A potentially useful trianionic ligand for the reduction of dinitrogen catalytically by molybdenum complexes is one in which one of the arms in a [(RNCH2CH2)(3)N](3-) ligand is replaced by a 2-mesitylpyrrolyl-a-methyl arm, that is, KRNCH2CH2)(2)NCH2(2-MesitylPyrrolyl)(3-) (R = C6F6, 3,5-Me2C6H3, or 3,5-t-Bu2C6H3). Compounds have been prepared that contain the ligand in which R = C6F5 ([C6F5N)(2)Pyr](3-)); they include [(C6F5N)(2)Pyr]Mo(NMe2), [(C6F5N)(2)Pr]MoCl, [(C6F5N)(2)Pyr]MoOTf, and [(C6F5N)(2)Pyr]MoN. Compounds that contain the ligand in which R = 3,6-t-Bu2C6H3 ([Art-BuN)(2)Pyr](3-)) include {[(Art-BuN)(2-)Pyr]Mo(N-2)}Na(15-crown-5), {[(Art-BuN)(2)Pyr]Mo(N-2)}[NBu4], [(Art-BuN)(2)Pyr]Mo(N-2) (nu(NN) = 2012 cm(-1) in C6D6), [(Art-BuN)(2)Pyr]Mo(NH3)}BR4, and [(Art-BuN)(2)Pyr]Mo(CO). X-ray studies are reported for [(C6F5N)(2)Pyr]Mo(NMe2), [(C6F5N)(2)Pyr]MoCl, and [(Art-BuN)(2)Pyr]MoN. The [(Art-BuN)(2)Pyr]Mo(N-2)(0/-) reversible couple is found at -1.96 V (in PhF versus Cp2Fe+/0), but the [(Art-BuN)(2)Pyr]Mo(N-2)(+/0) couple is irreversible. Reduction of {[(Art-BuN)(2)Pyr]Mo(NH3)}BPh4 under Ar at approximately -1.68 V at a scan rate of 900 mV/s is not reversible. Ammonia in [(Art-BuN)(2)Pyr]Mo(NH3) can be substituted for dinitrogen in about 2 h if 10 equiv of BPh3 are present to trap the ammonia that is released. [(Art-BuN)(2)Pyr]Mo N=NH is a key intermediate in the proposed catalytic reduction of dinitrogen that could not be prepared. Dinitrogen exchange studies in L(AuN)(2)Pyr]Mo(N-2) suggest that steric hindrance by the ligand may be insufficient to protect decomposition of KArl. `uN)2Pyr]Mo-N=NH through a variety of pathways. Three attempts to reduce dinitrogen catalytically with [(Art-BuN)(2)Pyr]Mo(N) as a "catalyst" yielded an average of 1.02 +/- 0.12 equiv of NH3.