Quantitative Assessment of B-B-B, B-Hb-B, and B-Ht Bonds: From BH3 to B12H122-

We report the thermodynamic stabilities and the intrinsic strengths of three-center-two-electron B-B-B and B-H-b-B bonds (Hb: bridging hydrogen), and two-center-two-electron B-H-t bonds (Ht: terminal hydrogen) which can be served as a new, effective tool to determine the decisive role of the intermediates of hydrogenation/dehydrogenation reactions of borohydride. The calculated heats of formation were obtained with the G4 composite method and the intrinsic strengths of B-B-B, B-H-b-B, and B-H-t bonds were derived from local stretching force constants obtained at the B3LYP-D2/cc-pVTZ level of theory for 21 boron-hydrogen compounds, including 19 intermediates. The Quantum Theory of Atoms in Molecules (QTAIM) was used to deepen the inside into the nature of B-B-B, B-H-b-B, and B-H-t bonds. We found that all of the experimentally identified intermediates hindering the reversibility of the decomposition reactions are thermodynamically stable and possess strong B-B-B, B-H-b-B, and B-H-t bonds. This proves that thermodynamic data and intrinsic B-B-B, B-H-b-B, and B-H-t bond strengths form a new, effective tool to characterize new (potential) intermediates and to predict their role for the reversibility of the hydrogenation/dehydrogenation reactions.