Cyclization of thiocarbonyl groups in binuclear homoleptic nickel thiocarbonyls to give ligands derived from sulfur analogues of croconic and rhodizonic acids.

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 ∼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 Ni2(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 Ni2 unit. Higher energy Ni2(CS)n (n = 7, 6, 5) structures contain dimeric C2S2 ligands, which can bridge the central Ni2 unit. Dimeric C2S2 ligands rather than tetrathiosquare C4S4 ligands are found in the lowest energy Ni2(CS)4 structures.