Lanthanide aqua ions hydration number

Metals that form relatively stable aqua ions in solntion have generally been well characterized. The hydration nnmbers of the transition metals, for example, have commonly been found to be six, consistent with the results of solid-state studies of crystalline hydrates. Exceptions inclnde Ag+, which has a hydration number of four, and Cn +, which has been found to have four short M-0 interactions and two longer ones, consistent with a Jahn-Teller distortion (see Jahn - Teller Effect) of the complex. The hydration numbers of the lanthanide ions in solution have been the subject of many investigations and some controversy recent resnlts snggest a decrease from nine to eight across the series from Ta + to... 

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. 

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