Matthias Bickelhaupt

The chemical bond is commonly viewed as the result of stabilizing interactions, including electron-pair bonding and, for example, favorable HOMO–LUMO orbital interactions. However, the strength of a bond and the course of a reaction are determined not only by stabilization, but also by the extent of destabilizing closed-shell–closed-shell interaction, or Pauli repulsion. Over the past several years, our work has demonstrated that modulation of Pauli repulsion provides a general and powerful framework for understanding trends in bonding, stability, and reactivity across a wide range of molecular systems. This lecture will illustrate how Pauli repulsion controls element–element single bonds in relation to atomic size and coordination number, and how it underlies the recently recognized phenomenon of the lone-pair-shielded radical (LPSR, see illustration). These examples reveal that subtle changes in occupied–occupied interactions can have profound consequences for bond strength and molecular structure. The same conceptual framework also sheds light on variations in the stability of organic radicals and the magnitude of reaction barriers. Taken together, these findings highlight that chemical bonding is shaped not only by attractive forces, but equally by the modulation of repulsive ones. Recognizing and exploiting this balance opens new avenues for a deeper and more predictive understanding of molecular structure and reactivity.