Si II -cation

Keywords silyliumylidene, Si(II)-cation, divalent silicon, pentamethylcyclopentadienylsilicon cation 

Silylene (S1H2) and its derivatives with small organic substituents R represent highly reactive species, which are key intermediates in many thermal and photochemical reactions. They have been characterized by matrix and dilute gas phase studies [1]. The last two decades have seen great progress in the kinetic and/or thermodynamic stabilization of divalent silicon compoimds and in the isolation of species persistent under ordinary conditions (Fig. 1) [2 - 6]. 

The even more reactive SiH cation ( silyliumylidene ) has been observed in the laboratory by discharge of SilL [7] and also in the solar spectrum. If there is at all a chance to prepare a persistent derivative, then this would probably be with pentamethylcyclopentadienyl as Ji-bonded substituent (Fig. 1). 

We investigated the MesCsSf cation by theoretical methods [b31yb/6-311g(d,p)]. The calculations reveal a pentagonal pyramidal structure with the MesCs unit being ri -bonded to the silicon atom (see Fig. 3). The H NMR data are in agreement with the calculated structure. 

Experiments to isolate the compound MesCsSr B(C6Fs)4 from the reaction of (MesC5)2Si with 

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