Generation of melded ligand (M=Mo) and mono(dithiolene) (M=Nb) complexes in the reactions of M(S2)(S2CNEt2)3 with dimethyl acetylenedicarboxylate

Reaction of brown Mo(S2)(S2CNEt2)3 with dimethyl acetylenedicarboxylate (DMAC) in refluxing 1,2-dichloroethane results in the formation of Mo{SC(CO2Me)C(CO2Me)SCNEt2}{S2C2(CO2Me)2}(S2CNEt2) and Mo{SC(CO2Me)C(CO2Me)C(NEt2)S}{S2C2(CO2Me)2}(S2CNEt2) (coordinating atoms italicised), which contain ligands formed by melding dithiocarbamate and alkyne moieties (J. Chem. Soc., Chem. Commun. (1994) 2579). In contrast, reaction of orange Nb(S2)(S2CNEt2)3 with DMAC in refluxing 1,2-dichloroethane produces purple, air-stable Nb{S2C2(CO2Me)2}(S2CNEt2)3, which possesses an eight-coordinate, distorted mmmm dodecahedral structure closely related to that of the starting material. The related thio-niobium complex, NbS(S2CNEt2)3, does not react with alkynes even under forcing conditions.