The Basics of Covalent Bonding in Terms of Energy and Dynamics

Abstract

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: H2+ and H2. 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.

Analysis of Students Understanding of Chemical Bonds Concept Using Three Tier Multiple Choice

“…The bond that occurs between atoms in the above structure is. The answer to this question is covalent bonds because of the electron sharing among atoms (Nordholm & Bacskay, 2020). The high level of misconceptions is caused by students not distinguishing between bonds between atoms and bonds between molecules.…”

From Electronegativity towards Reactivity—Searching for a Measure of Atomic Reactivity

“…In Hartree–Fock or DFT calculations, the interatomic electron transfer is seen in the formation of delocalized molecular orbitals, but the same effect can also be achieved in correlated calculations. The Heitler–London wavefunction for H 2 is a particularly clear example of how two polarized configurations, by addition, represent the unpolarized hydrogen atom environments in the H 2 molecule, but it should be noted that only when the electrons are able to oscillate rapidly between the atoms is the stabilization associated with partial occupation realized [ 49 ].…”

“…Upon closer examination, it is soon realized that it fails to account for the strong quantum mechanical coupling between the dynamics of the electrons and their correctly quantized kinetic energy. In fact, it is readily seen that this term is insensitive to whether parts of space integrated over are dynamically connected or not and therefore TF-DFT is unable to resolve what we have claimed to be the main mechanism of covalent stabilization in molecule formation [ 25 , 31 , 49 ].…”

X-ray absorption spectra of f-element complexes: insight from relativistic multiconfigurational wavefunction theory

“…It is familiar to chemists that covalent bonding manifests itself in calculations via the mixing of overlapping atomic orbitals (AOs) of neighboring atoms in the molecular orbitals (MOs) in a way that is accompanied by constructive quantum interference and energetic stabilization of the system. 28,29 Furthermore, it is textbook knowledge that the mixing of AOs and the (de)stabilization of the (anti)bonding MO is larger when the AOs match well energetically, as compared to the case when the AOs have very different energies. Because the transitions probed in X-ray spectroscopy of metal complexes are out of strongly localized atomic core shells, the pre-edge intensities can be very sensitive to the extent of donation bonding.…”