Dihydrogen bonding: donor-acceptor bonding (AH...HX) versus the H2 molecule (A-H2-X)

Dihydrogen bond or H2 molecule? The central H-H bond in linear H4 can exist in two qualitatively different bonding modes corresponding to two different electronic states, namely a donor–acceptor dihydrogen bond (DHB) and a central H2 molecule with an electron‐pair bond (see figure). This insight evolves from Kohn–Sham density functional analysis and it is further applied here to understand the bonding in more realistic model systems.
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Dihydrogen bonds (DHBs) play a role in, among others, crystal packing, organometallic reaction mechanisms, and potential hydrogen‐storage materials. In this work we have analyzed the central H-H bond in linear H4, LiH⋅⋅⋅HX, BH4−⋅⋅⋅HX, and AlH4−⋅⋅⋅HX complexes with various X by using the quantitative molecular orbital model contained in Kohn–Sham density functional theory at the BP86/TZ2P level of theory. First, we address the questions of if and how one can distinguish, in principle, between a H⋅⋅⋅H donor–acceptor DHB and the formation of an H2 molecule by using the simple H4 model system. The results of these analyses have been used to gain an understanding of the bonding in more realistic model systems (some of which have been studied experimentally), and how this differs from the bonding in H4.