Hypervalency, secondary bonding and hydrogen bonding: siblings under the skin

Area of Science:

• * Chemical bonding theories
• * Quantum chemistry
• * Materials science

Background:

• * Hypervalent bonding (HVB), secondary bonding (SB), and hydrogen bonding (HB) involve interactions between nucleophilic and electrophilic partners.
• * Existing models like the 'σ hole' and 'n→σ*' describe electrostatic and covalent contributions to these bonds.
• * The traditional view of HVB involves octet expansion, which is now considered largely formal.

Purpose of the Study:

• * To unify the understanding of HVB, SB, and HB under a single theoretical framework.
• * To explain the continuum of bonding strengths observed in these interactions.
• * To reconcile the 'σ hole' and 'n→σ*' models with the concept of octet retention.

Main Methods:

• * Theoretical analysis of bonding patterns in HVB, SB, and HB.
• * Application of the 'σ hole' model for electrostatic interactions.
• * Application of the 'n→σ*' model for covalent interactions.

Main Results:

• * A unified model combining 'σ hole' and 'n→σ*' contributions explains the continuum from weak SB/HB to strong HVB/HB.
• * Strong hydrogen bonds can be viewed as 'hypervalent hydrogen' analogous to HVB.
• * The central atom in these interactions retains its octet, with apparent expansion being formal.

Conclusions:

• * HVB, SB, and HB are interconnected phenomena explained by a combination of electrostatic and covalent interactions.
• * The 'σ hole' and 'n→σ*' models provide a comprehensive explanation for the observed bonding continuum.
• * Octet retention in the central atom is a key feature across these bonding types.