Three different types of organometallic compounds [Ni{C6Cl4)-2}2] (R = Ph 1 or Et 1'), [NiCl{C6Cl4(PR2)-2}L] (R = Ph 2 or Et 2') and [NiCl{C6Cl4(PR2)-2}L2] (R = Ph 3 or Et 3') have been obtained from 1 equivalent of PR2(C6Cl5) (R = Ph or Et), [Ni(cod)2] (cod = cis,cis-cycloocta-1,5-diene), and L = PMe2Ph a, PEt3 b, P(CH2Ph)3 c or PPh3 d. Complexes 2 evolve in solution, either to 1 and [NiCl2L2], or to 3 by breaking of the Ni-P bond of the four-membered ring by free phosphine. The selective preparation of compounds 1 or 3 can be achieved by performing the oxidative-addition reaction in the absence or with 2 equivalents of L respectively. When 1 equivalent of a diphosphine was used in the oxidative-addition reaction a mononuclear five-co-ordinate complex was obtained. [NiX{C6Cl4(PR2)-2}-(L-L)] 4 [L - L = Ph2Ph2(CH2)(n)PPh2, n = 2 or 3]. However, dppm (Ph2PCH2PPh2) acts as a monodentate ligand to give the five-co-ordinate compound [NiCl{C6Cl4(PPh2)-2}(dppm)2]. Complexes 2, 2' show preferentially a cis geometry, 1' is trans, and 3, 3' have the L ligands in trans position. Insertion of CO or alkynes into the Ni-C was not observed. Compounds 1 and 1' in the presence of neutral ligands L = CO or PR3 (PR3 = PMe2Ph a or PEt3 b) gave five-co-ordinate complexes [Ni{C6Cl4(PPh2)-2}L] without cleavage of the Ni-P bond of the ring. Stabilization of the four-membered ring is achieved when two bidentate ligands are present or in the five-co-ordinate compound [NiCl{C6Cl4(PPh2)-2}(dppm)2]. Two bidentate ligands are also needed to stabilize the formation of five-co-ordinate complexes. The molecular structures of complexes 1', 3b, and [Ni{C6Cl4(PPh2)-2}(PEt3)] were determined by single-crystal X-ray diffraction.