Motivated by the potential high-temperature superconductivity of "chemically precompressed" hydrogen-rich compounds, the high pressure phases of the ionic salt BaReH9 are explored theoretically. We find that the compound adapts to compression not only by structural distortions or increase of coordination number, but also through evolution of discrete H-2 units which fill up interspace gaps. This last structural change is associated with a dramatic lowering of metallization pressure, so that BaReH9 can be expected to turn metallic at the onset of the H-2-containing phase (51 GPa).