Synthesis and characterization of bis(phosphine)nickel(0) complexes containing nonsymmetrically substituted acetylenes

Article abstract of DOI:10.1021/om00039a033

Some new nickel π-complexes of the general formula L₂Ni(XC≡CY) (L = PCy₃, PPh₃, 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 properties of the π-bond, three-membered metallacycle was investigated. With X = Ph kept constant, the following substituent variations of Y have been tested Y = Me, Et, i-Pr, t-Bu (systematic α-methyl perturbation on the tetrahedral C atom), H, SiMe₃, and SiPh₃. It could be established that the quaternary C atoms of the three-membered metal ring system have very different electron densities (¹³C NMR analysis) that are caused by nonsymmetrical substitution. The strongest polarization of the acetylene bond between the sp carbon atoms could 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≡C²SiMe ₃) complex: monoclinic, space group P2₁/c; a = 12.200 (2) A?, b = 12.564 (2) A?, c = 26.607 (4) A?, α = 90°, β = 98.37 (1)°, γ = 90°, V = 4034.8 (1.1) A?³, Z = 4, R = 0.0477; Ni-C¹ and Ni-C² bond lengths 1.884 (6) and 1.926 (6) A?; bend-back angles 31.8 (5)° (on C¹) and 41.3 (5)° (on C²). The electron density of the complexed acetylene carbon atoms is influenced also by the properties of phosphine ligands on the transition metal. Donor phosphine ligands like PCy₃ caused an increase, while acceptor ones like P(O-o-Tol)₃ caused a decrease in polarization between the two C atoms of the metallacycle.