitals contract with increasing oxidation state leading to poorer dw-pw overlap, this would not be expected on the basis of a 7r-bonding mechanism. Similarly, one can compare /(Pt-P) for pairs of isomers in the +2 and +4 states in a planar platinum(II) complex, the platinum 6s orbital is shared by four ligands whereas in an octahedral platinum(IV) complex it is shared by six ligands. Therefore, the 6s character is expected to be only 2/3 as much in the platinum(IV) complexes, correlating well with the 7(Pt-P) values, which can be taken to be a measure of the a-character in the bond. 

A possible explanation for the small change in bond distance is that, with increasing oxidation state of the metal, the d orbitals contract. This contraction may cause overlap of d orbitals in tt bonding to become less effective. Thus, as 8 electrons are removed, the tt interactions become weaker the two factors (increase in bond order and increase in oxidation state of Re) very nearly offset each other. 

Pentavalent arsenic is well documented but prone to reduction to the trivalent state, a phenomenon attributed to d orbital contraction. 

In Table 2 the ratios of the size of the valence d to the valence s orbitals are given. Unlike the atomic separations, the trend in the ratios is obvious the d orbital contracts... 

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