Coordination numbers of 10 and above

   It is always dangerous to draw conclusions on the basis of the non-existence of structure types, but, from data available at the present time, it seems that a coordination of ^_10 is generally confined to the f-block metal ions. Complexes containing [BH₄]^- and related ligands are an exception, e.g. in [Hf(BH₄)₄] and [Zr(MeBH₃)₄] the ligands are tridentate  and the metal centres are 12-coordinate. Figure 1.1 shows the structure of [Hf(BH₄)₄] and the cubeoctahedral arrangement of the 12 hydrogen atoms around the metal centre. The same coordination environment is found in [Zr[MeBH₃)₄].

Fig. 1.1 (a) The structure of [Hf(BH₄)₄] determined by neutron diffraction at low temperature [R.W. Broach et al. (1983) Inorg. Chem., vol. 22, p. 1081]. Colour code: Hf, red; B, blue; H, white. (b) The 12-vertex cubeoctahedral coordination sphere of the Hf(IV) centre in [Hf(BH₄)₄].