Carbones as Ligands in Novel Main-Group Compounds E[C(NHC)2 ]2 (E=Be, B+ , C2+ , N3+ , Mg, Al+ , Si2+ , P3+ ): A Theoretical Study.

Abstract

Quantum chemical calculations of the main-group compounds E[C(NHCMe )2 ]2 (E=Be, B+ , C2+ , N3+ , Mg, Al+ , Si2+ , P3+ ) 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(NHCMe )2 ]2 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(NHCMe )2 ]2+ and C[C(NHCMe )2 ]22+ . 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(NHCMe )2 ]23+ 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(NHCMe )2 ]2 and Mg[C(NHCMe )2 ]2 possess strongly positively charged atoms Be and Mg. The carbodicarbene groups C(NHCMe )2 serve as acceptor ligands in the compounds and may be sketched with dative bonds (NHCMe )2 C←E→C(NHCMe )2 (E=Be, Mg). Dative bonds in which the carbones C(NHCMe )2 are donor ligands are suggested for the cations (NHCMe )2 C→E←C(NHCMe )2 (E=B+ , Al+ ). The dications and trications possess electron-sharing bonds in which the bonding situation is best described with the formula [(NHCMe )2 C]+ -E-[C(NHCMe )2 ]+ (E=C, Si, N+ , P+ ).