Trapping of H₂ ^- in aluminum hydride, Al₄H₁₄

Ever since our first experimental and computational identification of Al₄H₆ as a boron analog [X. Li et al., Science 315, 356 (2007)], studies on aluminum hydrides unveiled a richer pattern of structural motifs. These include aluminum-rich hydrides, which follow shell closing electron counting models; stoichiometric clusters (called baby crystals), which structurally correspond to the bulk alane; and more. In this regard, a mass spectral identification of unusually high intense peak of Al₄H₁₄ ^-, which has two hydrogen atoms beyond stoichiometry, has remained mostly unresolved [X. Li et al., J. Chem. Phys. 132, 241103 (2010)]. In this Communication, with the help of global minima methods and density functional theory-based calculations, we identify the lowest energy bound structure with a unique Al-H-H-Al bonding. Our electronic structural analysis reveals that two Al₂H₆ units trap a transient, metastable H₂ ^-. In other words, three stable molecules, two Al₂H₆ and an H₂, are held together by a single electron. Our studies provide a pathway to stabilize transient species by making them part of a more extensive system.