Preparation and Reactivity of Nickel(0) Complexes with η²-Coordinated Alkynylphosphines

The η 2 -alkynylphosphine complexes [Ni(η 2 -Ph 2 PC⋮CR)(dcpe)] (R = Me ( 1a ), CO 2 Me ( 1b ), Ph ( 1c )), which are formed by displacement of ethylene from [Ni(C 2 H 4 )(dcpe)] by the corresponding alkynylphosphine, react with HCl (1 equiv) to give five-coordinate nickel(II) complexes, [NiCl{C( CHR)PPh 2 -κ P, C 1 }(dcpe)] (R = Me ( 2a ), CO 2 Me ( 2b ), Ph ( 2c )), which contain a coordinated methylenephosphanickelacyclopropane fragment. In the case of 1a, the proton adds regiospecifically at the carbon atom bearing the methyl group. This mode of addition is favored for 1b, c, but small amounts of the addition products [NiCl{η 1 - C ( R ) CH(PPh 2 )}(dcpe)] (R = CO 2 Me ( 3b ), Ph ( 3c )), arising from addition at the PPh 2 -bearing carbon atoms, are also formed. Unsaturated molecules such as CS 2 and CO 2 insert into the Ni−P bonds of complexes 2a, c to give five-membered nickelacycles. Complexes 1a, 2a, c, and 3c have been structurally characterized by X-ray diffraction analysis.