Role of core and valence atomic orbitals in the formation of chemical bonds

Abstract

The general principles of the electronic structure of molecules and complexes are discussed. An analysis of photoelectron and x-ray photoelectron spectra reveals that in most cases, the interaction of the valence AO's results in the formation of delocalized MO's, which are populated by electrons, but are not bonding according to their energies (molecules with small atomic cores are exceptions). These MO's do not make a positive contribution to the exothermic effect of chemical bonding. The delocalization of the valence electrons in such MO's reduces their charge density in the region of the atomic cores and lowers the energy of the inner “atoms-in-molecules” levels as a result of the reduction of the internal shielding; the ESCA shifts of the inner electronic levels confirm this stabilization. In accordance with the virial theorem, the lowering of the energy of the core levels of the central atoms (with simultaneous, generally small changes in the energy of the core levels of the ligands and despite the significant destabilization of the delocalized MO's in comparison to the valence orbitals of the corresponding free atoms) makes up the main contribution to the energy of chemical bonds.