Dimensioning the Term Carbenoid.

The current use of the term carbenoid is discussed, particularly in the context of carbene transfer reactions from diazo compounds, in which intermediates of type Ln M=CR1 R2 , or one of its resonance forms, are tagged which such a name. We discuss this issue, on the basis of the data evidencing the metal-carbene nature of those intermediates, as well as the existence of carbenoids of type (N2 )(M)CR1 R2 en route to the formation to Ln M=CR1 R2' from diazo reagents. We propose the exclusive use of the carbenoid term to species of type (X)(M)CR1 R2 with a tetrasubstituted carbon center that upon loss of X afford an effective carbene transfer process.