Silyl anions structure

Unlike silyl radicals and silylium ions that have been elusive for a long time, silyl anions (silanides) are well known since the pioneering work of Gilman etal. in the 1960s. Two literature reviews on silyl anions by Lickiss and Smith as well as Tamao appeared in 1995. Many structural elucidations and reactivity studies of silyl anions have been... 

The chemistry of metalated organosilicon compounds has been the subject of several reviews1, the most recent ones by Lickiss and Smith13 and Tamao and Kawachilb, which cover the literature up to the year 1994. This chapter will now take into account the developments in the chemistry of metalated silanes up to the middle of 1996 however, for completeness there will be some overlap with former reviews. The emphasis of this review is on the synthesis and structure of these metalated silanes. However, some examples of their utilization for synthetic purposes will also be given where appropriate. For more information about synthetic applications of silyl anions the reader is referred to some leading references in this fieldla,b h k. 

Studies of the structure of peralkylated silyl anions in solution by spectroscopic methods such as IR, UV or NMR spectroscopy are exceedingly rare, but some information on their structure in the solid is available. 

IR, UV and, most importantly, NMR spectroscopy have proven to be excellent tools for elucidation of the molecular and electronic structures of silyl anions. Whereas 13C NMR data of metalated alkylsilanes are quite rare, the wealth of 13C NMR data which are available for metalated arylsilanes allows one to draw important conclusions concerning the question of charge delocalization in these compounds (Table 2)36,47. 

Optimization of geometry for some silylenium cations was made (22,24). Such calculations predict that silylenium ions will adopt a planar structure in contrast to silyl anions, which are predicted to be pyramidal. It should be noted that theoretical studies also indicate a high ability of silicon to accommodate negative charge. The parent silyl anion H3Si was calculated to be more stable than its carbon analog by about 50 kcal/mol (24). This implies a remarkable affinity of silylenium ions toward electron-rich species. 

Few structural data are available on type (B) silyl anions. The SiH3 anion may be prepared from SiH4 and the alkali metals in the solvents CH3OCH2CH2OCH3 or OP[N(CH3)2]3. While the vibrational spectrum of SiH3 has not yet been obtained, the nmr in CH3OCH2CH2OCH3 solutions has been reported 108). 

Some years ago Hengge et al. published a feasible synthesis of KSi6Men (1) and did basic research on the reactivity of this silyl anion including the preparation of transition metal complexes [1-3]. Our group contributed two crystal structures of such compounds [4, 5]. In an effort to get further insight into the reactivity of 1 and the structures of its transition metal conq>lexes, we reacted it with various transition metal conq>ounds. 

The distinct reactivity of silyl anions versus carbanions towards N20 was used149 to probe the structure(s) of [M —1]" generated via deprotonation of trimethylsilane with NH2. The fact that the [M — H] ion m/z 73 upon reaction with N20 generates an ion at m/z 75 and not at m/z 89 clearly demonstrates that deprotonation has generated ion 9 and not 10 (reaction 131). 

---