Redox behaviour of group VIIB metal—metal bonded carbonyls: substitution effects

Abstract

Electrochemical reduction parameters of M 2(CO) 10 (M 2 = Mn 2, MnRe, Re 2) (I) are compared with those of the related axially disubstituted M 2(CO) 8(PPh 3) 2 (II) compounds. With compounds II, the half-wave potentials are more cathodic than with compounds I. The same results are obtained from cyclic voltammetry on platinum electrode. From the electronic absorption spectra of compounds I and II, the energies of the σ → σ★ transitions associated with the metal—metal bond are calculated. In Mn 2(CO) 10 and MnRe(CO) 10 these energies correlate well with the electrochemical results: replacement of two axial CO groups by PPh 3 weakens the metal—metal bond. In contrast for Re 2(CO) 10 the same substitution reinforces the rhenium—rhenium bond.