Reaction of cis-bis[1,2-bis(dimethylphosphino)ethane]dicarbonylmetal complexes of chromium and molybdenum with alkyl halides. Studies of a mechanism for oxidative addition to d6 metal complexes

Abstract

The complexes cis-[M(CO)₂(dmpe)₂](M = Cr or Mo; dmpe is Me₂PCH₂CH₂PMe₂) react with alkyl halides RX [RX is CX₄(X = Cl or Br), C₂Cl₆, 4-YC₆H₄CH₂X (Y = H, X = Cl, Br, or I; Y = NO₂, X = Br), Ph₂CHBr, R₃CX (R = Ph, X = Cl; R = Me, X = I), or CH₂C(R)CH₂X (R = Me, X = Cl; R = H, X = Br)] to form trans-[M(CO)₂(dmpe)₂]X (M = Cr or Mo) and subsequently either cis-[M(CO)₂(dmpe)₂X]X or trans-[MH(CO)₂(dmpe)₂]X (M = Mo only). The reaction has been studied by i.r., e.s.r., and electronic spectroscopy and the rate of the reaction has been measured as a function of the concentration of RX in certain cases. Evidence for the intervention of para-magnetic (free-radical) species is provided by e.s.r. measurements and the observation of the effect of a spin trap (nitrosodurene). The results indicate that electron transfer by an outer-sphere process from the metal(0) complex to RX produces the metal (I) cation (M = Cr or Mo), which (when M = Mo) is then converted, by a second electron transfer to the RX substrate (which involves an inner-sphere process), into the molybdenum(II) final product. Comparisons are made with reactions of cis-[Mo(CO)₂(dppe)₂](dppe is Ph₂PCH₂CH₂PPh₂) and with oxidative-addition reactions of other metal complexes.