Coordination numbers

In this section, we give an overview of the coordination numbers and geometries found within d-block metal compounds.  It is impossible to give a comprehensive account, and several points should be borne in mind:

• most examples in this section involve mononuclear complexes, and in complexes with more than one metal centre, structural features are often conveniently described in terms of individual metal centres (e.g. in polymer 1.1, each Pd(II) centre is in a square planar environment);

(1.1)

• although coordination environments are often described in terms of regular geometries such as those in Table 1.1, in practice they are often distorted, for example as a consequence of steric effects;
• detailed discussion of a particular geometry usually involves bond lengths and angles determined in the solid state and these may be affected by crystal packing forces;
• where the energy difference between different possible structures is small (e.g. for 5- and 8-coordinate complexes), fluxional behaviour in solution may be observed; the small energy differencemay also lead to the observation of different structures in the solid state, e.g. in salts of [Ni(CN)₅]^3- the shape of the anion depends upon the cation present and in [Cr(en)₃][NiCN₅)].1.5H₂O, both trigonal bipyramidal and square-based pyramidal structures are present.

   We shall not be concerned with ionic lattices in this section.  Almost all the examples we discuss involve mononuclear species in which the metal centre is covalently bonded to the atoms in the coordination sphere. The metal–ligand bonding in complexes can generally be considered in terms of π-donor ligands interacting with a metal centre which acts as a π-acceptor. This may, in some complexes, be augmented with interactions involving π -donor ligands (with the metal as a π-acceptor) or π-acceptor ligands (with the metal as a π-donor). For a preliminary discussion of stereochemistry, it is not necessary to detail the metal–ligand bonding but we shall find it useful to draw attention to the electronic configuration of the metal centre.