NEW ROUTES TO CO(I)-BIS(DIPHENYLPHOSPHINO)METHANE CARBONYL-COMPLEXES CONTAINING MONOCOORDINATED AND CHELATING LIGANDS - STRUCTURE OF [CO(ETA-2-DPPM)2(CO)][CO(CO)4]

The mechanism of reduction of Co(II) by NaBH4 in the presence of CO and dppm (Ph2PCH2PPh2), which can give any one or more of several Co(I), Co0, or Co(I)- complexes depending upon experimental conditions, has been elucidated by detailed study of possible individual steps of the reaction. Reactions of CoX(eta-1-dppm)3 (X = Cl, Br) with CO lead in sequence to CoX(eta-1-dppm)2(CO)2, [Co(eta-2-dppm)(eta-1-dppm)(CO)2]X, and [Co(eta-2-dppm)2(CO)]X, all of which have been isolated and fully characterized, and the series of reactions can be reversed under CO at elevated temperatures. The related complex [Co(eta-1-dppm)(CO)4]Br is formed by a modification of the procedure. The synthesis and X-ray crystal structure of [Co(eta-2-dppm)2(CO)]-[Co(CO)4] (1), formed in the direct reduction of the CoX2/dppm/CO system by NaBH4, are reported, and evidence for the reaction sequence whereby 1 is formed is presented. The heterobinuclear complex [CoRh(mu-dppm)2(mu-CO)(CO)2(mu-Cl)]Cl has been synthesized by reaction of CoCl(eta-1-dppm)2(CO)2 with Rh2(CO)4Cl2. An X-ray diffraction investigation showed that 1 crystallizes in space group P2(1)/a with a = 25.737 (6) angstrom, b = 11.129 (2) angstrom, c = 18.085 (3) angstrom, alpha = 99.06 (2)-degrees, and Z = 4.