Pentacoordinated silicon

FIGURE 62. Schematic representation of the occupied molecular orbitals of SiH5 (D3h). 

Quantum-mechanical calculations have centered on anionic pentacoordinat, silicon species, mainly as models for the transition states or intermediates in bimolecular SN2 substitution reactions at silicon—a topic of current experimental interest7 9,359. 

TABLE 38. Calculated energies (kcal mol ) for the reactions X + SiH4 - SiHX-(equation 56) and SiH4X - SiHJ +HX (equation 57), for axial substituents X, calculated at MP2/6-31 + +G //6-31G fl 

Second-row substituents (X = CH3, OH, F) are more effective in stabilizing XSiH4 than is hydride, and much more effective than third-row substituents363. Similar calculations for FSiH4 were reported recently by Streitwieser and coworkers358. 

The stabilities of the pentacoordinated XSiH4 anions relative to elimination of HX (equation 57) were also evaluated and the results are also given in Table 38. Expulsion of HX is exothermic for X = H and CH3 but endothermic for X = OH, F, SH and Cl and nearly thermoneutral for X = SiH3363. 

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The reactivity of penta- and hexacoordinated silicon compounds has been described to be very different from the reactivity of the corresponding tetracoordinated derivatives [ 1], An increase in reactivity towards nucleophiles has been observed in the case of neutral and anionic pentacoordinated silicon compounds as exemplified by the following Schemes [2],... 

The mechanism of these reactions can be described as a nucleophilic attack on a pentacoordinate silicon compound. Supporting this assumption, neutral or anionic hexacoordinated structures have been isolated and their X-ray structures obtained [5]. 

As can be seen from the data presented, the high energies of complex formation decrease sharply the endothermicity of the retro-Wittig type decomposition and, moreover, fundamentally change the reaction mechanism. As has been shown for betaines (")X-E14Me2-CH2-E15( + )Me3 (X = S, Se E14 = Si, Ge E14 = P, As), the reaction occurs as bimolecular nucleophilic substitution at the E14 atom. For silicon betaines, the transition states TS-b-pyr with pentacoordinate silicon and nearby them no deep local minima corresponding to the C-b complexes can be localized in the reaction coordinate. 

Pentacoordinate silicon complexes can also be prepared via the reaction of diaryldichlorosilanes with carbenes 4 (R = Et, Pr R = Me). Interestingly, a 2-(trimethylsilyl)imidazolium salt (56) was formed when Me3SiI was treated with 4 (39). Unfortunately, no information regarding the crystal structures of these species is available. 

The literature on these structures is sparse, and generally the articles report preparations of several of them in similar ways. These compounds are constituted by oxygen, sulfur, or nitrogen adducts at the silicon atom, thus providing pentacoordinated silicon compounds or structural analogs (Scheme 33). 

The study of compounds containing pentacoordinate silicon atoms currently represents one of the main areas of research in silicon chemistry. This is evident from the numerous reviews and proceedings published on this topic in recent years.112 Most of the pentacoordinate silicon compounds described in the literature are either salts with A5.S7-silicate anions or neutral silicon complexes with a 4+1 coordination to silicon. This review deals with a completely different class of pentacoordinate silicon compounds zwitterionic A S /-silicatcs. These molecular compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. 

Generally, the pentacoordinate silicon compounds described in this chapter are sensitive to water and very easily undergo hydrolytic Si-O cleavage reactions in solution. This has been used for the synthesis of the octa(silases-quioxane) 46, which was obtained in 90% yield by treatment of compound 35 with water in boiling acetonitrile (Scheme 8).34... 

The pentacoordinate silicon compounds 50,43 51,43 and 529 are spirocyclic zwitterionic A557-silicates with an Si04C skeleton. They contain two ethane-l,2-diolato(2-) ligands or two meso-oxolane-3,4-diolato(2-) ligands. 

The pentacoordinate silicon compounds 81, 8,54 82,54 83,54 and 8455 are spirocyclic zwitterionic A5S7-silicates with an Si04C skeleton. The chiral zwitterions contain two diolato(2—) ligands that formally derive from aceto-hydroximic acid and benzohydroximic acid (tautomers of acetohydroxamic acid and benzohydroxamic acid). 

The spirocyclic pentacoordinate silicon compounds 85,56 86,39 and 8739 are zwitterionic A5S7-silicates with an S702N2C skeleton. The chiral zwitterions... 

As demonstrated by single-crystal X-ray diffraction, the /-coordination polyhedra of 85-87 are distorted trigonal bipyramids, with each of the axial positions occupied by the oxygen atoms. This is shown for compound 86 in Fig. 11. In all cases, the crystals are formed from pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 85-87 are listed in Table XIII. As can be seen from the Si-O [1.8004(10)-1.829(6) A], Si-N [1.741(7)-1.764(6) A], and Si-C distances [1.867(8)-1.915(2) A], the A/02N2C frameworks of 85-87 are built up by five normal covalent bonds and do not involve a bonding system in the sense of the 4+1 coordination usually observed for pentacoordinate silicon species with Si-N bonds. 

The spirocyclic pentacoordinate silicon compounds 90,45,58 91 45,58 and 9259 are zwitterionic A5Si-silicates with an Si05 skeleton. The chiral zwitter-ions contain two 2-methyllactato(2-) or two benzilato(2-) ligands. In these molecules, two five-membered SiC>2C2 rings are connected by the silicon spirocenter. Compounds 90 and 91 represent isoelectronic analogs of the zwitterionic A5Sz-silicates 58 and 59 (0/CH2 replacement see Section III,D). 

The pentacoordinate silicon compounds 94,23 95,23 96-98,60 99,60,61100,61,62 101-103,60 104,61,62 105,62 106,62 and 10761,63 are monocyclic zwitterionic A5S7-silicates with an Si02FC2 skeleton. The chiral zwitterions each contain one bidentate diolato(2-) ligand that formally derives from 1,2-dihydroxy-benzene, salicylic acid, glycolic acid, oxalic acid, benzohydroximic acid (tautomer of benzohydroxamic acid), 2-methyllactic acid, or (S)-mande-lic acid. 

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