Hydride chemistry of group 13 is well-known due to its significant divergence from classical organic chemistry. While carbon serves as the keystone of organic chemistry, forming the basis with classical two-center bonds, adjacent element boron reveals a fascinating ground of non-classical multicenter bonds. However, as in other groups, we can observe a significant disproportionality in hydride chemistry research. Extensive investigations have been dedicated to lighter elements, while heavier elements, Tl, In, Ga, and even partly Al have received limited attention. The complexity of experimental exploration arises from the inherent sensitivity and instability of binary hydrides of these elements.
In this theoretical research, we cast light upon these hydrides, investigating the structural evolution of the simplest hydrides with stoichiometries X₂H_y and X₃H_z, where X=B, Al, Ga, In, Tl, y = 0 - 6, and z = 0 - 9. Our methodology includes a Coalescence Kick global minimum search algorithm, AdNDP bonding analysis in conjunction with QTAIM, ELF, and NICS calculations; we employed semi-empirical xtb method, as well as DFT and ab initio approaches.
Our investigation reveals distinctive structural features. While saturated stoichiometries display uniform structures, unsaturated stoichiometries demonstrate profound differences. Boron, as the lightest element, distinguishes itself with unique structural motifs unobservable for heavier analogues. The transition from boron to aluminum leads to a significant shift in properties, while transition from aluminum to thallium is more moderate. These heavy elements exhibit a tendency to retain a lone pair or unpaired electron (i.e., 1c bonding elements), while this kind of bonds are remarkably unstable in the case of boron, which prefers 2c or 3c bonding motifs. For example, thallium hydrides become unstable toward dehydrogenation after losing even one lone pair. Aluminum and gallium hydrides are characterized by a competition among 1c, 2c, and 3c bonding elements, yielding a large array of closely lying isomers. Additionally, observed trends indicate a preference for nonclassical 3c-2e bonds in boron, and a transition toward classical 2c-2e bonds in heavier elements on the example of X₂H_y stoichiometry. In sum, this systematic exploration yields insights into the hydride chemistry of Group 13 elements, serving as a framework for further theoretical and experimental studies.

Steve Scheiner

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