Dihydrogen Bonding in Main Group Elements:  A Case Study of Complexes of LiH, BH₃, and AlH₃with Third-Row Hydrides

A systematic investigation of the occurrence of dihydrogen bonds in the main group elements is taken up. The complexes of LiH, BH 3, and AlH 3 with the third-row hydrides, viz. HCl, H 2 S, and PH 3, as well as their dimers are studied at the ab initio MP2 level of theory and compared with the corresponding second-row hydrides. The dihydrogen bonds in the third-row dimers are weaker than those in the second-row ones, with the H···H distance being greater than 2.4 Å in some complexes and dimers. The existence of a bond between such neighboring hydrogens is established on the basis of topographical analysis of electron density. The energy of dihydrogen bonds in the dimers of third-row complexes is similar to the conventional weak H-bonds. The decomposition analysis of interaction energy of dimers shows predominance of an electrostatic component followed by polarization and charge transfer. The present study also suggests reinvestigation of the structure of phosphine−borane (BH 3 PH 3 ) using the IR and NMR techniques.