On the existence of MH(n) species with M = Al, Ga and n = 4, 5, 6. Computational study of structures, stabilities and bonding.

Based on second-order perturbation theory (MP2) predictions with large 6-311 + + G(3df, 3pd) basis set we have reviewed the possible structures and stabilities of a series of neutral MH(n)(M = Al, Ga; n = 4, 5, 6) species. For AlH4 and AlH5, our results confirm the previous theoretical findings, which indicate the dihydrogen C(s) complexes (2A') AlH2(H2) and (1A') AlH3(H2), respectively, as the lowest energy isomers. We found, similarly, C(s) (2A') GaH2(H2) and (1A') GaH3(H2) van der Waals complexes as the most stable species of the gallium analogues GaH4 and GaH5. The calculated H2 dissociation energies (D(e)) for AlH2(H2) and AlH3(H2) are of the order 1.8-2.5 kcalmol(-1), whereas this range of values for GaH2(H2) and GaH3(H2) is 1.4-1.8 kcalmol(-1) . Symmetry-adapted perturbation theory (SAPT) was used to analyze the interaction energies of these dihydrogen complexes (for n = 5) to determine why the Ga species show a smaller binding energy than the Al species. The SAPT partitioning of the interaction energy showed significant differences between AlH3(H2) and GaH3(H2), resulting from the much stronger "hydride" character of the aluminum species. The experimental observation of AlH2(H2) and AlH3(H2), and likely GaH3(H2), via low-temperature matrix isolation has been reported recently by Pullumbi et al. and Andrews et al., supporting the theoretical predictions. For n = 6, we found the degenerate C2(2A) and C(s)(2A') MH2(H2)2 "double H2" type van der Waals complexes as the lowest energy species for both M = Al and Ga.