Thioxoethenylidene (CCS) as a Bridging Ligand

The reaction of [Mo­(CBr)­(CO)2(Tp*)] (Tp* = hydrotris­(3,5-dimethylpyrazol-1-yl)­borate)
with [Fe2(μ-SLi)2(CO)6] affords,
inter alia, the unsymmetrical binuclear thioxoethenylidene complex
[Mo2(μ,σ­(C):η2(C′S)-CCS)­(CO)4(Tp*)2], which may be more directly obtained from [Mo­(CBr)­(CO)2(Tp*)] and Li2S. The reaction presumably proceeds
via the intermediacy of the bis­(alkylidynyl)­thioether complex S­{CMo­(CO)2(Tp*)}2, which was, however, not directly observed
but explored computationally and found to lie 78.6 kJ mol–1 higher in energy than the final thioxoethenylidene product. Computational
interrogation of the molecules [M2(μ-C2S)­(CO)2(Tp*)2] (M = Mo, W, Re, Os) reveals
three plausible coordination modes for a thioxoethenylidene bridge
which involve a progressive strengthening of the C–C bond and
weakening of the M–C and M–S bonds, as might be expected
from simple effective atomic number considerations.