The sulfur analogue of the well-known Ni(CO)(4), namely, Ni(CS)(4), has been observed spectroscopically in low temperature matrices but is not known as a stable species under ambient conditions. Theoretical studies show that Ni(CS)(4) with monomeric CS ligands and tetrahedrally coordinated nickel is disfavored by similar to 17 kcal/mol relative to unusual isomeric Ni(C2S2)(2) structures. In the latter structures the CS ligands couple pairwise through C-C bond formation to give dimeric S-C-C-S ligands, which bond preferentially to the nickel atom through their C-S bonds rather than their C-C bonds. Coupling of CS ligands in the lowest energy binuclear Ni-2(CS)(n) (n = 7, 6, 5) structures results in cyclization to give remarkable CnSn (n = 5, 6) ligands containing five- and six-membered carbocyclic rings. Such ligands, which are the sulfur analogues of the well-known croconate (n = 5) and rhodizonate (n = 6) oxocarbon ligands, function as bidentate ligands to the central Ni-2 unit. Higher energy Ni-2(CS)(n) (n = 7, 6, 5) structures contain dimeric C2S2 ligands, which can bridge the central Ni-2 unit. Dimeric C2S2 ligands rather than tetrathiosquare C4S4 ligands are found in the lowest energy Ni-2(CS)(4) structures.