Cyclopentasilane

Cyclopentasilane can be made from diphenyldichlorosilane (C₆H₅)₂SiCl₂ reacting with lithium. This forms decaphenylcyclopentasilane. This reacts with aluminium chloride in benzene catalysed with hydrogen chloride to yield decachlorocyclopentasilane. Decachlorocyclopentasilane then has its chlorine replaced by hydrogen using lithium aluminium hydride.^[3]

Cyclopentasilane is pyrophoric. It has a σ-delocalization property. Cyclopentasilane is a liquid at standard conditions, but can be frozen to crystals at 173K. The monoclinic crystals are not isomorphic to cyclopentane. The silicon to silicon bond lengths in the solid vary from 2.3353 and 2.3377 Å, which is shorter than in the substituted cyclopentasilanes or linear silanes. In gas form, those bond lengths, as measured by electron diffraction, increased to 2.342 Å.^[3]

Cyclopentasilane is sensitive, and starts decomposing when heated over 84°C, when it loses hydrogen and polymerizes. As it is heated more silane is produced, and at 178°C it yields disilane. When heated to over 250°C it decomposes to a silicon hydrogen polymer.^[3]

A five member ring can adopt a number of shapes: planar with all the atoms arranged in a pentagon with symmetry D_5h; envelope symmetry C_s, with four atoms in a rectangular plane and one popped up like the flap, and twist with symmetry C₂. The planar form has slightly higher energy than the other forms.^[4]