To spot a Fischer carbene, be on the lookout for reasonable zwitterionic resonance structures like the one at right below.The clever reader may notice that we haven’t mentioned π-acidic R groups, such as carbonyls.What sets carbene precursors apart from free carbenes?What other kinds of molecules may act as both nucleophiles and electrophiles at the same atom?Fundamentally, this “ambi-electronic” behavior is useful for the creation of double bonds.One bond comes from “forward flow,” and the other from “reverse flow.” Naturally, the other reacting partner also needs to be ambi-electronic for this method to work.Vinylidenes are the organometallic analogues of allenes, and are best described as intermediate in behavior between Fischer and Schrock carbenes.They are electrophilic at the α carbon and nucleophilic at the β carbon—in fact, a nice analogy can be made between vinylidenes and carbon monoxide.
In practice, many carbenes are intermediate between the Fischer and Schrock ideals.
Fischer carbenes, Shrock carbenes, and vinylidenes are usually actor ligands, but they may be either nucleophilic or electrophilic, depending on the nature of the R groups and metal.
In addition, these ligands present some interesting synthetic problems: because free carbenes are quite unstable, ligand substitution does not cut the mustard for metal carbene synthesis.
Ylides (diazo compounds, specifically) are the most common precursors to metal carbene complexes. The electrophilic frontier orbital of an ylide is just the σ* orbital of the bond connecting the charged atoms, which makes sense if we consider the positively charged fragment as a good leaving group (it always is). The figure above depicts some of the most famous ylides of organic chemistry, including those used for alkene synthesis (Corey-Chaykovsky and Wittig) and cycloaddition reactions (the carbonyl ylide).
Although diazo compounds are most commonly drawn with charges on the two nitrogen atoms, the diazo carbon is a good nucleophile and can attack electrophilic metal centers to initiate metal carbene formation.