[Ar2N3]Mo(N)(O-t-Bu) (1), which contains the conformationally rigid pyridine-based diamido ligand [2,6-(ArNCH2)(2)NC5H3](2-) (Ar = 2,6-diisopropylphenyl), is a catalyst for the reduction of dinitrogen with protons and electrons. Various acids have been added in order to explore where and how the first proton adds to the complex. The addition of adamantol to 1 produces a five-coordinate bis(adamantoxide), [HAr2N3]Mo(N)(OAd)2 (2a), in which one of the amido nitrogens in the ligand has been protonated and the resulting aniline nitrogen in the [HAr2N3](-) ligand is not bound to the metal. The addition of [Ph2N1-12][0Tf] to 1 produces {[HAr2N3]Mo(N)(O-t-Bu)}(OTf) (3), in which an amido nitrogen has been protonated, but the aniline in the [1-1Ar(2)N(3)]- ligand remains bound to the metal. Last, the addition of (2,6-lutidinium)BArr4 (BAr84 = {B (3,5- (CF3)2C(6)H(3))(4)}(-)) to 1 yields {[Ar2N3]Mo (N)(LutH) (0- t-Bu)}BArF4, in which Lut1-1+ is hydrogen-bonded to the nitride in the solid state and in dichloromethane with Keg = 412 94 and AG = 3.6 0.8 kcal at 22 degrees C. A similar hydrogen-bonded adduct was formed through the addition of (2-methylpyridinium)BArF4 to 1, but the addition of (pyridinium)BArF, to 1 leads to the formation {[I-1Ar(2)N(3)]Mo(N)(O-t-Bu)}(BArF4), in which the amide nitrogen has been protonated. The addition of cobaltocene to 3 or {[Ar2N3]Mo(N)(LutH)(O-t-Bu)}(BAr84) leads only to the re-formation of 1. X-ray structural studies were carried out on 2a, 3, and {[Ar2N3]Mo(N)(LutH)(0-t-Bu)}(BArF4).