Doubly occupied orbitals, triplet carbenes

We can think of the singlet state of a free carbene, in terms of localized hybrid orbitals, as a bent molecule (6) harboring three sp2 orbitals (one of which is doubly occupied and non-bonding) and an empty, perpendicularly-situated 2p orbital. A triplet carbene (really a diradical) would be a less bent species (7) with two singly-occupied, orthogonal 2p orbitals and two sp orbitals that are involved in bonding to substituents. 

It is informative to compare methylene and nitrene in terms of simple molecular orbital theory and then to extend this comparison to their aromatic derivatives phenyl carbene and phenyl nitrene. Hoffman et al. [28] have described the molecular orbitals of CH2 in the following way. Methylene has two nonbonding molecular orbitals, one is an in-plane, a type, hybrid orbital, the other is an out-of-plane pure p type n orbital. Singlet methylene has a bond angle of 105° and the in-plane, sigma, orbital is doubly occupied. Triplet methylene has both a singly occupied n and a orbital and a bond angle of 135°. 

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