Structure of vanadium(III) complexes in aqucous solution and ability of oxo-bridged dinuclear complex formation

We prepared vanadium(III) complexes with several complexons (aminopolycarboxylates) and investigated based on the pH dependence of the visible absorption spectrum if the complex yields an oxo-bridged dinuclear complex by hydrolysis in aqueous solution. It was found that the complexes can be classified into two groups, one of which gives an oxo-bridged dimer and the other does not. The vanadium(III) complexes with 1, 3-pdta, edds, or eddda ligand belong to the former group while those with nta, edta, or dtpa to the latter one. This difference in the dimerization tendency of vanadium(III) complexes can be related to the difference in the structure of monomeric complexes. Namely, X-ray structure analysis and other methods revealed that the monomeric complexes belonging to the former group adopted a hexacoordinate structure and those belonging to the latter one a heptacoordinate structure. The dependence of the dimerization tendency on the coordination number of the monomeric complex can be interpreted assuming that the intermediate dihydroxo-bridged dimer will be formed by an associative mechanism in the dimerization reaction The monomeric complex with heptacoordination should give the intermediate with an octacoordination structure that is unlikely to exist and as a result would not yield an oxo-bridged dimer.