Nickel Complexes Containing New Carbon−Phosphorus Unsaturated Ligands:  First Examples of Phosphavinylidene−Phosphorane [R₃PCPR‘] and Phosphavinyl Phosphonium [C(H)(PR₃)P(R‘)]⁺ Ligands

Oxidative-addition reactions of Cl₂CPN(SiMe₃)₂ with 1:2 Ni(COD)₂/PPh₃, Ni(PPh₃)₄, or (Ph₃P)₂Ni(C₂H₄) initially yields the phosphavinyl phosphonium complex Cl(Ph₃P)Ni[η²-C(Cl)(PPh₃)PN(SiMe₃)₂] (IIa). Addition of another equivalent of Ni(0) reagent to IIa results in the formation of the novel, dinuclear, phosphavinylidene−phosphorane complex Ni₂Cl₂(PPh₃)₂[μ₂-η²:η²-C(PPh₃)PN(SiMe₃)₂] (IIIa); the structure of IIIa was established by X-ray diffraction and contains a Ph₃PCPR‘ ligand bridged between two four-coordinate, planar nickel atoms in a butterfly arrangement with a Ni−Ni distance that is too long for a significant bonding interaction. The Ph₃PCPR‘ ligand, which may be viewed as a phosphavinylidene (CPR) ligand with a phosphine-donor substituent, acts as a six-electron donor to the two nickel atoms. This contrasts with the known diphosphaallene compounds of the types R₃PCPR₃ and RPCPR that act as two-electron donor ligands. When the reaction of the Ni(0) reagent is performed with Cl₂CPMes* (Mes* = 2,4,6-tri-tert-butylphenyl), the mononuclear phosphonio−phosphavinyl complex Cl(Ph₃P)Ni[η²-C(H)(PPh₃)P(Mes*)] (Va) forms. The structure of Va was established by X-ray diffraction and contains a [C(H)(PR₃)P(R‘)] ligand that acts as a three-electron donor to the Ni(PPh₃)Cl fragment. This structure also exhibits a puckered, boat-shaped supermesityl ring. Both IIIa and Va exhibit labile PPh₃ groups on nickel which are easily substituted with PEt₃, but the carbon-bound PPh₃ groups could not be substituted with PEt₃.