View of Lone Electron Pairs and Their Role in Structural Chemistry

Nonbonding valence electrons, better known as lone pairs, are found in all anions as well as in cations in their lower oxidation states. In this paper the properties of lone pairs are analyzed using a bond valence model defined in terms of a core-and-valence-shell picture, which can be reduced to the point charges of the ionic model. A bond is defined in terms of the electrostatic flux linking the atoms. When the lone pairs are inactive, they are uniformly distributed around the valence shell and the ion behaves like a main group ion that has no lone pairs. In this state it can be assigned a bonding strength that obeys the valence matching rule (for stable bonds the cation and anion bonding strengths should not differ by more than a factor of 2). However, when an ion with a lone pair has a bonding strength less than half that of the counterion, it has the flexibility to form a stronger bond by converting lone pair electron density to bonding electron density in the region where the valence shells overlap. To conserve the number of lone pairs, bonding electron density elsewhere in the valence shell is converted to lone pairs. The result is the adoption of an anisotropic coordination environment with fewer bonds but bonds whose enhanced strength matches the higher bonding strength of the counterion. This analysis raises questions about the validity of a number of traditional ideas on the nature of chemical bonds. It shows that lone pairs do not form dative bonds. In a neutral molecule, the base function that is often attributed to a lone pair is always accompanied by an acid function, and both functions must be simultaneously activated. Electron-pair bonds, which are found only around strongly bonding cations, normally result in the formation of molecules, explaining why the models developed to describe molecular structures are unable to give good descriptions of extended crystal structures.