A transition metal thiocarbonyl complex is a coordination compound containing the ligand CS. Whereas metal carbonyl complexes are very common, even industrially important, only a few dozen thiocarbonyl complexes are known.
The main challenge for the preparation of thiocarbonyl complexes arises from the non-availability of carbon monosulfide. Thus, the CS ligand is often extruded from thiocarbonyl-containing precursors. One example involves thiophosgene, which reacts with disodium tetracarbonylferrate:
Instead of thiophosgene, chlorothioformates (ClC(S)OAr) have also been used as a source of CS ligand. The thiocarbonyl analogue of Vaska's complex is prepared in this way.
Carbon disulfide is another source of thiocarbonyl ligands, although CS<sub>2</sub> is less electrophilic than thiophosgene and its alkoxy derivative. Carbon disulfide forms ÷<sup>2</sup>-CS<sub>2</sub> complexes, which are susceptible to desulfurization. This pattern is illustrated by the reaction of Wilkinson's catalyst ():
The reaction of with carbon disulfide gives ca 30% yield of , a trimetallic cluster with a triply bridging thiocarbonyl ligands. Many other thiocarbonyl complexes arise from similarly complicated reactions in modest yield.
A variety of other routes have been developed, including addition of sulfur reagents to metal carbyne complexes.
According to the Covalent bond classification method, terminal CS is classified as an L ligand, i.e., a charge-neutral Lewis base. With respect to HSAB theory, it is classified as soft. According to spectroscopic measurements, CS is a superior pi-acceptor relative to CO, as indicated by the shortness of M-CS vs M-CO bonds.
Several complexes of CSe and CTe have been characterized. The first examples were prepared from the osmium dichlorocarbene complex.