  The structures of compressed rubidium polyhydrides, RbHn with n>1, and their
evolution under pressure are studied using density functional theory
calculations. These phases, which start to stabilize at only P=2 GPa, consist
of Rb+ cations and one or more of the following species: H- anions, H2
molecules, and H3- units. The latter motif, the simplest example of a
three-center four-electron bond, is found in the most stable structures, RbH5
and RbH3. Pressure induces the symmetrization of H3-. The thermodynamically
most stable polyhydrides metallize above 200 GPa. At the highest pressures
studied, our evolutionary searches find an RbH6 phase which contains polymeric
(H3-)$_\infty$ chains that show signs of one-dimensional liquid-like behavior.
