Silylenes singlet ground state

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. 

Silylenes (R2St). Silylenes, which are considerably more stable than the analogous carbenes, have a singlet ground state. With few exceptions, their reactions are analogous to those of carbenes, Silylenes are generated by a eliminations,... 

It has been theoretically and experimentally well established that silylenes have a singlet ground state [1]. Such species posses a free electron pair in a o-orbital and an empty orbital of Jt-symmetry therefore, they are a priori ambiphilic compounds, which can react either as an electrophile or as a nucleophile towards appropriate substrates. However, most silylenes have revealed a distinctive "electrophilic character". Dimethylsilyene, e g., adds to olefins and alkynes in the gas phase via a rate-controlling step that is accelerated by electron-donating substituents [2] these experimental results are in good agreement with a theoretical study of the reaction of SiH2 with ethylene, which shows that this cycloaddition proceeds via an initial electrophilic phase in which the silylene LUMO interacts with the 7t-electron system of the double bond [3]. Up to now, only some stable silylenes, such as recently described 1 [4] or silicocene 2 [5] have shown nucleophilic reactivity. 

Thermolysis of l,l-diadamantyl-2,3-dialkylsilacyclopropanes generated diadamantylsilylene, which was then trapped with cis- and /ra/M-2-butene and -3-hexene. The complete stereospecificity of silacyclopropane decomposition and silylene-alkene addition suggests an intermediate silylene in a singlet ground state (Equations (43) and (44)) <91JA1281). 

The various computational results agree well with these data. For silylene the situation is opposite to that for methylene in fact, a recent experimental nuclear recoil study seems to establish for SiH2 the existence of a singlet ground state. However, in this case, the energy separation with the Bj state is not known, since, in the various spectroscopic studies reported for SiH2, no transitions involving triplet electronic states have been identified. 

In 2006 Roesky et al. reported the base-stabilized chloro silylene, [PhC(NtBu)2]SiCl (1), with a three-coordinate silicon atom. The silicon atom in this compound exhibits a singlet ground state (1). Subsequently there was a report on the three-coordinate silylene with a tert-butoxide substituent, [PhC(NtBu)2]SiOtBu (2) (2). The silylenes 1 and 2 can act as Lewis bases and herein we describe the reactions of 1 and 2 with different Lewis acids. 

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