The nature of the chemical bond in the light of an energy decomposition analysis

This chapter summarizes the results of quantum chemical calculations where it has investigated the nature of the chemical bond in main-group and transition metal compounds with an energy decomposition analysis (EDA). The EDA decomposes the instantaneous interaction energy A-B between two fragments A and B into three terms that can be interpreted in a chemically meaningful way. The three terms are the quasi-classical electrostatic interaction between the frozen charges of the fragments -E_elstat, the exchange (Pauli) repulsion between electrons possessing the same spin ?E_Pauli, , and the orbital interaction term ΔE_orb. The latter term can be divided into contributions of orbitals having different symmetry, which allows an estimate of the strength of s, p, and d bonding. The results show that the quasi-classical electrostatic interaction significantly contributes to the bonding interactions in all molecules. The trend of the bond strength is in most cases correctly predicted by the orbital term ΔE_Pauli, but there are cases where the electrostatic attraction or the Pauli repulsion is more important for an understanding of the bonding interactions. The EDA is an unambiguously defined partitioning scheme that considers all terms yielding a chemical bond. The EDA can be considered as a bridge between the classical heuristic bonding models of chemistry and the physical mechanism of chemical bond formation.