The reduction of fac-[CrCl3((P3N3Bn)-N-Ph)], (1-(Cl-3)), ((P3N3Bn)-N-Ph = 1,5,9-tribenzyl-3,7,11-triphenyl-1,5,9-triaza-3,7,11-triphosphacyclododecane) with Mg in the presence of dmpe (dmpe = 1,2-bis(dimethylphosphino)ethane) affords the first example of a monodinitrogen Cr-0 complex, Cr(N-2)-(dmpe)((P3N3Bn)-N-Ph), (2(N-2)), containing a pentaphosphine coordination environment. 2(N-2) is supported by a unique facially coordinating 12-membered phosphorus macrocycle containing pendant amine groups in the second coordination sphere. Treatment of 2(N-2) at -78 degrees C with 1 equiv of [H(OEt2)(2)][B(C6F5)(4)] results in protonation of the metal center, generating the seven-coordinate Cr-II-N-2 hydride complex, [Cr(H)(N-2)(dmpe)((P3N3Bn)-N-Ph)][B(C6F5)(4)], [2(H)(N-2)](+). Treatment of 2(N-15(2)) with excess triflic acid at -50 degrees C afforded a trace amount of (NH4+)-N-15 from the reduction of the coordinated N-15(2) ligand (electrons originate from Cr). Electronic structure calculations were employed to evaluate the pK(a) values of three protonated sites of 2(N-2) (metal center, pendant amine, and N-2 ligand) and were used to predict the thermodynamically preferred Cr-NxHy intermediates in the N-2 reduction pathway for 2(N-2) and the recently published complex trans-[Cr(N-2)(2)((P4N4Bn)-N-Ph)] upon the addition of protons and electrons.