Fischer and Schrock carbene complexes in the light of global and local electrophilicity-based descriptors

The carbon atom (carbene) of Fischer and Schrock complexes are electrophilic and nucleophilic, respectively. The reactivity index electrophilicity is a global reactivity parameter and can tell only about the total electrophilicity of the complexes. To differentiate between the reactivity patterns of these two carbenes, the philicity and multiphilic descriptor are calculated. In Fischer complexes, it is found that the philicity of the nucleophilic attack (omega C+) is higher than that of philicity of the electrophilic attack (omega C-) implying the electrophilic nature. A reverse order is found in the Schrock complex pointing nucleophilic character. The multiphilic descriptor (Delta omega(C) = omega C+ - omega C-) is found to be positive in Fischer but negative in Schrock leading to the same conclusion. Fischer carbene complexes having general formula (CO)(5)CrCH-R (R = CH3, Ph, C(sic)CH, CHCH2, OCH3, OH, NHCH3, and NH2) the order of omega C+ and Delta omega(C) better describe the trend. The trend has been justified through energy decomposition in the purview natural orbital for chemical valence (EDA-NOCV) analysis owing to the pi contribution from the R group. The change in the reactivity patterns along the intrinsic reaction coordinate of two representative reactions is plotted. This way of understanding the reactivity parameters would help experimental chemists to predict the catalytic application of carbene complexes of transition metal without the classification of Fischer and Schrock type.

Automated summary

The carbon atom (carbene) in Fischer and Schrock complexes exhibits electrophilic and nucleophilic properties respectively. To differentiate the reactivity patterns of these two carbenes, the philicity and multiphilic descriptor are calculated. It is found that the nucleophilic attack has higher philicity in Fischer complexes, while the electrophilic attack has higher philicity in Schrock complexes. The multiphilic descriptor is positive in Fischer and negative in Schrock, confirming the same conclusion. This understanding of reactivity parameters can help predict the catalytic application of carbene complexes without the classification of Fischer and Schrock types.