Lanthanide elements aquo ions

In aqueous solutions the most common complex is aquo ion, [M(H20) ]3+ when the counter anion is an innocent or non-complexing anion. The value of n is known and is not constant in the lanthanide series. However a value of 9 for n for elements La to Nd and a value of 8 for elements Tb-Lu is proposed [90]. For ions from Nd3+ to Tb3+ steric factors play a critical role with an equilibrium mixture of 8-coordinated and 9-coordinated aquo... 

Two mechanisms were proposed for the hydrolysis reaction. Since the hydroxide ions were generated elect roly tically, the lanthanide ion may have reacted at the cathode surface with replacement of coordinated water molecules to form Ln(OH) or Ln(OH)2. Polynuclear complex formation could also have been formed by this mechanism. Alternatively, the aquo complexes could have been electrolyzed at the cathode surface to form the hydroxo complexes. The authors (Suzuki et al. 1986) suggested, without explanation, that hydrolysis of the elements with large ionic radii and higher hydration number occurs more likely by the first mechanism. However, the second mechanism would seem more likely for the formation of all hydrolyzed species. 

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