Catenation

Catenation is the ability of a chemical element to form a long chain-like structure via a series of covalent bonds. Catenation occurs most readily in carbon, which forms covalent bonds with other carbon atoms. Catenation is the reason for the presence of a large number of organic compounds in nature. Carbon is most well known for its properties of catenation, with organic chemistry essentially being 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, which decreases with more diffuse orbitals (those with higher azimuthal quantum number) overlapping to form the bond. Hence, carbon, with the least diffuse valence shell p orbital is capable of forming longer p-p sigma bonded chains of atoms than heavier elements which bond via higher valence shell orbitals. Catenation 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 carbon, the sigma overlap between adjacent atoms is sufficiently strong that perfectly stable chains can be formed. With other elements this was once thought to be extremely difficult in spite of plenty of evidence to the contrary.

The remarkable properties of elemental sulfur are largely due to catenation. In the native state, sulfur exists as S₈ molecules. On heating these rings open and link together giving rise to increasingly long chains, as evidenced by the progressive increase in viscosity as the chains lengthen. Selenium and tellurium also show variants of these structural motifs.

Silicon can form sigma bonds to other silicon atoms and (disilane is the parent of this class of compounds). However, it is difficult to prepare and isolate Si_nH_2n+2 (analogous to the saturated alkane hydrocarbons) with n greater than about 8, as their thermal stability decreases with increases in the number of silicon atoms. Silanes higher in molecular weight than disilane decompose to polymeric polysilicon hydride and hydrogen.^[1][2] But with a suitable pair of organic substituents in place of hydrogen on each silicon it is possible to prepare polysilanes (sometimes, erroneously called polysilenes) that are analogues of alkanes. These long chain compounds have surprising electronic properties - high electrical conductivity, for example - arising from sigma delocalization of the electrons in the chain. (Ref. R.D. Miller and J. Michl. Chem Rev 89 (1989), pp. 1359–1410.)

Phosphorus chains (with organic substituents) have been prepared, although these tend to be quite fragile. Small rings or clusters are more common.

Even silicon–silicon pi bonds are possible. However, these bonds are less stable than the carbon analogues. Disilane is quite reactive compared to ethane. Disilylenes are quite rare, unlike alkenes. Examples of disilynes, long thought to be too unstable to be isolated^[3] were reported in 2004.^[4]

In recent years a variety of double and triple bonds between the semi-metallic elements have been reported, including silicon, germanium, arsenic, bismuth and so on. The ability of certain main group elements to catenate is currently the subject of research into inorganic polymers.

See also

• Concatenation (disambiguation)

References

1. ^ W. W. Porterfield, Inorganic Chemistry: A Unified Approach, 2nd Ed.", Academic Press (1993), p. 219.
2. ^ Inorganic Chemistry, Holleman-Wiberg, John Wiley & Sons (2001) p. 844.
3. ^ Karni, M.; Apeloig, Y. (January 2002). "The quest for a stable silyne, RSi≡CR′. The effect of bulky substituents". Silicon Chemistry 1 (1): 59–65. doi:10.1023/A:1016091614005. 
4. ^ Akira Sekiguchi, Rei Kinjo, Masaaki Ichinohe (September 2004). "A Stable Compound Containing a Silicon-Silicon Triple Bond". Science 305 (5691): 1755–1757. doi:10.1126/science.1102209. PMID 15375262. [1]