Addition of Monodentate Charge-Neutral Nucleophiles

Basically, halocarbons should be capable of entering the silicon coordination sphere with a lone pair of one of their halogen atoms, as has been shown for the 1,2-dichlorobenzene solvate of a silicenium ion (/Pr3Si. .. Cl o-CeHiCl) [76]. To the best of our knowledge, this kind of solvate formation has not been proven relevant in hypercoordinated silicon chemistry yet. 

Ethers were shown to form adducts with silicon compounds (e.g., 8,9 and 10), but literature reports on crystallographic evidence are limited to few examples [77-80]. In the same manner, alcohols and water should be capable of forming related adducts. Surprisingly, no example of an alcohol solvate can be found in the CSD [81], but adduct formation with alcohols has already been reported [82]. Water, however, has already been demonstrated by X-ray crystallography to act as a ligand in hypercoordinated Si complexes (e.g., 11 and 12) [82-85]. 

So far, no crystallographic evidence for adducts of silanes with aldehydes, ketones, esters, or acyl halides has been reported [81]. Dimethylformamide [86-88] and tetramethylurea [89], however, are known to enter the silicon coordination sphere (e.g., in 13 and 14, respectively). In a similar manner amine-M-oxides (e.g., in 15) [90], phosphine oxides (e.g., in 16 and 17) [90,91], and phosphoric amides [92-94] form hypercoordinated Si complexes. Although dimethyl sulfoxide (DMSO) increases the silicon coordination number (as shown Si NMR spectroscopy) [49], crystallographic evidence for a siUcmi complex with DMSO Ugand(s) is still lacking [81]. 

Only recently, nitriles were shown to increase the silicon coordination number to up to five in cationic complexes (18) [95], even though nitriles had already been shown to form adducts with sihcenium ions, thus functioning as a donor moiety in a tetrahedral Si coordination sphere [96]. Amines are scarcely encountered in silicon coordination compounds, only few examples of crystallographically evidenced silicon complexes with monodentate amines have been reported so far, which include the adduct SiF4(NH3)2 [97-100], Imines, however, are well known to add to various halosilanes. Especially IV-heterocycles with imine functionahty, such as pyridines [101-103], imidazoles [104, 105], pyrazoles [106], and related compounds, can be found as ligands in various silicon complexes (e.g., in 19 and 20). 

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