The Preparatory (Anti)Bonding Character of Molecular Orbitals

The strengths of chemical bonds define our world by influencing chemical reactivity and molecular structure. This work presents unpredicted changes in bonding energies of covalent aluminum-carbon bonds in isolated molecular aluminum complexes while populating remote ligand-centered orbitals. Their chemical bonding situation was investigated computationally by energy decomposition analysis, which revealed that populating frontier molecular orbitals enables the relaxation (or preparation) of homolysis products, consequently influencing bonding strength. It was demonstrated in the case of several organic, inorganic, and organometallic standard molecules that generally bond dissociation energies can change upon the population of frontier preparatory (anti)bonding molecular orbitals, which do not necessarily have to show orbital coefficients at the chemical bond. The concept of preparatory character of orbitals allows qualitative predictions of trends in bonding energies upon oxidation or reduction of molecular compounds, being a valuable addition to molecular orbital theory across all fields of chemistry.