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)(2)Ni(C2H4) initially yields the phosphavi nyl phosphonium complex Cl(Ph3P)Ni[eta(2)-C(Cl)(PPh3)=PN(SiMe3)(2)] (I Ia). Addition of another equivalent of Ni(0) reagent to IIa results in the formation of the novel, dinuclear, phosphavinylidene-phosphorane complex PPh3)(2)[mu(2)-eta(2):eta(2)-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 lon g for a significant bonding interaction. The Ph3P=C=PR' ligand, which may be viewed as a phosphavinylidene (=C=PR) ligand with a phosphine-d onor 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 th e reaction of the Ni(0) reagent is performed with Cl2C=PMes (Mes* = 2 ,4,6-tri-tert-butylphenyl), the mononuclear phosphonio-phosphavinyl co mplex Cl(Ph3P)Ni[eta(2)-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 fra gment. This structure also exhibits a puckered, boat-shaped supermesit yl ring. Both IIIa and Va exhibit labile PPh3 groups on nickel which a re easily substituted with PEt3, but the carbon-bound PPh3 groups coul d not be substituted with PEt3.