Dependence of spectroscopic and electrochemical properties of dimolybdenum multiimine complexes on ligand structure

The complexes MoX4(Multiimine)2, where X = Cl, Br, I and Multiimine = dimethyl-bipyridine, bipyridine, phenanthroline, bipyrazine, bipyridazine and bipyrimidine, have been prepared. The product complexes apparently contain non-bridged quadruple molybdenum-molybdenum bonds. Each molybdenum is coordinated to a bidentate diimine and two halogen atoms. An electronic absorption study reveals an important trend that the intensity of the delta-->delta* transition increases with decreasing energy. This shows the energy of this band is determined by mixing of this transition with a metal-to-ligand charge transfer transition. An EEC type mechanism is proposed for the redox behavior of these compounds on the basis of an electrochemical study and some consistent results are obtained by correlating the oxidation potentials with the delta-->delta* transition energies. Also, fairly good correlations of both the delta-->delta* transition energies and the oxidation potentials with pk(a) of L are obtained.