skeletal atoms and two hydrogen atoms. Thus the Si atom is tetravalent (has four bonds). If n = 1, quasi-two-dimensional (corrugated sheets) or random three-dimensional silicon networks termed polysilynes are obtained in which each silicon atom is bonded to three other silicon skeletal atoms and one hydrogen atom giving, again, a tetravalent Si atom. In the polysilenes and the polysilynes, the backbone is made exclusively of silicon atoms; the pendent- or side-groups, are not shared between skeletal atoms of the backbone chain. Compression of silane itself under extremely high pressures (> 90 GPa) for long periods of time (~8 months) produces a material that differs from those shown above in that the silicon atoms are not bonded to one another but, rather, are connected via bridging hydrogen atoms. The silicon atoms are 8-fold coordinated by hydrogen (i.e. each silicon atom is bonded to 8 hydrogen atoms) forming a tetragonal structure. Inorganic derivatives – siloxene In addition to the polysilicon halides, another related compound, in which the hydrogen is partially replaced by an inorganic group, is siloxene. Siloxene was first observed before the polysilicon hydrides, but it was initially thought to be a polymeric silicon hydride itself. Siloxene is structurally derived from layered polysilyne by replacing one-half of the hydrogen atoms with OH groups. It has the chemical formula Si2H2O (or [Si6H3(OH)3]x) and it is prepared by reacting calcium disilicide (CaSi2), which has a puckered layer of silicon atoms, with dilute hydrochloric acid. This yellow solid was prepared from that route as early as 1900 by Charles Schenck Bradley (1853–1929), a one-time associate of Thomas Edison. However, its structure, the intra-polymer Si-Si bond order, and true chemical formula were not determined for some time afterwards Bradley had erroneously concluded it was the silicon analog, (Si2H2)x, of the unsaturated (double-bond containing) polymer Polyacetylene and referred to as silico-acetylene. Organic derivatives – polysilanes and polysilynes When hydrogen atoms in linear polysilene are replaced with organic substituents, particularly alkyl and aryl groups, polysilanes are obtained. Although organosilicon polymers can be considered structural derivatives of polysilicon hydrides, they are not synthesized directly from them. Traditionally, polysilanes are prepared by the Wurtz-like sodium or potassium metal-mediated reductive coupling (dehalogenation) of dihaloorganosilanes (R2SiX2) in an inert solvent (e.g. toluene), very much like Burkhard's original method. Polysilynes have been similarly prepared from trihaloorganosilanes (RSiX3). See also Polysilicon halides References Inorganic silicon compounds Silicon hydrides Inorganic polymers