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)_<i>n</i> (<i>n</i> = 7, 6, 5) structures results in cyclization to give remarkable C_<i>n</i>S_<i>n</i> (<i>n</i> = 5, 6) ligands containing five- and six-membered carbocyclic rings. Such ligands, which are the sulfur analogues of the well-known croconate (<i>n</i> = 5) and rhodizonate (<i>n</i> = 6) oxocarbon ligands, function as bidentate ligands to the central Ni₂ unit. Higher energy Ni₂(CS)_<i>n</i> (<i>n</i> = 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