Electrochemical reduction of dicyanobis(tertiary phosphine)nickel(II) complexes

Abstract

Redox reactions of trans-[Ni(CN)₂L₂] complexes [L = PEt₃, PPr₃, PBu^t₃, PEtPh₂, PEt₂Ph, or P(C₆H₁₁)₃] at mercury electrodes in acetonitrile solution have been investigated by cyclic voltammetry, controlled-potential electrolysis, and spectrophotometry. With the exception of [Ni(CN)₂{P(C₆H₁₁)₃}₂], which exhibits different behaviour, all the investigated complexes undergo, at least, two subsequent cathodic processes in which the charge-transfer step leads to unstable and unusual dicyanodi(phosphine)-nickel(I) and -nickel(O) complexes respectively, which both decay in a short time to a dimeric nickel(I) species by two different reaction paths. When the neutral ligands are phosphines with sufficient π-acceptor ability a third cathodic process can be detected in which the dimeric nickel(I) species is further reduced to phosphinenickel(O) complexes not containing cyanide as ligand. Reduction mechanisms are proposed which are consistent with the data. The dependence of the reduction potentials on the nature of the phosphine ligands and the stereochemistry of the primary nickel(I) and nickel(O) reduction products are discussed.