[2] Compounds containing silicon–oxygen bonds include materials of major geological and industrial significance such as silica, silicate minerals and silicone polymers like polydimethylsiloxane.
Because of this moderately large difference in electronegativities, the Si−O bond is polar but not fully ionic.
[4] Other compounds containing Si=O double bonds are normally very reactive and unstable with respect to polymerisation or oligomerization.
The main reasons are hyperconjugation (donation from an oxygen p orbital to an Si–R σ* sigma antibonding molecular orbital, for example) and ionic effects (such as electrostatic repulsion between the two neighbouring partially positive silicon atoms).
It increases progressively from 133° to 180° in Ln2Si2O7 as the size and coordination number of the lanthanide decreases from neodymium to lutetium.