Contents
Isomerism
Two or more compounds having the same molecular formula but different arrangement of atoms are called isomers and the phenomenon is called isomerism. Because of different arrangement of atoms, isomers differ in one or more physical or chemical properties.
Isomers can be broadly classified into two major categories :
(A) Structural isomers
(B) Stereoisomers
Structural isomers
1. Ionisation isomerism
2. Hydrate isomerism
3. Coordination isomerism
4. Linkage isomerism
Stereoisomers
1. Geometrical isomerism
2. Optical isomerism
(A) Structural isomerism
The isomers which have same molecular formula but different structural arrangement of atoms or groups of atoms around the central metal ion are called structural isomers.
(1) Ionisation isomerism
The compounds which have same molecular formula but give different ions in solution are called ionisation isomers. In this type of isomerism, the difference arises from the interchange of groups within or outside the coordination entity. This type of isomerism occurs when the counter ion in a coordination compound is itself a potential ligand.
For example: There are two isomers of the compound of the formula Co(NH3)5BrSO4
(a) One of these is red-violet and forms a precipitate with BaCl2 indicating that sulphate ion is outside the coordination entity.
(b) The second one is red and does not form precipitate with BaCl2 but forms a precipitate of AgBr with silver nitrate indicating that bromide ion is outside the coordination entity.
The structures of the two compounds and their mode of ionisation are :
[CoBr(NH3)5]SO4 → [CoBr(NH3)5]2+ + SO42-
Pentaamminebromido –
cobalt(III) sulphate Gives test of SO42- ions
cobalt(III) sulphate Gives test of SO42- ions
Structure
(Red-violet)
[CoSO4(NH3)5]Br → [CoSO4(NH3)5]+ + Br¯
Pentaamminesulphato-
cobalt(III) bromide Gives test of Br¯ ions
(Red)
Other compounds showing this type of isomerism are:
(i) [CoCl2(NH3)4]NO2 and [CoCl(NO2)(NH3)4]Cl
(ii) [Co(NO3)(NH3)]SO4 and [Co(SO4)(NH3)5]NO3
(iii) [PtCl2(NH3)4]Br2 and [PtBr2(NH3)4]Cl2
(iv) [CoCl(NO2)(NH3)4]Cl and [CoCl2(NH3)4]NO2
(2) Solvate or Hydrate isomerism
The compounds which have the same molecular formula but differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice are called solvate isomers.
It is also known as hydrate isomerism where water is involved as a solvent.Thus, hydrate isomers differ in the number of water molecules present as ligands or as molecules of hydration.
In type of isomerism water molecules may occur inside and outside the coordination sphere as a coordinated group or a water of hydration.
For example, there are three isomers having the molecular formula CrCl3⋅6H2O.
These are :
(CrCl3(H2O)3], [CrCl(H2O)5]Cl2⋅H2O and [CrCl2(H2O)4]Cl⋅2H2O
(i) [Cr(H2O)6]Cl3 : It does not lose water when treated with conc. H2SO4 and three chloride ions are precipitated with AgNO3.
(ii) [CrCl(H2O)5]Cl2⋅H2O
Blue green
It loses one water molecule when treated with conc. H2SO4 and 2Cl¯ ions are precipitated with AgNO3
(iii) (CrCl2(H2O)4]Cl.2H2O : It loses two water molecules on treatment with conc. H2SO4 dark green and one Cl¯ ion is precipitated with AgNO3.
Similarly, the following two isomers are hydrate isomers :
[CoCl(en)2(H2O)]Cl2 and [CoCl2(en)2]Cl.H2O
[CoCl(H2O)(NH3)4]Cl2 and [CoCl2(NH3)4]Cl.H2O
[CrCl2(C2H5N)2(H2O)2]Cl and [CrCl3(C2H5N5)2H2O].H2O
(3) Coordination isomerism
The type of isomerism occurs in compounds containing both cationic and anionic entities and the isomers differ in the distribution of ligands in the coordination entity of cationic and anionic parts. This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in the complex.
The examples are:
(i) [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6] [Co(CN)6]
(ii) [Cu(NH3)4[PtCl4] and [Pt(NH3)4][CuCl4]
This type of isomerism is also shown by compounds in which the metal ion is the same in both cationic and anionic complexes.
For example :
(i) [Cr(NH3)6][Cr(CN)6] and [Cr(CN)2(NH3)4][Cr(CN)4(NH3)2]
(ii) [(Pt(NH3)4] [PtCl4] and [PtCl(NH3)3][PtCl3(NH3)]
(4) Linkage isomerism
The compounds which have the same molecular formula but differ in the mode of attachment of a ligand to the metal atom or ion are called linkage isomers.
For example: In NO2¯ ion, the nitrogen atom as well as the oxygen atom can donate their lone pairs. This gives rise to isomerism.
