Catenation is the ability of a chemical element to form covalent bonds with itself, resulting in ring, chain and cage structures. The element most well known for its catenation is carbon, with organic chemistry being essentially the study of catenated carbon structures (otherwise known as catenae). However, carbon is by no means the only element capable of forming such catenae, and several other main group elements are capable of forming an expansive range of catenae.
The ability of an element to catenate is primarily based on the bond energy of the element to itself. This ability is also influenced by a range of steric and electronic factors, including the electronegativity of the element in question, the molecular orbital hybridization and the ability to form different kinds of covalent bonds. For example, carbon has the ability to form both sigma and pi bonds to itself. This is due to an overlap between pi-electron orbitals, allowing electron density to be shared and thus stabilising the bond. Silicon, on the other hand, has negligible overlap between pi-orbitals, and thus tends to not form pi-bonds by preference. As a result, silicon has a relatively poor capacity for catenation.
Silicon can form sigma bonds to other silicon atoms (disilane is the parent of this class of compounds). Even silicon–silicon pi bonds are possible. However, these bonds are less stable than the carbon analogs. Disilane is quite reactive compared to ethane. Disilylenes are quite rare, unlike alkenes. And disilynes, unlike alkynes, are too unstable to be isolated except (possibly) for specially designed molecular structures.[1]
The ability of certain main group elements to catenate is currently the subject of research into inorganic polymers.
^ Karni, M.; Apeloig, Y. (January 2002). "The quest for a stable silyne, RSi≡CR′. The effect of bulky substituents". Silicon Chemistry1 (1): 59–65. doi:10.1023/A:1016091614005.
