Nickel Complexes Containing New Carbon−Phosphorus Unsaturated Ligands:  First Examples of Phosphavinylidene−Phosphorane [R<sub>3</sub>PCPR‘] and Phosphavinyl Phosphonium [C(H)(PR<sub>3</sub>)P(R‘)]<sup>+</sup> Ligands

Oxidative-addition reactions of Cl<sub>2</sub>CPN(SiMe<sub>3</sub>)<sub>2</sub> with 1:2 Ni(COD)<sub>2</sub>/PPh<sub>3</sub>, Ni(PPh<sub>3</sub>)<sub>4</sub>, or (Ph<sub>3</sub>P)<sub>2</sub>Ni(C<sub>2</sub>H<sub>4</sub>) initially yields the phosphavinyl phosphonium complex Cl(Ph<sub>3</sub>P)Ni[η<sup>2</sup>-C(Cl)(PPh<sub>3</sub>)PN(SiMe<sub>3</sub>)<sub>2</sub>] (<b>IIa</b>). Addition of another equivalent of Ni(0) reagent to <b>IIa</b> results in the formation of the novel, dinuclear, phosphavinylidene−phosphorane complex Ni<sub>2</sub>Cl<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>[μ<sub>2</sub>-η<sup>2</sup>:η<sup>2</sup>-C(PPh<sub>3</sub>)PN(SiMe<sub>3</sub>)<sub>2</sub>] (<b>IIIa</b>); the structure of <b>IIIa </b>was established by X-ray diffraction and contains a Ph<sub>3</sub>P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 Ph<sub>3</sub>P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 R<sub>3</sub>PCPR<sub>3</sub> and RPCPR that act as two-electron donor ligands. When the reaction of the Ni(0) reagent is performed with Cl<sub>2</sub>CPMes* (Mes* = 2,4,6-tri-<i>tert</i>-butylphenyl), the mononuclear phosphonio−phosphavinyl complex Cl(Ph<sub>3</sub>P)Ni[η<sup>2</sup>-C(H)(PPh<sub>3</sub>)P(Mes*)] (<b>Va</b>) forms. The structure of <b>Va</b> was established by X-ray diffraction and contains a [C(H)(PR<sub>3</sub>)P(R‘)] ligand that acts as a three-electron donor to the Ni(PPh<sub>3</sub>)Cl fragment. This structure also exhibits a puckered, boat-shaped supermesityl ring. Both <b>IIIa</b> and <b>Va</b> exhibit labile PPh<sub>3</sub> groups on nickel which are easily substituted with PEt<sub>3</sub>, but the carbon-bound PPh<sub>3</sub> groups could not be substituted with PEt<sub>3</sub>.