In
chemistry, linkage isomerism or ambidentate isomerism is a form of
isomerism in which certain
coordination compounds have the same composition but differ in their metal atom's connectivity to a
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elec ...
.
Typical ligands that give rise to linkage isomers are:
*
thiocyanate
Thiocyanate (also known as rhodanide) is the anion . It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) thiocyanate was formerly used in pyr ...
, –
isothiocyanate
In organic chemistry, isothiocyanate is the functional group , formed by substituting the oxygen in the isocyanate group with a sulfur. Many natural isothiocyanates from plants are produced by enzymatic conversion of metabolites called glucosi ...
,
*selenocyanate, – isoselenocyanate,
*
nitrite,
*
sulfite,
Examples of linkage isomers are violet-colored and orange-colored . The isomerization of the S-bonded isomer to the N-bonded isomer occurs intramolecularly.
The complex ''cis''-
dichlorotetrakis(dimethylsulfoxide)ruthenium(II) () exhibits linkage isomerism of dimethyl sulfoxide ligands due to S- vs. O-bonding. ''Trans''-dichlorotetrakis(dimethylsulfoxide)ruthenium(II) does not exhibit linkage isomers.
History
Linkage isomerism was first noted for
nitropentaamminecobalt(III) chloride, . This
cationic cobalt
complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
can be isolated as either of two linkage isomers. In the yellow-coloured isomer, the nitro ligand is bound through nitrogen. In the red linkage isomer, the nitrito is bound through one oxygen atom. The O-bonded isomer is often written as . Although the existence of the isomers had been known since the late 1800s, only in 1907 was the difference explained. It was later shown that the red isomer converted to the yellow isomer upon UV-irradiation. In this particular example, the formation of the nitro isomer () from the nitrito isomer () occurs by an intramolecular rearrangement.
References
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Coordination chemistry
Chemical bonding
Isomerism