Silsesquioxane

220x220px|thumbnail|right|A cubic silsesquioxane A silsesquioxane is an organosilicon compound with the chemical formula [RSiO3/2]n (R = H, alkyl, aryl, alkenyl or alkoxyl.). Silsesquioxanes are colorless solids that adopt cage-like or polymeric structures with Si-O-Si linkages and tetrahedral Si vertices. Silsesquioxanes are members of polyoctahedral silsesquioxanes ("POSS"), which have attracted attention as preceramic polymer precursors to ceramic materials and nanocomposites. Diverse substituents (R) can be attached to the Si centers. The molecules are unusual because they feature an inorganic silicate core and an organic exterior. The silica core confers rigidity and thermal stability.

==Structure== thumb|222x222px|Molecular structure of imine-silsesquioxane Silsesquioxanes are known in molecular form as well as polymers. The 6-, 8-, 10-, and 12-Si cages are sometimes labeled T6 T8, T10, and T12, respectively (T = tetrahedral vertex). The T8 cages, the most widely studied members, have the formula [RSiO3/2]8, or equivalently R8Si8O12. In all cases each Si center is bonded to three doubly bridging oxo groups, which in turn connect to other Si centers. The fourth group on Si is usually an alkyl, halide, hydride, alkoxide, etc. In the cubic clusters with Oh symmetry the Si-O-Si angles are in the range 145–152°, being bowed out, allowing the Si centers to better adopt tetrahedral geometry. The O-Si-O angle are in the range: 107–112°, Si-O bond: 1.55–1.65 Å.