Laser-ablated Zn and Cd atoms react with molecular hydrogens upon condensation at 4.5 K to form the metal dihydride molecules in solid hydrogen and neon. Ultraviolet irradiation increases these absorptions by an order of magnitude. Natural zinc- and cadmium isotopic splittings are resolved in solid p-H-2, o-D-2, and HD. Weaker absorptions are assigned to the linear HMMH molecules and to metal cluster monohydrides with the assistance of DFT calculations. Upon sublimation of the hydrogen or neon matrix, the metal dihydride molecules polymerize into solid hydrides with broad absorption bands characteristic of hydrogen-bridge bonding.