Triplet carbenes Carbenium ions

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

The Walsh diagram provides a satisfying analysis of the electronic structure of carbenes, and the essential features of the system can be summarized by the simple representations shown in the margin. The triplet has two electrons in two very different orbitals, what we have called p and o-(out). We might expect its reactivity to be similar to that of radicals, and indeed this is the case. The singlet state is quite different. It contains a lone pair of electrons in an MO [a(out)] that is reminiscent of an sp hybrid. It also contains an empty p orbital, just like a simple carbenium ion. Its reactivity patterns should be quite different from radicals, and we will see that they are (Chapter 10). 

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