Energetics of hydride and electron pair attachment to EX30/+ (E = B, C, Al, Si and X = F, Cl, Br, I) and the study of bonding trends among EX30/+, EX32−/−, and EX3H−/0 by use of ELF and NBO analyses

A theoretical gas-phase “ligand-free” or “electron pair affinity” (EPA) approach, based on CCSD(T)/(SDB-)cc-pVTZ//MP2/(SDB-)cc-pVTZ electronic structure calculations, is introduced as a possible means for determining Lewis acidity trends among planar EX30/+ (E = B, C, Al, Si; X = F, Cl, Br, I) species. In this treatment, the free electron pair is considered to be an extreme Lewis base. The calculated EPA values are compared with experimental Lewis acidities, previously calculated fluoride ion affinity (FIA) and hydride ion affinity (HA) trends, and are found to exhibit reasonable correlations in all cases. The bonding in the planar and trigonal pyramidal conformations of EX30/+ and of the trigonal pyramidal Lewis base EX32−/− anions are assessed by use of natural bond orbital (NBO) and natural resonance theory (NRT) analyses. The NBO charges of the CX3+ (X = Cl, Br, I, OTeF5) cations are shown to correlate with the cation–anion and cation–solvent contacts in the recently determined crystal structures of [CCl3][Sb(OTeF5)6], [CBr3][Sb(OTeF5)6]·SO2ClF, [CI3][Al(OC(CF3)3)4], and [C(OTeF5)3][Sb(OTeF5)6]·3SO2ClF and known fluoro-carbocation structures. Topological electron localization function (ELF) basin lobe isosurfaces and volumes are used to rationalize the Lewis acidity trends and bond ionicities of the EX30/+ species, and Lewis basicities of the EX32−/− species.