Silylenes singlet/triplet states

STRUCTURES OF SILYLENES AND GERMYLENES 3.1. Singlet and Triplet States... 

Kalcher and Sax also examined the lowest singlet and triplet states of cyc/o-(H2Si)2Si , a three-membered cyclic silylene. The Si—Si—Si angles at the divalent silicon are constrained to be much smaller than those in the open-chain (H3Si)2Si for both the singlet (56.2° versus 95.9°) and the triplet (62.8° versus 125.4°) states. It is surprising that AZ st is nearly the same in the cyclic (8.8 kcal mol 1) and open-chain (8.3 kcal mol 1) silylenes208. 

The existence and the relatively high stability of the silylene can be explained as a result of the singlet state. If the silylene were in a triplet state, polymerization would proceed faster and the formation of addition products would not be preferred. Herzberg postulated a singlet state, Nefedov433) found poor reactivity in all electrophilic reactions, and the difference in the reactivity in comparison to organic alkylcarbenes makes the triplet state unlikely. 

The prototypical carbene, CH2, and other simple alkyl and aryl carbenes have triplet ground states, with two unpaired electrons. In striking contrast, all silylenes known to date have singlet ground states, with the two nonbonding electrons paired. This difference is of the greatest importance for the properties, spectra, and chemical behavior of silylenes. 

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