An account of chronological computational investigations to ascertain the role of pπ-pπ bonding in influencing the Lewis acidity of BX3 (X = F, Cl, Br and I): Evolution of novel parameters and relegation of π-type back bonding concept

Abstract

Experimentally observed Lewis acidity order of the boron trihalides (BI3 > BBr3 > BCl3 > BF3) towards majority of the bases barring the very weak type (CO, HCN, CH3CN, CH3F etc.) is an abiding theme of chemical interest over several decades. The mainstream explanation exclusively invoked in the chemistry textbooks, is the π-type back-donation from filled pπ orbital of the halogen into the vacant pπ orbital of boron. A plethora of computational studies have been performed at various levels of sophistication (LCAO MO to DFT) from the late 1950s till date, in order to obtain a theoretical insight into the experimentally observed Lewis acidity order. Theoretical investigations uniformly suggest an insignificant role of the π-type back donation, rather the reorganisation energy of boron halides appears to be the parameter of profound impact. The pliability of the molecular acids and subsequent boron-halogen σ-bond elongation upon Lewis base coordination, have been authenticated as the prime factors in influencing the reorganisation energy. However, out-of-plane deformation of the ground state structure of boron halide to the geometry resembling the adduct, primarily accounts for the reorganisation energy. The origin and magnitude of the reorganisation energy have been probed from the standpoint of energy partitioning analysis (EPA). Quantitative Lewis acidity scales developed so far have been also discussed on the basis of NMR spectral features and quantum-chemically derived ion affinity calculations. Recently Parr's electrophilicity index for the boron halides have been computed, which can serve as a base-free metric to address the inherent Lewis acidity. The electrophilicity index and various ion affinity parameters have been found to bear well-behaved linear relations. Eluding the conventional spectral characterisation data related to the much explored N-donor ligands, we have herein purposely reported the X-ray structural features of the boron trihalide adducts with the trialkyl substituted phospines, arsines and stibines (π-acceptor ligands) as well as with the seleno and telluro ethers (π-donor ligands). The documented donor-boron bond length values in all the complexes have been found to attest the established acidity order unambiguously. Tetrahedral character (THC) of all those complexes have been calculated with the formula proposed by Hopfl and it reflects substantial covalent character in the boron-donor (P/As/Sb/Se/Te) bonds. This work primarily encompasses the theoretical investigations almost exclusively in a chronological way to provide a comprehensive understanding of underlying principles of the observed acidity order. After a rigorous computational research period of more than sixty years, it can be unequivocally inferred that the use of widely advocated π-type back-donation concept has been relegated over the passage of time. Instead, benchmark computational studies indicate that the electron accepting property should be interpreted in terms of polarisability, electron affinity and energy of LUMO of the boron halides. The subtle interplay of electronegativity and radius of the bonded halogen atom also contributes to the Lewis acidic order. Several theoretical models, such as, ligand close packing (LCP), minimum electrophilicity principle (MEP) and activation strain model (ASM), have emerged to treat the ruling factors of Lewis acidity in a quantitative way. So, the boron-halogen σ-bonding effects have practically outweighed the long-standing concept of π-type back-donation and nowadays the latter is discounted.