The Basics of Covalent Bonding in Terms of Energy and Dynamics

• Published in: Molecules (Basel, Switzerland) Switzerland), 2020-06, Vol.25 (11), p.2667
• Main Authors: Nordholm, Sture, Bacskay, George
• Format: Article
• Language: English
• Subjects: Agreements; atomic reactivity; Biochemistry & Molecular Biology; Biochemistry and Molecular Biology; Biokemi och molekylärbiologi; Chemical bonds; Chemical Sciences; chemical-bond; Chemistry; Chemists; Computer Simulation; Covalence; covalent bond; Covalent bonds; delocalization; electronic-structure; Electrons; Electrostatic properties; Electrostatics; Energy; Geometry; hydrogen; Hydrogen Bonding; interatomic electron; interatomic electron motion; Kemi; Kinetic energy; Kinetics; mechanism; Models, Molecular; molecules; motion; physical nature; Physics; Quantum mechanics; Quantum Theory; Review; Theory; valence; virial-theorem
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules25112667

We address the paradoxical fact that the concept of a covalent bond, a cornerstone of chemistry which is well resolved computationally by the methods of quantum chemistry, is still the subject of debate, disagreement, and ignorance with respect to its physical origin. Our aim here is to unify two seemingly different explanations: one in terms of energy, the other dynamics. We summarize the mechanistic bonding models and the debate over the last 100 years, with specific applications to the simplest molecules: H and H . In particular, we focus on the bonding analysis of Hellmann (1933) that was brought into modern form by Ruedenberg (from 1962 on). We and many others have helped verify the validity of the Hellmann-Ruedenberg proposal that a decrease in kinetic energy associated with interatomic delocalization of electron motion is the key to covalent bonding but contrary views, confusion or lack of understanding still abound. In order to resolve this impasse we show that quantum mechanics affords us a complementary dynamical perspective on the bonding mechanism, which agrees with that of Hellmann and Ruedenberg, while providing a direct and unifying view of atomic reactivity, molecule formation and the basic role of the kinetic energy, as well as the important but secondary role of electrostatics, in covalent bonding.