Cohesion of BaReH$_9$ and BaMnH$_9$: Density Functional Calculations and Prediction of (MnH$_9$)$^{2-}$ Salts*

Density functional calculations are used to calculate the structural and electronic properties of BaReH$_9$ and to analyze the bonding in this compound. This compound has an exceptionally high H to metal ratio of 4.5. The high coordination of Re in BaReH$_9$ is due to bonding between Re 5d states and states of d-like symmetry formed from combinations of H s orbitals in the H$_9$ cage. This explains the structure of the material, its short bond lengths and other physical properties, such as the high band gap. We compare with results for hypothetical BaMnH$_9 $, which we find to have similar bonding and cohesion to the Re compound. This suggests that it may be possible to synthesize (MnH$_9$)$^{2-}$ salts. Depending on the particular cation, such salts may have exceptionally high hydrogen contents, in excess of 10 weight \%.