Catenation

In chemistry, catenation is the bonding of atoms of the same element into a series, called a chain.^[1] A chain may be open if its ends are not bonded to each other (an open-chain compound), or closed if they are bonded in a ring (a cyclic compound).

Catenation occurs most readily with carbon, which forms covalent bonds with other carbon atoms to form longer chains and structures. This is the reason for the presence of the vast 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 (and known as catenae). Carbon chains in biochemistry combine any of various other elements, such as hydrogen, oxygen, and biometals, onto the backbone of carbon, and proteins can combine multiple chains encoded by multiple genes (such as light chains and heavy chains making up antibodies).

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, including silicon, sulfur and boron.

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 versatile chemistry of elemental sulfur is 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.^[2][3] 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.^[4]

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^[5] were reported in 2004.^[6]

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.

Etymology

The words to catenate and catenation reflect the Latin root catena, "chain".

See also

• Backbone chain

References