Silylenes ground state

Holthausen, M. C., Koch, W., Apeloig, Y., 1999, Theory Predicts Triplet Ground-State Organic Silylenes , J. Am. 

West and coworkers38 isolated a silylene (13) stable enough to be distilled at 85°C/0.1 Torr and reported 29Si (and other nuclei) NMR chemical shift (S + 78.3). The structure was also confirmed by X-ray crystallographic analysis (Figure 5) and quantum chemical calculations. It was suggested that the compound has an aromatic ground state. 

Silicon substituted silylenes attract the chemists interest for a special reason. According to ab initio calculations 85 137 176], substituents acting as cr-donors should induce a relatively large red-shift of the UV maximum or, in other words, the n(Si) —> 3p(Si) transition energy should be relatively small. Therefore these species are potential candidates for the long-sought triplet ground state silylenes, especially if this electronic effect is supported by a steric one. Nevertheless, reports on the matrix isolation of silicon substituted silylenes are comparatively scarce. 

Whereas much effort has been devoted to the search for triplet silylenes in the ground and excited states both experimentally189-194 and theoretically195 196, no successful result has been reported so far. Introduction of two bulky trialkylsilyl substituents to a silylene is predicted to lead to a ground state triplet196. 

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