Electrostatics Explains the Reverse Lewis Acidity of BH3 and Boron Trihalides: Infrared Intensities and a Relative Energy Gradient (REG) Analysis of IQA Energies.

Leonardo J Duarte,Roy E Bruns,Paul L A Popelier,Wagner E Richter

doi:10.1021/acs.jpca.1c05766

Abstract

The reaction path for the formation of BX3–NH3 (X = H, F, Cl, Br) complexes was divided into two processes: (i) rehybridization of the acid while adopting a pyramidal geometry, and (ii) the complex formation from the pyramidal geometries of the acid and base. The interacting quantum atom (IQA) method was used to investigate the Lewis acidity trend of these compounds. This topological analysis suggests that the boron–halogen bond exhibits a considerable degree of ionicity. A relative energy gradient (REG) analysis on IQA energies indicates that the acid–base complex formation is highly dependent on electrostatic energy. With increasing halogen electronegativity, a higher degree of ionicity of the B–X is observed, causing an increase in the absolute value of X and B charges. This increases not only the attractive electrostatic energy between the acid and base but also enhances the repulsive energy. The latter is the main factor behind the acidity trend exhibited by trihalides. Changes in geometry are relevant only for complexes where BH3 acts as an acid, where lower steric hindrance facilitates the adoption of the pyramidal geometry observed in the complex. The CCTDP analysis shows that infrared intensities of BX3–NH3 are determined mostly by the atomic charges and not by the charge transfer or polarization. The opposite is observed in covalent analogues.

Highlights

An intriguing fact in inorganic chemistry is the reverse acidity of boron trihalides with respect to a strong base such as NH3
We present a novel approach to understanding the unexpected Lewis acidity scale of boron trihalides using the recently proposed relative energy gradient (REG)[14] method combined with a topological energy partition scheme called the interacting quantum atoms (IQA).[15]
ΔErehybridization is equal to the energy necessary to force the planar sp[2] boron atom to adopt a pyramidal sp3-like structure (Figure 1A) and is given by the difference between the BX3 energy when adopting its geometry as it is in the equilibrium state of the complex and the equilibrium geometry of the isolated BX3 molecule

Summary

Introduction

An intriguing fact in inorganic chemistry is the reverse acidity of boron trihalides with respect to a strong base such as NH3. The most accepted explanation for this behavior, presented in undergraduate-level inorganic chemistry textbooks,[1,2] invokes π-backbonding, where p occupied orbitals from the halides overlapping with the boron’s empty p orbital. This effect confers some double-bond character to the B−X bond. The stronger the π-backbonding effect, the more difficult the adoption of a pyramidal geometry explaining the reverse acidity order. The IQA partitioning scheme utilizes the QTAIM definition of atomic basins to calculate intra- and interatomic energy terms that sum to the total energy of the system. We note that the above interpretation of IQA terms results in ionicity and covalency not being opposite of each other.[18]

Methods

Results

Conclusion