The electrochemical reduction of the cis-dioxo Mo(VI)-catechol complex, MoO2(cat2^2-, is investigated at a mercury electrode in pH 3.5-10 aqueous buffers utilizing the techniques of cyclic voltammetry and controlled potential coulometry. The reduction of MoO2(cat2^2- proceeds
by successive two-proton, one-electron and two- proton, two-electron
transfer steps which produce monomeric Mo(V) and Mo(III) species. At
pH 3.5-7 the Mo(V) monomer is unstable and undergoes dimerization. The
mechanism of the dimerization reaction and the structure of the Mo(V)
dimer produced are determined. At pH 7-10 the monomeric Mo (V) and
2- Mo(III) complexes produced by reduction of MoO2(cat2^2- are stable. A
stable Mo(IV) species is obtained upon reoxidation of the Mo( III )
monomer. The epr spectrum of the monomeric Mo(V) complex and visible
spectra of all oxidation states in alkaline solution arc reported.