Stabilization of high oxidation states in transition metals. An electrochemical and computational study of structurally comparable molybdenum and tungsten complexes

Molybdenum and tungsten chlorides, chloride oxides and chloride nitrides as well as thiazene and phosphazene metallacycles were studied by electrochemical methods in CH₂Cl₂. The different ligand systems have a profound influence on the redox potentials. The stabilization of high valency increases from the hexachloro to pentachlorooxo to chloronitrido complexes and the metallacycles. There is a systematic difference in the electrode potentials between molybdenum and tungsten redox couples. The geometries of the d¹ and d⁰ species, optimized using density functional methods, are in good agreement with the available experimental data and trends in redox data are reproduced by the computed energy difference between the appropriate d¹/d⁰ pair. The relative stability of the higher oxidation state is directly linked to the donor properties of the ligands present, and increases in the order: Cl₆ < F₆ < Cl₅O < Cl₄(N₃S₂) < Cl₃(N₃P₂) < Cl₄N.