Activation of dinitrogen (N-2) by well-defined metal hydrides is of much interest and importance, but studies in this area have remained limited to date. We report here N-2 activation and hydrogenation by C5Me4SiMe3 -ligated di- and trinuclear chromium polyhydride complexes. Hydrogenolysis of [Cp'Cr(mu-Me)(2)CrCp'] (Cp' = C5Me4SiMe3) (1) with H-2 in a dilute hexane solution under N-2-free conditions affords the dichromium dihydride complex [Cp'Cr(mu-H)(2)CrCp'] (2), while hydrogenolysis of 1 in a concentrated solution or without solvent provides the trinuclear chromium tetrahydride complex [(Cp'Cr)(3)(mu(3)-H)(mu-H)(3)] (3). When the reaction is carried out in the presence of N-2 in a dilute hexane solution, the tetranuclear diimide/dihydride complex [(Cp'Cr)(4)(mu(3)-NH)(2)(mu(3)-H)(2)] (4) is formed via N-N bond cleavage and N-H bond formation. The reaction of 2 with N-2 at room temperature gives the tetranuclear imide/nitride/dihydride complex [(Cp'Cr)(3)(C5Me(CH2)SiMe3)Cr(mu(3)-NH)(mu(3)-N)(mu-H)(2)] (5) via N-2 cleavage and hydrogenation and C-H bond activation of a Cp methyl group. At -30 degrees C, the reaction of 2 with N-2 affords the dinitride intermediate [(Cp'Cr)(4)(mu(3)-N)(2)(mu(3)-H)(2)] (6), which is quantitatively transformed to 5 at room temperature. Complex 5 reversibly converts to the stereoisomer 5'. The hydrogenation of a mixture of 5 and 5' with H-2 affords 4. The reaction of 3 with N(2 )proceeds at 100 degrees C to afford [(Cp'Cr)(3)(mu(3)-NH)(2)] (7). This transformation has also been investigated by DFT calculations. Both experimental and computational studies suggest that N-2 incorporation into the chromium hydride cluster is involved in the rate-determining step. This work represents the first example of N-2 cleavage and hydrogenation by well-defined chromium hydride complexes.