Four-electron reduction of dinitrogen has been achieved in the reaction of N-2 With [{p-Bu-t-calix-[4](O)(4)}(2)Nb-2(mu-M)(2)] (M = Li, 3; M = Na, 4; M = K, 5), which contain a very reactive Nb=Nb unit [Nb=Nb = 2.659(1) Angstrom; 4], in THF. The reaction leads to the dinuclear complexes [{p-Bu-t-calix[4]-(O)(4)}(2)(Nb=N-N=Nb](2-) M-2(+) (M=Li, 6; M = Na, 7; M = K, 8) [Nb=N = 1.747(12) Angstrom, N-N = 1.390(17) Angstrom; 7], containing the hydrazido [N-2](4-) anion. Two-electron reduction of the hydrazido tetraanion in 7 using sodium metal leads to the nitrido species 11, [{p-Bu-t-calix[4]-(O)(4)}(2)Nb-2(mu-N)(2)(mu-Na)(2)Na-2], which in solution is in equilibrium with the monomeric form 18, [{p-Bu-t-calix[4]-(O)(4)}Nb equivalent to N-NaSn]. Two distinct pathways have been identified as a function of the solvent used (THF or DME), In the case of DME, the key intermediate 12 has been intercepted as green crystals: The formation of 12, [{p-Bu-t-calix[4]-(O)(4)}(2)Nb-2(mu-eta(2):eta(2)-N-2)(mu-Na)(3)Na], results from the reduction of Nb(V) to Nb(IV), which temporarily stores the two electrons in an Nb-Nb bond [N-N = 1.403(8) Angstrom, Nb-Nb, 2.635(1) Angstrom]. Complex 12 contains a mu-eta 2:eta(2) side-on N-2 group, bonded between two niobiums and preorganized to be transformed into two nitrido anions. Heating in pyridine (90 degrees C) converts 12 to 11, with simultaneous cleavage of the Nb-Nb and N-N bonds, the heating causing two-electron transfer from the Nb-Nb to the N-N bond. The reaction of 4 with N-2, carried out in toluene instead of THF or DME, gives a nitrido complex, [{p-Bu-t-calix[4]-(O)(4)}(3)Nb-3(mu-N)(2)Na-3(THF)(2)], 15, which can be formed equally well from the reaction of 4a with 7a carried out in a 1:2 molar ratio in toluene. Complex 15 breaks up in CHCl3 or toluene, upon addition of TMEDA, into [{p-Bu-t-calix[4]-(O)(4)}(2)Nb-2(mu-N)(2){mu-Na-2-(TMEDA)(2)}Na-2], 16, and [{p-Bu-t-calix[4]-(O)(4)}(2)Nb-2(mu-N)][Na(TMEDA)(2)], 17.