Carbones as Ligands in Novel Main-Group Compounds E[C(NHC) ] (E=Be, B , C , N , Mg, Al , Si , P ): A Theoretical Study.

Quantum chemical calculations of the main-group compounds E[C(NHC ) ] (E=Be, B , C , N , Mg, Al , Si , P ) have been carried out using density functional theory at the BP86/def2-TZVPP and BP86-D3(BJ)/def2-TZVPP levels of theory. The geometry optimization at BP86/def2-TZVPP gives equilibrium structures with two-coordinated species E and bending angles C-E-C between 152.5° (E=Be) and 110.5° (E=Al). Inclusion of dispersion forces at BP86-D3(BJ)/def2-TZVPP yields a three-coordinated beryllium compound Be[C(NHC ) ] as the only energy minimum form. Three-coordinated isomers are found besides the two-coordinated energy minima for the boron and carbon cations B[C(NHC ) ] and C[C(NHC ) ] . The three-coordinated form of the boron compound is energetically lower lying than the two-coordinated form, while the opposite trend is calculated for the carbon species. The theoretically predicted bond dissociation energies suggest that all compounds are viable species for experimental studies. The X-ray structure of the benzoannelated homologue of P[C(NHC ) ] that was recently reported by Dordevic et al. agrees quite well with the calculated geometry of the molecule. A detailed bonding analysis using charge and energy decomposition methods shows that the two-coordinated neutral compounds Be[C(NHC ) ] and Mg[C(NHC ) ] possess strongly positively charged atoms Be and Mg. The carbodicarbene groups C(NHC ) serve as acceptor ligands in the compounds and may be sketched with dative bonds (NHC ) C←E→C(NHC ) (E=Be, Mg). Dative bonds in which the carbones C(NHC ) are donor ligands are suggested for the cations (NHC ) C→E←C(NHC ) (E=B , Al ). The dications and trications possess electron-sharing bonds in which the bonding situation is best described with the formula [(NHC ) C] -E-[C(NHC ) ] (E=C, Si, N , P ).