Singlet carbene bond angles

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. 

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. - ... 

The chemical and physical properties of DMFL contrast most sharply with those of FL. The geometrical features, in particular the carbene-carbon bond angle, of these two carbenes are expected to be identical. The most important difference between DMFL and FL is that the spin multiplicities of the lowest electronic states appear to have been inverted. The experiments indicate that the ground-state of DMFL is the singlet. This conclusion is outlined in the reactions shown in Scheme 4. 

PCH bond angle is broader (133.9°) than that of the singlet state (123.5°), as classically predicted for most carbenes.17... 

Analysis of the product distributions arising from both sensitized and non-sensitized irradiation of 2-allyloxyphenyldiazo species (8) showed that the C—H insertion product and much of the cyclopropanation arise from the triplet carbene.16 For the singlet carbene, intermolecular 0—H insertion with methanol is about 50 tunes faster than intramolecular addition to the double bond, hi this system, intramolecular reactions and intersystem crossing of the triplet carbene proceed at similar rates, hi the closely related indanyl system (9), the smaller RCR angle stabilizes the singlet state relative to the triplet and the intramolecular reactivity is dominated by the singlet state.17... 

FIGURE 5. Possible structures of phosphinocarbenes and phosphino(silyl)carbenes and the calculated structures for singlet and triplet species bond lengths (A) and bond angles (°) are given... 

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). 

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