Five-coordinate complexes stereochemistry

Five-coordinate complexes are far more common than was once supposed and are now known for all configurations from d to d. Two limiting stereochemistries may be distinguished (Fig. 19.4). One of the first authenticated examples of 5-coordination was [VO(acac)2] which has the square-pyramidal 4 structure with the =0 occupying the unique apical site. However, many of the complexes with this coordination number have structures intermediate between the... 

Five-coordination is now quite common in nickel(II) complexes and many polydentate ligands such as polyamines, salicylaldimines, polyarsines and polyphosphines have been designed with the purpose of favouring this stereochemistry.7,8 However, five-coordinate complexes with monodentate ligands ([Ni(CN)s]3 and [Ni(OAsMe)s]2+) are also known. 

As outlined in Section 50.5.4.6 the tripod-shaped tetradentate ligands have the most favourable geometry for the formation of five-coordinated complexes with trigonal bipyramidal geometry. However, the presence of different donor groups in the donor set may give rise either to coordination distortion or to stereochemistries other than trigonal bipyramidal. 

In the solid state the stereochemistry of copper(I) in its mononuclear complexes, as determined by X-ray crystallography (Figure 4.1), is dominated by four coordination. A significant number of three- and two-coordinate complexes are known, very few five-coordinate complexes exist and six coordination (or above) is unknown. This contrasts with the predominance of six coordination in the chemistry of copper(II) (see Section 53.4.2) and the absence of two or three coordination in the solid state, and with the formation of a significant number of seven- and eight-coordinate geometries.47,48... 

Crystallographic studies of transition metal hydride complexes Stereochemistry of six-coordination Five-coordinate structures Stereochemistry of five-coordinate Co complexes Absolute stereochemistry of chelate complexes Stereochemistry of optically-active transition metal complexes Electron density distributions in inorganic compounds... 

In five coordinate complexes, both associative and dissociative substitution pathways are possible. Five coordination leads to two limiting stereochemistries, trigonal bipyramidal (4.7) and square pyramidal (4.8) which normally have similar energies. Many square... 

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