A metal carbido complex is a coordination complex that contains the ligand C. Carbido complexes are a molecular subclass of carbides, which are prevalent. Carbido complexes represent models for intermediates in Fischer–Tropsch synthesis and related catalytic processes. They are also used as precursors for the synthesis of more complicated carbides. They are analogous to metal nitrido complexes.
Most molecular carbido complexes are clusters, usually featuring carbide as a six-fold bridging ligand. Examples include [Rh6C(CO)15]2-, and [Ru6C(CO)16]2−. The iron carbonyl carbides exist not only in the encapsulated carbon ([Fe6C(CO)16]2−) but also with exposed carbon centres as in Fe5C(CO)15 and Fe4C(CO)13.
Clusters without CO are also known.
Bridging carbido ligands can be subdivided into three classes, called cumulenic, metallocarbyne, and polar covalent. Cumulenic compounds generally bridge two metal atoms of the same element and are symmetrical. However, there are exceptions to this.
In rare cases, carbido ligands are terminal. One example is RuC(PCy3)2Cl2 with a Ru-C distance of 163 pm, typical for a triple bond. The complex can be obtained by metathesis of vinyl acetate to give [Ru(CH-p-C6H4Me)(PCy3)2Cl2] results in a metastable Ru(Cl2)(PCy3)2C2HOAc complex, which eliminates acetic acid.
The "naked" carbido ligand is weakly basic, forming complexes with other metal centers. The C-M bond is typically found to be around 1.65 angstroms. The 13CNMR resonance values for the carbido carbons vary widely, but range from δ211-406. Another example of a terminal carbido complex is Li[MoC(NR2)3] (Mo-C distance of 172 pm), which forms upon deprotonation of the methylidyne precursor.
- Polar covalent
Three classes of
binuclear carbido ligands:
and polar covalent.