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

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 sp^x(σ) and p(π) orbitals of Be and Mg which leads to a triple bond Ae F⁻. 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 Ae F⁻ (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. The ELF calculations provide monosynaptic and disynaptic basins of AeF⁻ which nicely agree with the analysis of the occupied molecular orbitals and with the charge density difference maps. A particular feature of the covalent bonds in AeF⁻ concerns the inductive interaction of F⁻ with the soft valence electrons in the (n)s valence orbitals of Ae. The polarization of the (n)s² electrons induces a (n)sp^x hybridized lone-pair orbital at atom Ae, which yields a large dipole moment with the negative end at Ae. The concomitant formation of a vacant (n)sp^x AO of atom Ae, which overlaps with the occupied 2p(σ) AO of F⁻, leads to a strong covalent σ bond.