The interaction of complexes [NiCl2L2](L = PMe3 or PMe2Ph) with Grignard reagents derived from Me2PhCCH2Cl and Me3SiCH2Cl yields thermally stable monoalkyl derivatives of the type [NiR(Cl)L2], which, when L = PMe3, afford upon carbonylation at room temperature and pressure the corresponding acyls [Ni(COR)Cl(PMe3)2]. Metathesis reactions with KX give [NiR(X)(PMe3)2] and [Ni(COR)X(PMe3)2](X = Br, I, NCS, or NCO), while with Tl[C5H5], [Ni(CH2CMe2Ph)(η-C5H5)(PMe3)] and [Ni(COCH2CMe2Ph)(η-C5H5)(PMe3)] can be isolated. The synthesis of [Ni(COCH2CMe2Ph)Cl(PMe2Ph)2] is also reported. The acyls, [Ni(COR)X(PMe3)2], readily carbonylate solutions of the alkyl complexes, [NiR′(X)(PMe3)2], providing under certain conditions [NiR(X)(PMe3)2] and [Ni(COR′)X(PMe3)2] in almost quantitative yields. Structures for the new compounds are proposed on the basis of i.r. and 1H n.m.r. spectroscopic studies. Those of [Ni(CH2SiMe3)Cl(PMe3)2] and [Ni(COCH2SiMe3)Cl(PMe3)2] have been confirmed by single-crystal X-ray determinations. Thus [Ni(CH2SiMe3)Cl(PMe3)2] crystallizes in the monoclinic space group Pc with a= 15.419(3), b= 6.406(2), c= 18.711(3)Å, β= 92.02(3)°, Z= 4, and R 0.031 based on 1 906 independent observed reflections. The phosphine ligands are trans[P–Ni–P 164(2)°] in a square-planar arrangement. The complex [Ni(COCH2SiMe3)Cl(PMe3)2] belongs to the orthorhombic space group Pbca with a= 12.668(3), b= 21.312(4), c= 28.925(4)Å, Z= 16, and R 0.049 for 1 394 independent observed reflections. The oxyzen atom is not co-ordinated to the nickel atom [Ni O 3.121(4)Å].