Transition metal complexes with sulfur atoms as ligands. 6. Synthesis, properties, and structure of the nickel chalcogenide complexes [Ni3(.mu.3-X)2(PR3)6]2+ (R = Me, X = S; R = Et, X = S, Se) and [Ni9(.mu.4-S)3(.mu.3-S)6(PEt3)6]2+