Laser-ablated Sc, Y, and La atoms react with molecular hydrogen upon condensation in excess argon, neon, and deuterium to produce the metal dihydride molecules and dihydrogen complexes MH(2) and (H(2))MH(2). The homoleptic tetrahydrometalate anions ScH(4)(-), YH(4)(-), and LaH(4)(-) are formed by electron capture and identified by isotopic substitution (D(2), HD, and H(2) + D(2) mixtures). Doping with CCl(4) to serve as an electron trap virtually eliminates the anion bands, and further supports the anion identifications. The observed vibrational frequencies are in agreement with the results of density functional theory calculations, which predict electron affinities in the 2.8-2.4 eV range for the (H(2))ScH(2), (H(2))YH(2), and (H(2))LaH(2) complexes, and indicate high stability for the MH(4)(-) (M = Sc, La, Y) anions and suggest the promise of synthesis on a larger scale for use as reducing agents.