Heats of hydrogenation of compounds featuring main group elements and with the potential for multiply bonding

Reaction enthalpies are calculated for the hydrogenation reactions of main group hydrides with the potential for multiple bonding, and thus the unsaturated character of these species is determined. In addition to the global minimum structures, which leave in some cases no hope for even a single E-E bond (E=Group 13, 14, or 15 element), calculations are also performed for geometries with maximum potential for multiple bonding. The trends down the groups and the periods are established. Interpretations have to take several factors into account. These factors sometimes work hand in hand but also against each other. We also include in our survey the species [HGaGaH]2- as a free anion and Na2[HGaGaH] as well as their hydrogenation products [H2GaGaH2]2- and Na2[H2GaGaH2]2-. The results show that the presence of the Na+ ions has a significant impact on their chemistry, and thus suggests that they are involved to a large extent in the bonding. Our results indicate that the compounds should be described as cluster compounds.