Coordination geometry common transition metal

The metal complexes discussed thus far bear little resemblance to the vast majority of common transition-metal complexes. Transition-metal chemistry is dominated by octahedral, square-planar, and tetrahedral coordination geometries, mixed ligand sets, and adherence to the 18-electron rule. The following three sections introduce donor-acceptor interactions that, although not unique to bonding in the d block, make the chemistry of the transition metals so distinctive. 

Figure 13 Common transition metal coordination geometries.

The most common coordination numbers for transition metal complexes of relevance in catalysis are four, five, and six. However, a few metal complexes with coordination numbers two, three, and seven are also of relevance. Structures 2.3-2.8 show the common coordination geometries for coordination numbers four, five, and six. 

For late transition metals, it is common for one of the pyrazolyl arms of a tris(pyrazolyl)borate to dissociate, adopting a bidentate coordination. This is especially common for the electronically favorable d8 square planar geometry, where facial coordination is less advantageous. 

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