Carbide complexes as π-acceptor ligands

A terminal carbide complex binds as a π-acceptor towards electron-rich metal centers, mirroring CO, and provides the first homoleptic, carbide-ligated complex.

The π-accepting character of a terminal carbide complex acting as a ligand is demonstrated experimentally and corroborates earlier theoretical predictions. As a result, coordination of a terminal ruthenium carbide complex to electron-rich metal centres is shown to provide a facile and versatile route to carbide-bridged heterometallic complexes. Synthesis, reactivity, spectroscopic and structural characterization are reported for heterobimetallic systems with auxiliary metals from groups 9–11: Rh(i), Ir(i), Pd(ii), Pt(ii), Ag(i), and Au(i) coordinated by [Ru(C)Cl₂(PCy₃)₂] (RuC). This encompasses the first example of a homoleptic carbide-ligated transition metal complex: [{(Cy₃P)₂Cl₂RuC}₂Au]⁺. Kinetics of substitution on Pt(ii) by RuC ranks the carbide complex as having intermediate nucleophilicity. The ¹³C-NMR signals from the carbide ligands are significantly more shielded in the bridged heterobimetallic complexes than in the parent terminal carbide complex. Structurally, RuC forms very shorts bonds to the heterometals, which supports the notion of the multiple bonded complex acting as a π-backbonding ligand. Reactions are reported where RuC displaces CO coordinated to Rh(i) and Ir(i). A strong trans influence exerted by RuC indicates it to be a stronger σ-donor than CO. The geometries around the carbide bridges resemble those in complexes of electron-rich metals with carbonyl or bridging nitride-complex-derived ligands, which establishes a link to other strong π-acceptor ligands.