Computational Investigation of Carbene-Phosphinidenes: Correlation between 31P Chemical Shifts and Bonding Features to Estimate the π-Backdonation of Carbenes.

Detailed investigations of the electronic structure and bonding scenario in different carbene-phosphinidenes have been presented using state-of-the-art computational methods (BP86/def2-TZVPP//BP86/def2-SVP). We have endeavored to find the correlation of the calculated 31P chemical shifts with different bonding parameters of compounds to access the relative π-acceptor strengths of the carbenes. 31P chemical shifts exhibit a weak correlation with σ-polarizations of Ccarb-P bonds toward phosphorus; however excellent correlations are obtained in the case of π-polarizations of Ccarb-P bonds toward the carbene carbon (Ccarb) and NPA charges on phosphorus atoms. 31P chemical shifts also show excellent correlations with the electron densities and energy densities of Ccarb-P bonds at BCPs, as suggested by QTAIM calculations. Moreover, EDA-NOCV analysis is implemented to gain brief insight into the bonding scenario in this class of compounds. Good correlation exists between the interaction energies between the carbene and PPh fragments and 31P chemical shifts. Additionally, we have investigated the correlations of calculated 31P chemical shifts with different bonding parameters of the corresponding free carbenes. The bonding scenario in different carbene-substituted phosphinidenes is also explored to see how the bonding situation depends on various substituents on phosphinidenes. The other substituted carbene-phosphinidenes show correlations similar to those of carbene-phenylphosphinidenes.