Introduction

Just as an infinite number of ways exist to assemble huge number of dissimilar coloured shoes, there exists different ways on the molecular scale in that a specified number of ligands can be arranged in 3 dimensional array round a central metal. This is the essential idea of isomerism in coordination chemistry. Different coordination numbers, shapes and points of attachment of ligands give an almost infinite number of three-dimensional structurally related complexes usually said isomers. Isomerism doesn't exist in complexes through identical monodentate and general donor atom ligands. Nevertheless, wherever more than one kind of ligand is bound, and even where either one kind of ligand through a set of more donor atoms than required, or a number of identical chelating ligands bind, the possibility of isomerism requires to be considered. It is significant to note that the number of isomers rises through enhance in coordination. The idea of isomerism is well expanded in complexes by coordination numbers four and six while no established cases of isomerism is connected through coordination numbers one, two and three. The idea of isomerism is extremely significant in drug design as different isomers have certain differences in reactivity, particularly optical isomers. An anticancer drug, Budotitane (Titanium complex), lost its market value when it was determined to possess optical isomers. This since only one isomer might be effective in the treatment and the other isomer might cause damaging consequence. This underscores require understanding idea of isomerism.

Isomerism

Isomerism is a phenomenon utilized to explain complexes through the similar molecular weight and molecular formula but dissimilar structural and/or spatial arrangement of donor atoms around the central metal in the coordination sphere. Such complexes are said isomers. In coordination chemistry, Isomers are of different types, they comprise hydrate or solvent isomers, ionization isomers, and coordination isomers having the similar overall formula but through dissimilar donor atoms of the equivalent or different ligands connected to the central metal atom or ion. The nomenclature of different types of isomerism is an indication of whether solvent, anions, or other coordination compounds imposes the isomerism in the structure. The terms linkage (ambidentate) isomerism is used when ambidentate ligands impose structural differences in complexes due to use of different donor atoms on a ligand. Stereoisomers have the same ligands with the same donor atoms, but differ in the geometric (spatial) arrangement of the ligands. Some stereoisomers are optically active hence they are classified as optical isomers. 

Isomerism in coordination chemistry is split into structural isomerism and stereoisomerism. Structural isomers fluctuate in the way ligands or donor atoms of ligands are arranged round a central metal. Stereoisomers differ in the spatial arrangement of ligands round the central metal. The figure below demonstrates dissimilar shapes of isomerism.

Fig: isomerism.  

Structural isomerism

As well recognized as constitutional isomers, structural isomers have the same empirical formula but differ in the arrangement of their constituent atoms. This consequence in difference in physical properties these as colour. Many dissimilar types of structural isomerism take place in coordination chemistry and several of them shall be conversed

i. Ionization isomerism

Ionization isomers provide dissimilar ions in aqueous solution. This is since different anions coordinated to the metal in the coordination sphere. The isomeric pairs differ in that there is a swap of 2 anionic groups inside and outside the coordination sphere. Instances; [Co(NH₃)₅Br]SO₄ (violet) and [Co(NH₃)₅SO₄]Br (red) Note that in [Co(NH₃)₅Br]SO₄ the sulphate is the counter ion and can be noticed via treating the solution of the complex through BaCl₂ to precipitate the sulphate in the form of BaSO₄ (qualitative test for sulphate). The bromide ion is coordinated and won't precipitate through silver nitrate because it is not free.

In [Co(NH₃)₅SO₄]Br, the test for bromide will be positive since Br- isn't coordinated to the metal while the test for sulphate will be negative since the sulphate is in the coordination sphere and not free.

[Pt(en)₂Cl₂]Br₂               and      [Pt(en)₂Br₂]Cl₂

[Cr(NH₃)₄ClBr]NO₂         and       [Cr(NH₃)₄ClNO₂]Br

[Cr(NH₃)₅Cl]NO₂            and       [Cr(NH₃)₅NO₂]Cl

[Co(NH₃)₄Br₂]Cl            and      [Co(NH₃)₄ClBr]Br

ii. Hydrate isomerism

As many compounds are prepared in aqueous solutions where water is abundant, complexes can precipitate or crystallize by water of crystallisation outside the coordination sphere or through coordinated water (ligand) inside the coordination sphere. There are many isomers that differ in the position of water molecules in their formula. For instance, there are three recognized hydrate isomers of CrCl₃(H₂O)₆: [Cr(H₂O)₆]Cl₃ (violet), [Cr(H₂O)₅Cl]Cl₂.H₂O (pale green) and [Cr(H₂O)₄Cl₂]Cl.2H₂O (dark green). 

