[HIPTN3N]Mo(N-2) (MoN2) ([HIPTN3N](3-)=[(HIPTNCH2CH2)(3)N](3-) where HIPT=3,5-(2,4,6-i-Pr3C6H2)(2)C6H3) reacts with dihydrogen slowly (days) at 22 degrees C to yield [HIPTN3N]MoH2 (MoH2), a compound whose properties are most consistent with it being dihydrogen complex of Mo(III). The intermediate in the slow reaction between MoN2 and H-2 is proposed to be [HIPTN3N]Mo (Mo). In contrast, MoN2, MoNH3, and MoH2 are interconverted rapidly in the presence of H-2, N-2, and NH3, and MoH2 is the lowest energy of the three Mo compounds. Catalytic runs with MoH2 as a catalyst suggest that it is competent for reduction of N-2 with protons and electrons under standard conditions. [HIPTN3N]MoH2 reacts rapidly with HD to yield a mixture of [HIPTN3N]MoH2, [HIPTN3N]MoD2, and [HIPTN3N]MoHD, and rapidly catalyzes H/D exchange between H-2 and D-2. MoH2 reacts readily with ethylene, PMe3, and CO to yield monoadducts. Reduction of dinitrogen to ammonia in the presence of 32 equiv of added hydrogen (vs Mo) is not catalytic, consistent with dihydrogen being an inhibitor of dinitrogen reduction.