Desulfurization and carbon-carbon coupling in the reaction of the anions [MnM(CO)6(.mu.-H)(.mu.-S2CPCy3)]- (M = manganese, rhenium) with carbon disulfide. X-ray structure of [Li(THF)3][Mn2(CO)6(.mu.-H){.mu.-S(S)C:C(PCy3)S}]