If nitrogen donates its lone pair, one particular compound will be formed.
If oxygen donates its lone pair, a different compound (although having the same molecular formula) is obtained. If the bonding is through N, the ligand is named as nitrito-N(or nitro) and if it is through O, it is named as nitrito-O (or nitrito).
NO2¯ nitrito-N (or nitro)
ONO¯ nitrito-O (or nitrito)
For example: Jorgensen discovered such behaviour in the complex [(Co(NH3)5(NO2)]Cl. He prepared two different pentaamminecobalt(II) chloride each containing the NO2 group in the complex ion. These are:
The unidentate ligands which can bind to the central atom through two donor atoms are also called ambidentate ligands.
Other examples of ligands are:
CN Cyano (through C)
NC Isocyano (through N)
SCN Thiocyanato (through S)
NCS Isothiocyanato (through N)
(B) Stereoisomers
Stereoisomers are those isomers which have the same position of atoms or groups but they differ in the spatial arrangements around the central atom. Two types of isomerism viz., geometrical isomerism and optical isomerism.
(1) Geometrical isomerism
Geometrical isomerism arises in heteroleptic complexes due to ligands occupying different positions around the central ion. The ligands occupy positions either adjacent to one another or opposite to one another. These are referred to as cis- form (ligands occupy adjacent positions) and trans- form (ligands occupy opposite positions). This type of isomerism is, therefore, also referred to as cis-trans isomerism.
(a) Geometrical isomerism in complexes of coordination number 4
The complexes having coordination number 4 adopt tetrahedral or square planar geometry. The geometrical isomerism is not possible in tetrahedral complexes. This is because in tetrahedral geometry all the positions adjacent to one another in these complexes.
However, square planar complexes show geometrical isomerism.
(1) Square planar complexes of the type MA2X2 , MA2XY, MABX2, MABXY can exist as geometrical isomers (Here A and B are neutral ligands such as H2O, NH3, CO, NO, C5H5N whereas X and Y are anionic ligands such as Cl¯, NO2‾, CN¯, SCN¯ etc.)
Example:
(i) [PtCl2(NH3)] exists in cis and trans formas:
![[Pt(Cl2(NH3)2]](https://classnotes.org.in/wp-content/uploads/PtCl2NH32-700x245.jpeg)
(ii) [PtCl(C5H5N)2 (NH3)] exists in cis and trans form as:
![[PtCl(C5H5N)2(NH3)]](https://classnotes.org.in/wp-content/uploads/PtClC5H5N2NH3-700x198.jpeg)
(iii) Square planar complexes of the type MABCD show three isomers. The structures of these isomers can be written by fixing the position of one ligand (say A) and placing the other ligands B, C and D trans to it.
The complex [Pt(NO2)(py) (NH2OH)(NH3)]+ exists in three geometrical isomers as represented below:
![[Pt(NO2)py (NH2OH)(NH3)]+](https://classnotes.org.in/wp-content/uploads/PtNO2py-NH2OHNH3-700x141.jpeg)
(iv) Geometrical isomerism cannot occur in complexes of the type MA4 , MA3B or MAB3 because all possible spatial arrangements for any of these complexes will be exactly equivalent.
(v) The square planar complexes containing unsymmetrical bidentate ligands such as [M(AB)2] also show geometrical isomerism. For example, the complex [Pt(gly)2] where gly = NH2CH2COO¯ exists in cis and trans form :
![cis and trans isomers of [Pt)gly)2]](https://classnotes.org.in/wp-content/uploads/cis-and-trans-isomers-of-Ptgly2-700x186.jpeg)
(vi) Geometrical isomerism is also shown by bridged binuclear complexes of the type M2A2X4. For example: the complex [PtCl2 P(C2H5)3]2 exhibits geometrical isomers as :
![cis [PtCl2 P(C2H5)3]2](https://classnotes.org.in/wp-content/uploads/cis-PtCl2-PC2H532-700x213.jpeg)
![Trans [PtCl2 P(C2H5)3]2](https://classnotes.org.in/wp-content/uploads/Trans-PtCl2-PC2H532-700x253.jpeg)
(b) Geometrical isomerism in complexes of coordination number 6
The complexes having coordination 6 adopt octahedral geometry.
(1) The octahedral complexes of the type MA4X2, MA2X4 , MA4XY, etc. exhibit geometrical isomerism. Some common examples are :
An octahedral complex [CoCl2(NH3)4]+can exist as cis- and trans- isomers :
![cis and trans isomers of [CoCl2(NH3)4]+](https://classnotes.org.in/wp-content/uploads/cis-and-trans-isomers-of-CoCl2NH34.jpg)
Similarly, the complex [Fe(CN)4(NH3)2]¯ can exist as cis- and trans isomers.
