The strongest dative bond in main-group compounds. Theoretical study of OAeF^- (Ae = Be-Ba)

Quantum chemical calculations of the anions OAeF^- (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 equilibrium structures have linear geometries for Ae = Be and Mg but they are strongly bent for Ae = Sr and Ba while the calcium species has a quasi-linear structure with a very low bending potential. The calculated bond dissociation energies suggest a record-high BDE of D_e = 144.08 kcal mol^-1 for OBeF^- at the CCSD(T)/def2-TZVPP level, which is the strongest BDE for a dative bond that has been found so far. The BDE of the heavier homologues have a continuously decreasing order for Ae with Be > Mg (113.01 kcal mol^-1) > Ca (84.06 kcal mol^-1) > Sr (72.06 kcal mol^-1) > Ba (60.00 kcal mol^-1). The calculation of the charge distribution reveals a significant charge donation OAe ← F^- with a declining sequence for the heavier atoms Ae. The oxygen atom in OAeF^- carries always a higher partial charge than the fluorine atom, which contradicts the standard electronegativities of the atoms. The surprising partial charges are explained with the bonding situation of the atoms in the actual electronic structure. The bonding analysis of the OAe-F^- bonds using the EDA-NOCV method shows that the bonds have much more electrostatic character than the Ae-F^- bonds in the diatomic anions. This finding is supported by the results of the LED partitioning approach. The dative interactions have three major and one minor component. The assignment of a quadruple bond for the heavier species with Ae = Ca, Sr, Ba is not reasonable. The driving force for the bent geometries is the accumulation of electronic charge in the lone-pair region at the Ae atoms, which enhances the electrostatic attraction with the other atoms. An adequate description of the bonding situation is given by the formula O^--Ae⁺ ← F^-.