SYNTHESIS AND CHARACTERIZATION OF BIS(PHOSPHINE)NICKEL(0) COMPLEXES CONTAINING NONSYMMETRICALLY SUBSTITUTED ACETYLENES

Some new nickel pi-complexes of the general formula L2Ni(XC = CY) (L = PCy3, P(O-o-Tol)3; X = Ph, Me: Y = Me, Et, i-Pr, t-Bu, H, SiMe3, SiPh3) were prepared, and the influence of various substituents and ligands on the prorperties of the pi-bond, three-membered metallacycle was investigated. With X = Ph kept constant, the following substituent variations of Y have tested Y = Me, Et, i-Pr, t-Bu (systematic alpha-methyl pertubation on the tetrahedral C atom), H, SiMe3,. It could be established that the quaternary C atoms of the three-membered metal ring system have very different electron densities (C-13 NMR analysis) that are caused by nonsymmetrical substitution. The strongest ploarization of the acetylene bond between the sp carbon atoms couls be obtained with X = Ph, Y = SiMe3 and X = Ph, Y = SiPh3 substituted acetylenes. The differences between the interaction of these C atoms with the metal center have been confirmed by an X-ray diffraction study of the (PPh3)2Ni(PhC1 = C2SiMe3) complex: monoclinic, space group P2(1)/c; a = 12.200 (2) angstrom, b = 12.564 (2) angstrom, b = 12.564 (2) angstrom, c = 26.607 (4) angstrom, alpha = 90-degrees, beta = 98.37 (1)degrees, gamma = 4034.8 (1.1) angstrom(3), Z = 4, R = 0.0477; Ni-C1 and Ni-C2 bond lengths 1.884 (6) and 1.926 (6) angstrom; bend-back angels 31.8 (5)degrees (on C2). The electron density of the complexed acetylene carbon atoms is influemnced alsol by the the properties of phosphine legands on the transition metal metal. Donor phosphine ligands like PCy3 caused an increase, while acceptor ones like P(O-o-Tol)3 caused decrease in polarizationl between the two C atoms of the metallacycle.