The rates of hydrogenation of the N-2 ligand in the side-on bound dinitrogen compounds, [(eta(5)-C5Me4H)(2)Zr](2)(mu(2),eta(2),eta(2)-N-2) and [(eta(5)-C5Me5)(eta(5)-C5H2-1,2-Me-2-4-R)Zr](2)(mu(2),eta(2),eta(2)-N-2) (R = Me, Ph), to afford the corresponding hydrido zirconocene diazenido complexes have been measured by electronic spectroscopy. Determination of the rate law for the hydrogenation of [(eta(5)-C5Me5) (eta(5)-C5H2-1,2,4-Me-3)Zr](2)(mu(2),eta 2,eta(2-)N(2)) establishes an overall second-order reaction, first order with respect to each reagent. These data, in combination with a normal, primary kinetic isotope effect of 2.2(1) for H-2 versus D-2 addition, establish the first H-2 addition as the rate-determining step in N-2 hydrogenation. Kinetic isotope effects of similar direction and magnitude have also been measured for hydrogenation (deuteration) of the two other zirconocene dinitrogen complexes. Measuring the rate constants for the hydrogenation of [(eta(5)-C5Me5)(eta(5)-C5H2-1,2,4-Me-3)Zr](2)(mu(2), eta(2),eta(2)-N-2) over a 40 degrees C temperature range provided activation parameters of Delta H-double dagger = 8.4(8) kcal/mol and Delta S-double dagger = -33(4) eu. The entropy of activation is consistent with an ordered four-centered transition structure, where H-2 undergoes formal 1,2-addition to a zirconium-nitrogen bond with considerable multiple bond character. Support for this hypothesis stems from the observation of N-2 functionalization by C-H activation of a cyclopentaclienyl methyl substituent in the mixed ring dinitrogen complexes, [(eta(5)-C5Me5)-(eta(5)-C5H2-1 2-Me-2-4-R)Zr](2) (mu(2),eta(2,)eta(2-)N(2)) (R = Me, Ph), to afford cyclometalated zirconocene diazenido derivatives.