Analysis of the Unusual Chemical Bonds and Dipole Moments of AeF⁻ (Ae=Be−Ba): A Lesson in Covalent Bonding

Abstract

Quantum chemical calculations of the anions AeF‐ (Ae = Be ‐ Ba) have been carried out using ab initio methods at the CCSD(T)/def2‐TZVPP level and density functional theory employing BP86 with various basis sets. The detailed bonding analyses using different charge‐ and energy partitioning methods show that the molecules possess three distinctively different dative bonds in the lighter species with Ae = Be, Mg and four dative bonds when Ae = Ca, Sr, Ba. The occupied 2p atomic orbitals (AOs) and to a lesser degree the occupied 2s AO of F‐ donate electronic charge into the vacant spx(σ) and p(π) orbitals of Be and Mg which leads to a triple bond AeF‐. The heavier Ae atoms Ca, Sr, Ba use their vacant (n‐1)d AOs as acceptor orbitals which enables them to form a second σ donor bond with F‐ that leads to quadruply bonded AeF‐ (Ae = Ca – Ba). The presentation of molecular orbitals or charge distribution using only one isodensity value may give misleading information about the overall nature of the orbital or charge distribution. Better insights are given by contour line diagrams.