The role of LixHy(CsxHy) molecules, which can be formed from seeding an alkali into a hydrogen plasma, is examined throughab initiocalculations of the structure and stability of such species. The simplest alkali hydride, LiH, supports a bound anion for all internuclear separations. Larger structures, such as Li2H2and Li3H, do not support stable anions but exhibit thermodynamic stability as gas phase molecules. Their possible roles in dissociative attachment to form H−and/or Li−is examined.