The isomers can be differentiated by the mole of silver chloride precipitated by using silver nitrate on molar solution of the isomers. In [Cr(H₂O)₆]Cl₃, 3 mole of silver chloride will be precipitated indicating that none of the Cl^- ion is in the coordination sphere, 2 mole will be precipitated in [Cr(H₂O)₅Cl]Cl₂.H₂O and 1 mole in [Cr(H₂O)₄Cl₂]Cl.2H₂O indicating the number of Cl- not coordinated to the metal ion. The water of crystallisation outside the coordination sphere can be detected on dehydration of the complexes. Other instances are:

[Co(NH₃)₄(H₂O)Cl]Cl₂    and     [Co(NH₃)₄Cl₂]Cl.H₂O

[Co(NH₃)₅(H₂O)](NO₃)₃     and    [Co(NH₃)(NO₃)](NO₃)₂.H₂O

iii. Coordination isomerism   

These isomers enclose pairs of ionic complexes that exchange ligands through each other. Many isomeric pairs are possible by redistribution of ligands between 2 metal centres. Illustrations are:

[Co(NH₃)₆][Cr(CN)₆]                       and   [Cr(NH3)₆][Co(CN)₆]

[Co(NH3)₅(CN)][Cr(CN)₅(NH₃)]    and   [Cr(NH3)₅(CN)][Co(CN)₅(NH₃)]

[Co(en)₃][Cr(CN)₆]                          and  [Cr(en)₃][Co(CN)₆]

[Cu(NH₃)₄][PtCl₄]                            and   [Pt(NH3)₄][CuCl₄]

Note that the cationic complex is written first.

iv. Linkage isomerism

This kind of isomerism is examined in complexes containing ambidentate ligands which can coordinate through at least two different binding sites. Example of such ligand is nitrite (NO2-) which can coordinate through nitrogen or oxygen.

[Co(NH₃)₅(NO₂)]²⁺ (red)              and          [Co(NH₃)₅(ONO)]²⁺ (yellow)

The yellow complex, [Co(NH₃)₅(ONO)]²⁺, is unstable and it is switched into [Co(NH₃)₅(NO₂)]²⁺ both in solution and the solid state either by heating or by exposure to ultraviolet light. The two isomer can be differentiated through Infrared spectroscopy, for the O-bonded ligand, characteristic absorption bands at 1065 and 1470 cm^-1 are examined, while for the N-bonded ligand, the corresponding vibrational wave numbers are 1310 and 1430 cm^-1. Other instances are [Co(CN)₅(SCN)]^3- and [Co(CN)(NCS)]3-, where the sulphur and nitrogen of  the thiocyanate ligand imposes the observed linkage isomerism.

v. Polymerization isomerism

Polymerization isomers are isomers with the same simplest unit called monomer. The combination of two or more monomer units results in polymeric complex isomer. An example is the unit [Pt(NH₃)₂Cl₂] which on combination can give; [Pt(NH₃)₄][PtCl₄], [Pt(NH3)3Cl]2[PtCl4] and [Pt(NH₃)₄][Pt(NH₃)Cl₃]2. As well the unit [Co(NH₃)₃Cl₃] on combination will produce  

[Co(NH₃)₆][CoCl₆], [Co(NH₃)₄Cl₂][Co(NH₃)2Cl₄]        and    [Co(NH₃)₅Cl][Co(NH₃)Cl₅].

VI. Ligand isomerism

This is a shape of isomerism due to the isomeric nature of the ligands. Typical instances are [Pt(NH₂CH₂CH₂CH₂NH₂)₂]²⁺and [Pt(NH₂CH(CH₃)CH₂NH₂)₂]²⁺The ligand propane-1, 3-diamine and its isomer methylethylenediamine impose the isomerism. 

Stereoisomerism

Stereoisomerism occurs in complexes due to difference in spatial arrangement of ligands round the central metal. Stereoisomerism is divided into geometric and optical isomerism.

Geometric isomerism

The most common type of geometrical isomerism involves cis and trans isomers in square planar and octahedral complexes. In square planar complex [Pt(NH₃)Cl₂], the cis- and trans- isomers are shown below. Note that in the trans- form, identical ligands are divided via bond angle of 180⁰ while in cis- the bond angle between identical ligands is 90⁰.

In Octahedral compound [Co(NH₃)₄Cl₂]⁺, cis- and trans-isomers take place even as in [Co(NH₃)₃Cl₃], facial and meridional isomers happen. Such are shown below.

Fig: facial and meridional isomers  

Optical isomerism

Optical isomers survive in complexes that aren't super imposable on their mirror images. Such isomers have capability to turn around the plane of polarized light in conflicting direction. A mixture of optical isomers of the similar quantity won't rotate the plane of polarized light since the effect of one isomer is cancelled out via the other. Such a mixture is said racemic mixture. Optical isomerism is possible in tetrahedral and octahedral complexes (cis-isomers) where centre of symmetry is absent but not in square planar complexes. Optical isomers are said enantiomers. A solution of enantiomer that rotates the plane of polarized light in clockwise direction is designated as positive (+) or dextrorotatory (d) enantiomer while the isomer that rotates the plane of polarized light in anticlockwise direction is allocated as negative (-) or leavorotatory (l) enantiomer. Example is dichlorobis (ethylenediammine) cobalt (III). The optical isomers have identical chemical and physical properties but differ in their ability to rotate the plane of polarized light unlike diastereoisomer that differ in both chemical and physical properties and lacks ability to rotate the plane of polarized light.

Fig: optical isomerism  

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