Isomerism

Isomerism

Coordination compounds often exist as isomers—i.e., as compounds with the same chemical composition but different structural formulas. Many different kinds of isomerism occur among coordination compounds. The following are some of the more common types.

Cis-trans isomerism

Cis-trans (geometric) isomers of coordination compounds differ from one another only in the manner in which the ligands are distributed spatially; for example, in the isomeric pair of diamminedichloroplatinum compounds
the two ammonia molecules and the two chlorine atoms are situated next to one another in one isomer, called the cis (Latin for “on this side”) isomer, and across from one another in the other, the trans (Latin for “on the other side”) isomer. A similar relationship exists between the cis and trans forms of the tetraamminedichlorocobalt(1+) ion:
 

Enantiomers and diastereomers

So-called optical isomers (or enantiomers) have the ability to rotate plane-polarized light in opposite directions. Enantiomers exist when the molecules of the substances are mirror images but are not superimposable upon one another. In coordination compounds, enantiomers can arise either from the presence of an asymmetric ligand, such as one isomer of the amino acid, alanine (aminopropionic acid),
or from an asymmetric arrangement of the ligands. Familiar examples of the latter variety are octahedral comp

lexes carrying three didentate ligands, such as ethylenediamine, NH₂CH₂CH₂NH₂. The two enantiomers corresponding to such a complex are depicted by the structures below.

The ethylenediamine ligands above are indicated by a curved line between the symbols for the nitrogen atoms.
Diastereomers, on the other hand, are not superimposable and also are not mirror images. Using AB as an example of a chelating ligand, in which the symbol AB implies that the two ends of the chelate are different, there are six possible isomers of a complex cis-[M(AB)₂X₂]. For example, AB might correspond to alanine [CH₃CH(NH₂)C(O)O]⁻, where both N and O are attached to the metal. Alternatively, AB could represent a ligand such as propylenenediamine, [NH₂CH₂C(CH₃)HNH₂], where the two ends of the molecule are distinguished by the fact that one of the Hs on a C is substituted with a methyl (CH₃) group.

Ionization isomerism

Certain isomeric pairs occur that differ only in that two ionic groups exchange positions within (and without) the primary coordination sphere. These are called ionization isomers and are exemplified by the two compounds, pentaamminebromocobalt sulfate, [CoBr(NH₃)₅]SO₄, and pentaamminesulfatocobalt bromide, [Co(SO₄)(NH₃)₅]Br. In the former the bromide ion is coordinated to the cobalt(3+) ion, and the sulfate ion is outside the coordination sphere; in the latter the sulfate ion occurs within the coordination sphere, and the bromide ion is outside it.

Linkage isomerism

Isomerism also results when a given ligand is joined to the central atom through different atoms of the ligand. Such isomerism is called linkage isomerism. A pair of linkage isomers are the ions [Co(NO₂)(NH₃)₅]²⁺and [Co(ONO)(NH₃)₅]²⁺, in which the anionic ligand is joined to the cobalt atom through nitrogen or oxygen, as shown by designating it with the formulas NO₂⁻(nitro) and ONO⁻(nitrito), respectively. Another example of this variety of isomerism is given by the pair of ions [Co(CN)₅(NCS)]³⁻ and [Co(CN)₅(SCN)]³⁻, in which an isothiocyanate (NCS)⁻ and a thiocyanate group (SCN)⁻ are bonded to the cobalt(3+) ion through a nitrogen or sulfur atom, respectively.

Coordination isomerism

Ionic coordination compounds that contain complex cations and anions can exist as isomers if the ligands associated with the two metal atoms are exchanged, as in the pair of compounds, hexaamminecobalt(3+) hexacyanochromate(3–), [Co(NH₃)₆][Cr(CN)₆], and hexaamminechromium(3+) hexacyanocobaltate(3–), [Cr(NH₃)₆][Co(CN)₆]. Such compounds are called coordination isomers, as are the isomeric pairs obtained by redistributing the ligands between the two metal atoms, as in the doubly coordinated pair, tetraammineplatinum(2+) hexachloroplatinate(2–), [Pt(NH₃)₄][PtCl₆], and tetraamminedichloroplatinum(2+) tetrachloroplatinate(2–), [PtCl₂(NH₃)₄][PtCl₄].

Ligand isomerism

Isomeric coordination compounds are known in which the overall isomerism results from isomerism solely within the ligand groups. An example of such isomerism is shown by the ions, bis(1,3-diaminopropane)platinum(2+) and bis(1,2-diaminopropane)platinum(2+),