Non-Covalent Interactions Involving Alkaline-Earth Atoms and Lewis Bases B: An ab Initio Investigation of Beryllium and Magnesium Bonds, B···MR2 (M = Be or Mg, and R = H, F or CH3)

Ibon Alkorta,Anthony Legon

doi:10.3390/inorganics7030035

Abstract

Geometries, equilibrium dissociation energies (De), intermolecular stretching, and quadratic force constants (kσ) determined by ab initio calculations conducted at the CCSD(T)/aug-cc-pVTZ level of theory, with De obtained by using the complete basis set (CBS) extrapolation [CCSD(T)/CBS energy], are presented for the B···BeR2 and B···MgR2 complexes, where B is one of the following Lewis bases: CO, H2S, PH3, HCN, H2O or NH3, and R is H, F or CH3. The BeR2 and MgR2 precursor molecules were shown to be linear and non-dipolar. The non-covalent intermolecular bond in the B···BeR2 complexes is shown to result from the interaction of the electrophilic band around the Be atom of BeR2 (as indicated by the molecular electrostatic potential surface) with non-bonding electron pairs of the base, B, and may be described as a beryllium bond by analogy with complexes such as B···CO2, which contain a tetrel bond. The conclusions for the B···MgR2 series are similar and a magnesium bond can be correspondingly invoked. The geometries established for B···BeR2 and B···MgR2 can be rationalized by a simple rule previously enunciated for tetrel-bonded complexes of the type B···CO2. It is also shown that the dissociation energy, De, is directly proportional to the force constant, kσ, in each B···MR2 series, but with a constant of proportionality different from that established for many hydrogen-bonded B···HX complexes and halogen-bonded B···XY complexes. The values of the electrophilicity, EA, determined from the De for B···BeR2 complexes for the individual Lewis acids, A, reveal the order A = BeF2 > BeH2 > Be(CH3)2—a result that is consistent with the −I and +I effects of F and CH3 relative to H. The conclusions for the MgR2 series are similar but, for a given R, they have smaller electrophilicities than those of the BeR2 series. A definition of alkaline-earth non-covalent bonds is presented.

Highlights

The non-covalent interactions of closed-shell molecules represent an important subject in many areas of chemistry and biology
We show, from the molecular electrostatic surface potentials, that there is a positive belt around the central Group 2 atom which can act as the electrophilic region when forming either a beryllium or a magnesium bond [19] to the most nucleophilic region of the Lewis base
The geometries belong to the point groups D∞h, D∞h and D3d, respectively, and are consistent with two

Summary

Introduction

The non-covalent interactions of closed-shell molecules represent an important subject in many areas of chemistry and biology. The central position of the hydrogen bond in these disciplines is well known. Since the 1950s, there has been a rapid growth of interest in other non-covalent interactions. The halogen bond was first named and identified experimentally in the solid state in the 1950s by Hassel [1], and in the gas phase as a weak interaction involving simple Lewis bases with di-halogen molecules in the 1990s [2]. The halogen bond was shown [2,3] to have properties similar to those of the hydrogen bond. Interest in the halogen bond has grown rapidly within chemistry, biology

Methods

Results

Conclusion