Genuine quadruple bonds between two main-group atoms. Chemical bonding in AeF− (Ae = Be–Ba) and isoelectronic EF (E = B–Tl) and the particular role of d orbitals in covalent interactions of heavier alkaline-earth atoms

Abstract

Quantum chemical calculations of anions AeF⁻ (Ae = Be–Ba) and isoelectronic group-13 molecules EF (E = B–Tl) have been carried out using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory employing BP86 various basis sets. The equilibrium distances, bond dissociation energies and vibrational frequencies are reported. The alkali earth fluoride anions AeF⁻ exhibit strong bonds between the closed-shell species Ae and F⁻ with bond dissociation energies between 68.8 kcal mol⁻¹ for MgF⁻ and 87.5 kcal mol⁻¹ for BeF⁻ and they show an unusual increasing trend MgF⁻ < CaF⁻ < SrF⁻ < BaF⁻. This is in contrast to the isoelectronic group-13 fluorides EF where the BDE continuously decreases from BF to TlF. The calculated dipole moments of AeF⁻ are very large between 5.97 D for BeF⁻ and 1.78 D for BaF⁻ with the negative end always at the Ae atom (Ae→F⁻). This is explained by the location of the electronic charge of the lone pair at Ae, which is rather distant from the nucleus. The analysis of the electronic structure of AeF⁻ suggests significant charge donation Ae←F⁻ into the vacant valence orbitals of Ae. A bonding analysis with the EDA-NOCV method suggests that the molecules are mainly covalently bonded. The strongest orbital interaction in the anions comes from the inductive polarization of the 2p_σ electrons of F⁻, which leads to a hybridization of the (n)s and (n)p_σ AOs at Ae. There are two degenerate π donor interactions Ae←F⁻ in all anions AeF⁻, which provide 25–30% to the covalent bonding. There is another σ orbital interaction in the anions, which is very weak in BeF⁻ and MgF⁻. In contrast, the second stabilizing σ orbital interaction in CaF⁻, SrF⁻ and BaF⁻ yields a strongly stabilizing σ orbital, because the Ae atoms use their (n − 1)d_σ AOs for bonding. The energy lowering of the second σ interaction in the latter anions is even stronger than the π bonding. The EDA-NOCV results suggest that BeF⁻ and MgF⁻ have three strongly polarized bonds, whereas CaF⁻, SrF⁻ and BaF⁻ have four bonding orbitals. The quadruple bonds in the heavier alkaline earth species are made possible because they use s/d valence orbitals like transition metals for covalent bonding. The EDA-NOCV analysis of the group-13 fluorides EF gives a conventional picture with one very strong σ bond and two rather weak π interactions.