Coordination polyhedra transition metal cations

Filling anion vacancies with inserted fluorine anions completes the coordination polyhedron of the transition metal cation to an octahedron, which in the case of Jahn-Teller active cations leads to anisotropic coordination with four short equatorial and two longer apical metal-oxygen bonds. 

In some cases, the CFSE attained by a transition metal ion in a regular octahedral site may be enhanced if the coordination polyhedron is distorted. This effect is potentially very important in most silicate minerals since their crystal structures typically contain six-coordinated sites that are distorted from octahedral symmetry. Such distortions are partly responsible for the ranges of metal-oxygen distances alluded to earlier, eq. (6.6). Note, however, that the displacement of a cation from the centre of a regular octahedron, such as the comparatively undistorted orthopyroxene Ml coordination polyhedron (fig. 5.16), also causes inequalities of metal-oxygen distances. 

Octahedral coordination is frequently adopted by the important 3d transition-metal ions. In this coordination polyhedron, each cation is surrounded by six anions, to form an octahedral [MOe] group, (Figure 7.14). The cubic structure of rhenium trioxide, ReC>3, a = 0.3750 nm, is... 

The majority of inorganic substances contain bonded atoms of three or more different elements. Salts with mixed anions (LaOF, PbFCl, BiOX, etc) have been discussed in Sect. 5.2, those with mixed cations are discussed here, as well as complex compounds which are combinations of intrinsically stable molecules. In complex compounds the bonding is essentially covalent within the coordination polyhedron but essentially ionic outside it. Complex compounds where the central atom is metal (especially transition metal) are known as coordination compounds. Thus, KNO3 and BaSOa are complex compounds, but not coordination compounds. The amount of structural information on such compounds is immense, therefore we shall concentrate on a few problems. 

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