Carbene ligands possess a metal-carbon double bond and are closely related to alkylidenes. Carbene ligands (B) have a heteroatom substituent unlike alkylidenes (A) which usually have alkyl substituents on the alpha carbon atom:
These are sometimes called Fischer carbenes in honor of E.O. Fischer, who reported the first example in 1964 and later won a Nobel Prize for his pioneering work on ferrocene with Wilkinson.
Fischer carbenes are typically found on electron-rich, low oxidation state metal complexes (mid to late transition metals) containing pi-acceptor ligands. The presence of the heteroatom on the alpha carbon allows us to draw a resonance structure that is not possible for an unsubstituted (Schrock-type) alkylidene:
If we look at this from a molecular/atomic orbital perspective, one lone pair is donated from the singlet carbene to an empty d-orbital on the metal (red), and a lone pair is back-donated from a filled metal orbital into a vacant pz orbital on carbon (blue). There is competition for this vacant orbital by the lone pair(s) on the heteroatom, consistent with our second resonance structure. Overall, the bonding closely resembles that of carbon monoxide. Therefore, carbene ligands are usually thought of as neutral species, unlike dianionic Schrock alkylidenes (which usually lack electrons for back-donation). But remember that electron counting is just a formalism!
The stabilization of a partial positive charge on the carbene ligand has predictable consequences that show up in the reactivity and spectroscopic features (see below).
Single crystal x-ray diffraction studies confirm that the second resonance form shown above plays a major role in describing the bonding in metal carbene complexes. The metal double carbon bonds in these complexes tend to be longer than typical M=C double bonds, but shorter than M-C single bonds. Likewise, the carbon-heteroatom bond length is somewhat shorter than a typical M-E bond. For example, in the case below, a "normal" C-N bond is 145 pm:
The stronger the pi-donor on the carbene carbon, the lower the M=C bond order and lower the barrier to rotation around the M-C bond.
Proton and carbon NMR data are similar to those observed for alkylidene complexes, however, Fischer carbenes do not typically display any unusual JCH values that might indicate an agostic interaction between the carbene proton and the metal.
Fischer carbenes often contain carbonyl ligands which can provide very useful NMR and IR data.
There are many synthetic methods for the synthesis of carbene complexes. The four most common ones are shown here.
Acetylides can be easily synthesized from Li or SiMe3 acetylides. These species are quite reactive towards electrophiles at the beta carbon, i.e. they are easily protonated or alkylated:
Fischer carbenes exhibit such a wide manifold of reactivity, that an entire text could be devoted to them. Some of the more common reaction pathways are shown here.
These reactions go in good yield and are stereospecific. A second example is by Helquist et al. J. Am. Chem. Soc. 1982, 104, 1869:
Substituting a phosphine for one CO in the above reaction leads to good enantiomeric excess. One should note that there is well-developed cyclopropanation chemistry using diazoalkanes (explosive!) as the carbene source; organometallic reagents are much safer.
The metal can be easily cleaved from the naphthol. Dötz-type chemistry is well-developed and has been used to prepare Vitamin E as well as some antibiotics. What is the mechanism of this "termolecular" reaction? Experiments show that the rate is inhibited by excess CO or phosphine, so the first step must be dissociation of CO. Coordination of the alkyne, formation of a metallacyclobutene, insertion of CO into the ring, rearrangement to a pi-complex, ring closure and a proton shift account for our observed product (whew!):
