Electron transfer studies of dithiolate complexes: effects of ligand variation and metal substitution

Solution redox potentials and heterogeneous electron transfer rate constants have been measured for mono-ene-1,2-dithiolate complexes of the type (Tp*)M(E)(S ∩ S) [Tp*=hydrotris(3,5-dimethyl-1-pyrazolyl)borate; M=Mo, W; E=O, NO]. The dithiolate ligands (S ∩ S) equatorially coordinated to the central metal include: 1,2-benzenedithiolate (bdt); 3,6-dichloro-1,2-benzenedithiolate (bdtCl 2 ); and 3,4-toluenedithiolate (tdt). Cyclic voltammograms reveal quasi-reversible one-electron reductions for all of the compounds; and, a one-electron quasi-reversible oxidation process is also observed for (Tp*)MoO(bdt), (Tp*)MoO(tdt), and (Tp*)WO(tdt). Electrochemical potentials are strongly dependent upon the nature of the substituents on the benzene ring of the dithiolate ligand. The nitrosyl–molybdenum complexes (E=NO) are more difficult to reduce than their corresponding oxo-molybdenum complexes. The tungsten compound, (Tp*)WO(tdt), has the most negative electrochemical potentials among the complexes investigated. Heterogeneous electron transfer rates are insensitive to the variations of the central metal and ligands. These results support a highly covalent bonding interaction between the metal and the dithiolate ligands that modulates electron transfer reactions within these compounds.