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(SiMe 3)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[μ 2-η2:η2-C(PPh3)=PN(SiMe 3)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.