The stereochemistry of silicon

There are several compounds in which Si is 5-coordinated. The most straightforward case would be an ion SiXJ such as occurs in salts (NR4XSiFs), in which R must be larger than CH3, but their structures are not yet known. Structural studies have been made of various chelate species, for example, the anion (c) in its tetramethylammonium salt, and the molecules (d) and (e). (There are some unexpected bond lengths in (c)-a very short C-C, 1 -29 A, in the phenylene 

The action of metallic K on SiH4 or Si2H in aprotic solvents such as 1 2-dimethoxyethane produces KSiH3, which forms colourless crystals with the NaCl structure. The H atoms were not located, but the SiHJ ion would contain 3-covalent Si. For the unstable Sip2 and SiCl2 species see p. 785. 

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Although the stereochemistry of silicon in these compounds presents no points of special interest, the bond arrangement being the normal tetrahedral one, the siloxanes include some interesting ring systems. 

The elucidation of reaction mechanisms in organic chemistry has involved both kinetic and stereochemical studies. Similarly, the stereochemistry of silicon (1,2) has played a determining role in understanding the reaction mechanism in organosilicon compounds. 

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