Study of substituent effects, isomerization and cross redox reactions associated with electrochemical oxidation of Mo(CO)3P3 systems

Article abstract of DOI:10.1021/om00082a005

Homogeneous and heterogeneous aspects of the redox couple [Mo(CO)₃P₃]^+/0 (P = monodentate phosphorus ligand) have been examined by polarographic, voltammetric, and spectroscopic methods in dichloromethane solution. The tricarbonyl compounds in each oxidation state can exist in both facial (fac⁺, fac⁰) and meridional (mer⁺, mer⁰) forms. The cyclic voltammograms at room temperature often showed the form of quasi-reversible couples; however, they did not fit theoretical models for simple electron transfer. With use of low temperatures and a variety of scan rates, it was deduced that the electrode processes could be explained in terms of parts of the square reaction scheme fac⁰ ? fac⁺ + e^- (upward harpoon with barb leftwards) + ? (upward harpoon with barb leftwards) + ? mer⁰ ? mer⁺ + e^- and the cross redox reaction. fac⁺ + mer⁰ ?^κ fac⁰ + mer⁺ At low temperatures it was possible to calculate E°(fac^+/0) and E°(mer^+/0) and hence calculate the equilibrium constant, K, for the cross redox reaction. Both K and the E° values showed a very large dependence on the nature of the phosphorus ligand. For fac-Mo(CO)₃P₃, a satisfactory linear correlation between carbon-13 NMR chemical shift (δ(¹³C)) of the carbonyl ligands and E°(fac^+/0) was obtained, but similar correlations were not observed between E°(fac^+/0) and either δ(³¹P) of the phosphorus ligands or δ(⁹⁵Mo) of the central molybdenum atom.