The reaction between carbene 2 R (R = CF 3 , CN , H, M e, OM e, P h, tBu) and N 2 is presented in Figure . Here, 2A R is the pre-complex. The activation energies for the formation of 2B R from 2A R follow the trend CN < CF 3 < P h < tBu < H < M e < OM e which is very similar to the trend observed for carbene 1 R except for M e, tBu and H. The order of the stabilities of activated complexes shows a different trend; CN > CF 3 > P h > OM e > H > M e > tBu. The change in the order of stability may be due to the presence of the bridge at the carbene backbone. Here the steric factor of tBu reduces its stability. Additionally, due to the steric hindrance, the observed activation energies are higher for all substitutions corresponding to their 1 R analogue. The highest and the lowest activation energies for these carbenes are 41.0 kcal/mol and 30.1 kcal/mol. The highest activation energy is for the OM e substituents and the lowest activation energy is for the CN substituents. Interestingly, for the 1 R carbene, these two substituents show the highest and the lowest activation energy respectively.