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polysilicon halides
Polysilicon halides are silicon backbone polymeric solids. At room temperature, the polysilicon fluorides are colorless to yellow solids while the chlorides, bromides, and iodides are, respectively, yellow, amber, and red-orange.〔Inorganic Chemistry, Holleman-Wiberg, Academic Press (2001), p. 850.〕 Polysilicon dihalides (perhalo-polysilenes) have the general formula (Si''X''2)n while the polysilicon monohalides (perhalo-polysilynes) have the formula (Si''X'')''n'', where ''X'' is F, Cl, Br, or I and ''n'' is the number of monomer units in the polymer.
==Macromolecular Structure==
The polysilicon halides can be considered structural derivatives of the polysilicon hydrides, in which the side-group hydrogen atoms are substituted with halogen atoms. In the monomeric silicon dihalide (aka dihalosilylene and dihalosilene) molecule, which is analogous to carbene molecules, the silicon atom is divalent (forms two bonds). By contrast, in both the polysilicon dihalides and the polysilicon monohalides, as well as the polysilicon hydrides, the silicon atom is tetravalent with a local coordination geometry that is tetrahedral, even though the stoichiometry of the monohalides (()n = SinXn) might erroneously imply a structural analogy between perhalopolysilynes and () polyacetylenes with the similar formula (C2H2)n. The carbon atoms in the polyacetylene polymer are sp2-hybridized and thus have a local coordination geometry that is trigonal planar. However, this is not observed in the polysilicon halides or hydrides because the Si=Si double bond in disilene compounds are much more reactive than C=C double bonds. Only when the substituent groups on silicon are very large are disilene compounds kinetically non-labile.〔R. West "The Chemistry of the Silicon-Silicon Double Bond" ''Angewandte Chemie'' Vol. 26, No. 12 (1987) pp. 1201-1211.〕
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