Distortions Caused by Electronic Effects

An important group of cations that shows electronically distorted environments are those of the main group elements in lower oxidation states. These contain nonbonding electron pairs in their valence shells, the so-called lone pairs . Such atoms are usually found displaced from the center of their coordination sphere so as to form between 3 and 5 strong bonds and a number of weaker ones. The effect can be described using the Valence Shell Electron Pair Repulsion (VSEPR) Model [43] in which it is assumed that the cation is surrounded uniformly by between 4 and 7 electron pairs occupying valence shell orbitals. One or more of these is a lone pair 

As one moves down the Periodic Table the atoms become more polarizable and the stereoactivity of the lone pair is sometimes suppressed, presumably because the lone pair can reside in a symmetric s orbital which essentially forms part of the core. In these cases, the factor that determines whether the lone pair will be stereoactive is the base strength of the counterion. Strongly basic counterions (e.g. phosphate, Sb = 0.25 v.u.) favour the formation of strong bonds and hence a low coordination number which stabilizes an asymmetric environment as, for example, in three-coordinate TI3PO4 [52]. Weakly basic counterions (e.g. nitrate, Sb = 0.11 v.u.) form weak bonds and hence stabilize the higher (symmetric) environment, as observed in, e.g. ten-coordinate TINO3 [38,45,51]. 

A number of transition metals also show electronically driven asymmetries in their coordination environments. Typical of these are and U which form one or two strong bonds to O to give the vanadyl (VO) and uranyl (U02) complexes. Cu typically shows an elongation of two trans bonds in its octahedral coordination sphere [46]. Cu and Hg , though not strictly transition metals, tend to form two strong colinear bonds. Other distortions tend to be weaker and are only expressed when other factors favor a distorted crystal structure. The distortion of the Ti environment in BaTiOi is primarily a steric effect (see Section 10.6.2), but the polarizability of the Tr cation probably contributes. Similar effects are found for other cations with a d° configuration [47]. Most of these effects are not well understood in detail but their presence must be taken into account when using the bond valence model. 

Although by their nature, cations with electronically distorted environments do not obey the Equal Valence Rule, they are found to obey the Valence Sum Rule, indicating that the atomic valence is still shared between the bonds, albeit unequally. 

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