Synthesis of some cationic 16- and 18-electron nickel alkyl phosphine complexes containing bidentate ligands: ligand-dependent square-planar, trigonal-bipyramidal, and square-pyramidal co-ordination geometries, their reactions with CO and alkynes, and the crystal structure of [Ni(CH2SiMe3)(PMe3)3]BF4

Abstract

Treatment of [NiMe(PMe₃)₄]BF₄(1) with chelating ligands L–L gives the complexes [NiMe(PMe₃)₂(L–L)]BF₄ which may be rigid, with a trigonal-bipyramidal 18-electron structure [L–L = 2,2′-bipyridyl (bipy) or 1,10-phenanthroline (phen)], or show fluxional behaviour [L–L = 1,2-bis(diphenylphosphino)ethane (dppe) or 1,2-bis(dimethylphosphino)ethane (dmpe)].The latter two complexes are in equilibrium with square-planar [NiMe(PMe₃)(L–L)]BF₄. Treatment of (1) with excess of dmpe leads to complete substitution of PMe₃ to give square-pyramidal [NiMe(dmpe)₂BF₄. In [Ni(CH₂SiMe₃)(PMe₃)₃]BF₄(9) the co-ordination of a fourth PMe₃ ligand to give a five-co-ordinate complex analogous to (1) is precluded for steric reasons. The crystal structure of (9) shows the metal centre in a tetrahedrally distorted square-planar environment, with P–Ni–P and P–Ni–C angles of 135.5(2) and 157.8(7)°, respectively. While [NiMe(PMe₃)₂(dmpe)]BF₄ and [NiMe(dmpe)₂]BF₄ react with CO to give stable nickel acyl complexes, the dppe complex [Ni(COMe)(PMe₃)(dppe)]BF₄ is only observed at low temperature. Treatment of (1) with 2 equivalents of PPh₃ or with an excess of bipy, phen, or dppe leads to reductive elimination and formation of zerovalent nickel complexes. Similarly, PhCCPh induces the reductive methyl migration in (1) from nickel to PMe₃via spectroscopically observable ylidic intermediates to give [Ni(C₂Ph₂)(PMe₃)₂] and [PMe₄]BF₄.