Dinitrogen is reduced to ammonia by the molybdenum complex of L = [HIPTN3N](3-) [Mo; HIPT = 3,5-(2,4,6-iPr(3)C(6)H(2))(2)C6H3]. The mechanism by which this occurs involves the stepwise addition of proton/electron pairs, but how the first pair converts MoN2 to MoN=NH remains uncertain. The first proton of reduction might bind either at N-beta of N-2 or at one of the three amido nitrogen (N-am) ligands. Treatment of MoCO with [2,4,6-Me3C5H3N]BAr'(4) [Ar' = 2,3-(CF3)(2)C6H3] in the absence of reductant generates HMoCO+, whose electron paramagnetic resonance spectrum has greatly reduced g anisotropy relative to MoCO. H-2 Mims pulsed electron nuclear double-resonance spectroscopy of (HMoCO+)-H-2 shows a signal that simulations show to have a hyperfine tensor with an isotropic coupling, a(iso)(H-2) = -0.22 MHz, and a roughly dipolar anisotropic interaction, T(H-2) = [-0.48, -0.93, 1.42] MHz. The simulations show that the deuteron is bound to N-am, near the Mo equatorial plane. not along the normal, and at a distance of 2.6 angstrom from Mo, which is nearly identical with the (N-am)H-2(+)-Mo distance predicted by density functional theory computations.