Steric effects triplet carbenes

The stability of azole carbenes can be attributed to electronic factors which operate in both the Tran d CT-frameworks (92JA5530). In the TT-framework, electron donation into the carbene out-of-plane p-orbital by the electron-rich system moderates the typical electrophilic reactivity of carbenes. In the o-framework, additional stability for the carbene electron pair may be gained from the o-electron-withdrawal effects on the carbene center by the more electronegative nitrogens, which moderates the carbene nucleophilic reactivity. The combination of these a- and TT-effects serves to increase the singlet-triplet gap and stabilize the singlet carbene over the more reactive triplet state. For carbenes with bulky substituents (tert-butyl, 1-adamantyl, etc.) steric effects provide additional stabilization. 

The chemistry found for those carbenes is thus in sharp contrast with that found for other diarylearbenes and is interpreted in terms of steric effects, which kinetically stabilize triplet states to a significant degree. 

Stabilization of triplet carbenes is shown to be better accomplished by steric protection than by electronic effect. However, the latter strategy can be used to stabilize singlet carbenes. ... 

On the other hand, kinetic stabilization (steric protection) is more effective in stabilizing the triplet counterpart because the introduction of bulky groups around the carbene center results in an increase in the carbene bond angle and therefore must make AGjt larger. In this case, the triplet state is thermodynamically stabilized with respect to the singlet. - ... 

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