Bond angle stable singlet carbenes

The simplest carbene CH2 ( methylene, Figure 3.14, center) is a bent molecule with an H,C,H bond angle of 135° and has a triplet ground state. The singlet CH2 is less stable by 8 kcal/mol. Its free electron pair occupies the sp AO (because in this orbital it is nearer to the nucleus and therefore more stabilized than in the 2pz AO), and the H,C,H bond angle amounts to 105° (two electrons in the sp AO as compared to one electron in the sp AO of triplet CH2 cf. the discussion of VSEPR theory in Section 1.1.1). 

Spectroscopic investigations of a number of carbenes of differing structures have shown that they fall broadly into two groups (1) those (which you will learn to call triplets ) that ESR spectroscopy demonstrates have unpaired electrons and whose bond angles are 130-150° and (2) those (like the stable crystalline carbene above which you will learn to call a singlet ) that have bond angles of 100-110° but cannot be observed by ESR. Many carbenes, like CH2 itself, can be found in either style, though one may be more common. 

In organorhodium compounds, reaction with diazomethane [50-52], and rearrangement with acetylene compounds, etc. [53-62], afford a variety of carbene complexes. The carbenes are a general term for a chargeless divalent carbon (X(Y)C ) and the triplet state bond angle is ca. 150°) is more stable by ca. lOkcal/moI than the singlet state (bond angle ca. 100°). The metal complexes of a carbene are represented as M=CRR and when R and R are C or H, it is called an alky-lidene complex. The carbene is labile, but the carbene complexes are relatively stable. 

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