Theoretical investigation of the weakly bonded donor—acceptor complexes X3B—H2, X3B—C2H4, and X3B—C2H2 (X = H, F, Cl)

Quantum mechanical calculations at the MP2/6-311G(2df,2pd)//MP2/TZ2P level of theory are reported for the compounds X3B—D with X = H, F, Cl, and D = H2, C2H4, and C2H2. The calculated binding energies show the trend BH3 >> BF3 > BCl3 for the Lewis acids and C2H4 > C2H2 > H2 for the Lewis bases. BCl3 and BF3 form weakly bonded van der Waals complexes with C2H4 and C2H2 with similar binding energies De between 4.2 kcal mol-1 for F3B—C2H4 and 3.2 kcal mol-1 for Cl3B—C2H2. The dihydrogen complexes have lower bond dissociation energies of 0.9 kcal mol-1 for Cl3B—H2 and 0.7 kcal mol-1 for Cl3B—H2. The BH3 complexes are significantly more strongly bonded and show the onset of true chemical bonding. The NBO method finds a polarized 2-electron 3-centre bond for the three H3B—D compounds. The most strongly bonded complex is H3B—C2H4 (De = 15.0 kcal mol-1), while H3B—C2H2 (De = 8.5 kcal mol-1) and H3B—H2 (De = 5.7 kcal mol-1) are more weakly bonded species. The contribution of the zero-point vibrational energy shows H3B—H2 to be scarcely bound. The CDA method suggests that H2, C2H4 and C2H2 are stronger donor than acceptor ligands. Calculated vibrational spectra are reported also.