Singlet carbene analog structures

There are two possibilities to describe the structure of these singlet carbene analogs ... 

The aromatic stabilization of the planar bond configuration may qualitatively be interpreted in terms of a model of the electronic structure of pseudo-olefines (87JA5303), in which the planar structure (249) is viewed as resulting from the interaction between two analogs of carbene XH2 in the triplet state, whereas the frans-pyramidalized structure (250) results from that between two carbene-type units XH2 in the singlet state. 

The nitrogen analogs of carbenes are called nitrenes. As with carbenes, both singlet and triplet electronic states are possible. The triplet state is usually the ground state for simple structures, but either species can be involved in reactions. The most common... 

The structural analogy between carbenes and their isoelec-tronic carbocations is also observed for singlet cyclopentylidene (8), which has a C2 symmetric structure (Figure 8) similar to the cyclopentyl cation. " Similarly to 2, two CH bonds stabilize the carbene center in 8. As the system is incorporated into a five-membered ring, the CCC angle at the carbene center is smaller than in 2 (104° vs. 112°). Thus, the. singlet. state... 

---