Nine block metal compounds

Compounds of boron and the less electronegative elements (metals and metalloids) are loosely referred to as borides, a very large class encompassing hundreds of known compounds. Well-characterized binary transition metal borides number over 120, and include compounds of all of the d-block elements except Zn, Ag, Cd, Au, and Hg (Cr alone forms nine known borides). 

The involvement of d orbitals leading particularly to sp d" hybrids (1 < n < 5) for coordination numbers five through nine (Table 2). This typically occurs in the coordination compounds of the d-block transition metals. The polyhedra for coordination numbers five and six can be generated by addition of one or two d orbitals, respectively, to a four-orbital spherical sp manifold. Similarly the polyhedra for coordination numbers eight and seven can be generated by subtraction of one or two d orbitals, respectively, from a nine-orbital spherical sp d manifold. 

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