Complexes with coordination number eight

It is interesting that the salt (NH4)3(HfF7) is actually (NH4)2[HfF6] + NH F this is one of the few clear-cut distinctions between the chemistries of zirconium and hafnium—the zirconium compound contains the [ZrF7] anion. 

Both the dodecahedron and square antiprism may be regarded as distortions of a cubic arrangement of ligands (Fig. 3.19). They are favoured because a cubic configuration would involve greater interligand steric 

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Most of the nickel compounds in the solid state and almost all in aqueous solution contain the metal in the oxidation state +2, which, by consequence, can be considered the ordinary oxidation state for nickel in its compounds. The electronic structure and stereochemistry of nickel(II) were reviewed in 1968.6 The most stable electronic configuration of the free Ni ion is [Ar]3d8 which is also the ground state configuration in its complexes. The overwhelming majority of nickel(II) complexes have coordination numbers of four, five and six. Complexes with coordination numbers of three, seven and eight are still quite rare. 

A number of chloride complexes include Pr(terpy)Cl3.8H2O X-ray diffraction established the presence of a [Pr(terpy)C1(H20)5] + ion. Lanthanide nitrates react with 1 mole of terpy in MeCN to give [La(terpy)(N03)3(H20) ] (Ln = La, = 2 Ln = Ce-Ho, n = 1 Ln = Er-Lu, n = 0) with coordination numbers decreasing from 11 (La) to 9 (Er-Lu) if these are crystallized from water [Ln(terpy)(N03)2(H20)4] NO3 (Ln = La-Gd) and [Ln(terpy)(N03)2(H20)3]N03.2H20 (Ln = Tb-Lu) are formed these have both biden-tate and ionic nitrate and nine- and eight-coordinate lanthanides, respectively. Thiocyanate complexes Ln(terpy)2(NCS)3(Ln = Pr, Nd) are also nine coordinate. 

Indeed, the majority of the higher-covalent silicon compounds studied which have an increased coordination number are formed by bi-, tri-, and tetradentate ligands similar effects are apparent with other Group IVA elements. It is likely that the structural factors are responsible for the existence of pentacoordinate pentavalence carbon species expanding the valence shell to 10 (Sect. 3.2). For the same reason that there are about twenty structures of the organotin complexes with seven- and eight-coordination... 

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