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 ([SiX]n = SinXn) might erroneously imply a structural analogy between perhalopolysilynes and [linear] 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.
[3] Since that time, it has been shown that gaseous silicon dihalide molecules (SiX2) are formed as intermediates in the Si/SiX4 reactions.
[5] The polymerization is believed to occur via paramagnetic di-radical oligomeric intermediates like Si2F4 (•SiF2-F2Si•) and Si3F6 (•SiF2-SiF2-F2Si•),[6][7] The polysilicon dihalides also form from the thermally-induced disproportionation of perhalosilanes (according to: x SinX2n+2 → x SiX4 + (n-1) (SiX2)x where n ≥ 2).
Under conditions of high vacuum and fast pumping, SiCl2 may be isolated by rapidly quenching the reaction products or, under less stringent vacuum conditions, (SiCl2)n polymer is deposited just beyond the hot zone while the perchlorosilanes SinCl2n+2 are trapped farther downstream.
For example, (SiCl2)n undergoes substitution by alcohols to give poly(dialkoxysilylene)s.[12] The polysilicon monohalides are all stable to 400°C, but are also water and air sensitive.