Investigation of electronic stabilization mechanisms in carbenes and boron analogs by quantum theory of atoms in molecules analysis of the topology of the electron density

This work presents the results of a thorough investigation into electronic properties and mechanisms of stabilization in carbenes and boron analogs as revealed by the quantum theory of atoms in molecules, QTAIM. The conclusions drawn from this theoretical approach are based upon the physics of the systems and the balance of attractive and repulsive Ehrenfest forces, determined by the topology of the real-space electron density. The electronic structure of the unusual molecules of this study are not always completely understood from the orbital point of view, and conflicting mechanisms are sometimes seen with the use of QTAIM.;In the three membered ring (3MR) cyclopropenylidene, the carbene carbon (C2) is stabilized upon amino substitution through inductive withdrawal by nitrogen, back polarization of electron density into the ring, preferential accumulation of charge in the ring plane due to σ-aromaticity, and subsequent σ-π polarization of the non-bonding valence shell charge concentration of C2. These mechanisms lead to the protection of C2 from nucleophilic attack and are at odds with the orbital picture of extended π-hyperconjugation of the nitrogen lone pair two bonds removed from C2. The electronic tuning of C2 properties of N-heterocyclic carbenes (NHCs) through the variation of ring size and ring substituents remote from C2 is determined by QTAIM analysis of free NHCs and NHC-transition metal (NHC-TM) complexes. The influence of nucleophilic and electrophilic ligands on experimental properties of NHC-TM complexes is uncovered by changes in the binding forces of the trans -carbonyl groups. The electronic mechanisms of tuning are revealed by changes in the delocalization of electrons across atomic basins and by the virial of the Ehrenfest forces acting on the interatomic surfaces. QTAIM properties of models of boryl anion analogs of carbenes reveal the electronic mechanisms that stabilize the reactive boron center, as well as the nucleophilic nature of the boron despite its positive atomic charge. Finally, models of orbital-based concepts of lone-pair inversion and double aromaticity are approached from the QTAIM perspective. Again, mechanisms are uncovered that conflict with the orbital point of view. QTAIM based mechanisms are confirmed by comparison with calculated and experimental spectroscopic data.