![cis and trans isomer of [Fe(CN)4(NH3)2]-](https://classnotes.org.in/wp-content/uploads/cis-and-trans-isomer-of-FeCN4NH32--700x335.jpeg)
(2) Octahedral complexes of the type [MA3B3] like [Co(NO2)3(NH3)3] also exist in two geometrical isomers.
![fac and mer isomers of [Co(NO2)3(NH3)3]](https://classnotes.org.in/wp-content/uploads/fac-and-mer-isomers-of-CoNO23NH33-300x201.jpg)
When the three ligands (with same donor atoms) are on the same triangular face of the octahedron, the isomer is called facial or fac isomer.
When the three ligands are on the same equatorial plane of the octahedron i.e., around the meridian of the octahedron, the isomer is called meridional or mer isomer.
In facial isomer, the three ligands are at the corners of a triangular face while in meridional isomer, the three ligands are at the three corners of a square plane.
[RhCl3(py)3] exist as fac and mer isomers.
(3) Octahedral complexes having didentate ligands of the type M(AA)2X2and M(AA)2XY can also exist as cis and trans isomers, where AA represents a symmetrical bidentate ligand such as ethylenediamine (en), oxalate ion (ox).For example, an octahedral complex [Co(en)2Cl2]+ exists as two isomers :
![[Co(en)2Cl2]+](https://classnotes.org.in/wp-content/uploads/Coen2Cl2-700x289.jpeg)
(4) Octahedral complexes having six different ligands of the type M(ABCDEF) would exhibit geometrical isomerism. These isomers may be written by fixing a ligand at one position and then placing the other ligands trans to it.
15 different isomers are possible for such type of complexes. The only compound of this type that has been prepared is [Pt(Br)(Cl)()(NO2)(py)(NH3)].
(5) The complexes containing unsymmetrical bidentate ligands also show geometrical isomerism. For example: the complex triglycinatochromium (III), [Cr(gly)3], where gly is H2NCH2COO¯, exists in cis and trans forms.
(2) Optical isomerism
The isomers which rotate the plane of polarised light equally but in opposite direction are called optically active isomers. These are also called enantiomorphs.
The isomer which rotates the plane of polarised light to the right is called dextro rotatory designated as (d) and the one which rotates the plane of polarised light to the left is called laevo rotatory designated as (l). A 1:1 equilibrium mixture of d and l isomers gives a net zero rotation and is also called racemic mixture.
The d and l isomers are mirror images of each other just as left hand is mirror image of the right hand. These mirror images compounds are non-superimposable on each other and do not possess the plane of symmetry.
These optical isomers also possess the property of chirality (handedness). The essential condition for a substance to show optical activity is that the substance should not have a plane of symmetry in its structure. The optical isomers have identical physical and chemical properties. They differ only in the direction in which they rotate the plane of polarised light.
The common examples of complexes showing optical isomerism are octahedral complexes having bidentate ligands.
The common examples of complexes showing optical isomerism are octahedral complexes having bidentate ligands.
For example,
(i) Complexes of the type M(AA)3 (where AA is symmetrical bidentate ligands) such as [Co(en)3]3+ and [Cr(ox)3]3- exist as optical isomers.
![[Co(en)3]3+ and [Cr(ox)3]3-](https://classnotes.org.in/wp-content/uploads/Coen33-and-Crox33--700x445.jpeg)
(ii) Complexes of the type [M(AA)2X2] or [M(AA)2XY] (where AA is symmetrical bidentate and X and Y are monodentate ligands) also exhibit optical activity.
Some common examples are [CoCl2(en)2]+, [RhCl2(en)2]+, etc.
![trans[CoCl2(en)2]+](https://classnotes.org.in/wp-content/uploads/transCoCl2en2-700x281.jpeg)
![cis-[CoCl2(en)2]+](https://classnotes.org.in/wp-content/uploads/cis-CoCl2en2-1-700x302.jpeg)
The trans form does not show optical isomerism, i.e., it cannot be resolved into optical isomers. The reason is that the molecule has a plane of symmetry. On the other hand, the cis-isomer is unsymmetrical and can be resolved into optical isomers.
The coordination entity, [PtCl2(en)2]2+ forms cis and trans-isomers. Only the cis-isomers show optical activity.
![[PtCl2(en)2]2+](https://classnotes.org.in/wp-content/uploads/PtCl2en22-700x478.jpeg)
(iii) Complexes of the type [M(AA)X2Y2] containing one symmetrical bidentate ligand show optical isomerism. For example [CoCl2(en)(NH3)2]+ exists in d- and l- forms.
(iv) Octahedral complexes containing hexadentate ligands such as ethylenediaminetetracetato (EDTA) also show optical isomerism.
Example : [Co(edta)]¯ exists as two optical isomers.
![Optical isomers of [Co(edta]-](https://classnotes.org.in/wp-content/uploads/Optical-isomers-of-Coedta--700x446.jpeg)
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