Coordination numbers greater than six

The fifth regular solid, the icosahedron, and other 5-connected systems are omitted from Table 3.7 because there are no 5-connected plane or 3D nets composed of polygons of one kind only. For coordination numbers greater than six only 3D nets are possible. Two aspects of 3D nets which are important in structural chemistry should be mentioned here they will be discussed in more detail later. 

In recent years, many examples of compounds with tmusual coordination numbers, with novel structures, and with metal-metal bonds have been reported. Examples of complexes with odd coordination numbers and with coordination numbers greater than six are presented in Table 3.2. 

It might be expected that, since the Sc + ion has a considerably greater crystal radius than the corresponding tripositive ions of the 3d metals (compare Sc + at 0.885 A with Ti +, Cr +, and Fe + at 0.810, 0.755, and 0.785 A, respectively), coordination numbers greater than six might be conunon in scandium complexes. With the exception of the aqua ion (and nitrate complexes), this has not been realized, but studies of scandium chemistry are still relatively rare (partly on account of cost). Aqua complexes and simple hydrates have been discussed in Solvento Complexes of the Lanthanide Ions. 

A few examples have been reported in which presumably silicon has a coordination number greater than six . All of the hepta- and octa-coordinated silicon complexes for which single-crystal X-ray diffraction structures were obtained have a basic tetrahedral geometry, with three or four distantly coordinated ligand groups positioned against faces of the tetrahedron, in analogy to the bicapped tetrahedron structure mentioned earlier (Section VI.A.2). 

Coordination Numbers Greater than Six Among Fluorides and Oxofluorides... 

Main-Group Fluorides mth Coordination Numbers Greater Than Six... 

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