Elongation factors crystal forms

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. 

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