Eight-coordinate species

Xenon hexafluoride can act as a Lewis acid. It reacts with the heavier alkali fluorides to form seven-coordinate anions, which in turn can rearrange to form eight-coordinate species ... 

These are readily prepared by reaction of the lanthanide oxide or carbonate with the acid. Salts of noncoordinating anions most often crystallize as salts [Ln(OH2)9]X3 (X e.g. bromate, triflate, ethyl sulfate, tosylate). These contain the [Ln(OH2)9] + ion (Figure 4.1), even for the later lanthanides, where in aqueous solution the eight-coordinate species [Ln(OH2)8] predominates. 

Overall, then, the nature of the hydrated lanthanide ion in solution now appears to be well understood. In summary, the coordination number of [Tn(H20) ] + (aq) is believed to be 9 for the early lanthanides (La-Nd or Sm) and 8 for the later metals (Dy-Lu, Y), with the intermediate metals exhibiting a mixture of eight- and nine-coordinate species. The nine-coordinate species are assigned TTP structures and the eight-coordinate species square antiprismatic coordination. 

Melby et al., 1964). Even though the tetrakis-dipivaloylmethane complex does not exist, the tris-chelate is still capable of adding one or two additional ligands, such as pyridine, picoline, or o-phenanthroline, to form seven- and eight-coordinate species (Selbin et al., 1971). 

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