Isomerism in Coordination Compounds Preparation and Properties
Isomerism in Coordination Compounds :
→ Compounds having the same molecular formula but differ in structures or spatial arrangements are called isomers and the phenomenon is referred to as isomerism.
→ Isomerism is commonly considered in organic compounds but in coordination compounds also. Isomers have same chemical formula but different chemical properties.
→ Structural isomerism These isomers have different arrangement of ligands around the central metal atom/ion. It is of the following types: 1. Ionization Isomerism : This type of isomerism is due to the exchange of groups or ions between the coordination sphere and the ionisation sphere.
Example : Molecular formula Pt(NH3)4 Cl2 Br2 can represent three ionisation isomers:
Such types of isomers yield different ions in solution:
→ Hydrate Isomerism : Compounds which have the same molecular composition but differ in the number of water molecules present as ligands are called hydrate isomers and phenomenon is known as hydrate isomerism.
→ Water is such a neutral species, which may appear in variety of ways. It may coordinate to metal ions or it may appear in interstitial lattice position. So, when different number of water molecules are present inside and outside the coordination sphere, this isomerism is arised.
Example : Molecular formula Cocl, 61,0 can represent following four hydrate isomers:
→ Coordination Isomerism: This type of isomerism is observed in the coordination compounds having both cationic and anionic complex ions. The ligands are interchanged in both the cationic and anionic ions to form isomers.
→ Linkage isomerism or Salt isomerism : This type of isomerism arises due to the presence of two different donor atom in the same ligand which may thus attack through either of the two atoms ie, this type of isomerism occurs in complex compounds which contain ambidentate ligands like NO2–, CN–, SCN– etc.
→ Ligand Isomerism: Those co-ordination compounds in which ligands itself show isomerism are known as ligand isomerism.
→ Coordination Position Isomerism: This isomerism occurs in coordination compounds having bridging ligands where molecular formula is same but bonding of ligand is different in structural formula.
→ Stereo Isomerism : Compounds are stereoisomers when they contain same ligand in their co-ordination sphere but differ in the way that these ligands are arranged in space. Stereo isomerism is of two types:
→ Geometrical Isomerism : This isomerism is due to the ligands occupying different position around the central metal atom.
→ When identical ligands occupy positions near to each other called cis-isomers and when identical ligands occupy opposite to each other called trans-isomers. This type of isomerism is also known as cis-trans isomerism.
→ This isomerism is not possible with co-ordination number 2, 3 and 4 (tetrahedral complexes).
→ It is quite common in coordination number 4 (Square planar complexes) and 6 (octahedral complexes).
In square planar Complexes
- cis-isomers:When same ligands are at positions 1 – 2, 2 – 3, 3 – 4, 1 – 4.
- trans-isomers: When same ligands are at positions 1 – 3, 2 – 4.
→ Complexes with general formula Ma2b2 (where a and bare monodentate ligands) can show : Example:
→ Complexes with general formula Ma2 bc can show
→ Complexes with general formula Mabcd can have three geometrical isomers:
→ Square planar complexes having unsymmetrical bidentate ligands i.e., complexes with general formula Mab), (where (ab) is unsymmetrical ligand) can also show geometrical isomerism.
→ Square planar complexes having general formula Ma4, Ma3b, M(aa)2, M(aa)b2 does not show this isomerism.
→ In octahedral Complexes : In these complexes, the metal atom lies at the centre and centre of octahedral and 1 to 6 positions are occupied by the ligands.
→ cis-positions: 1 – 2, 1 – 3, 1 – 4, 1 – 5, 2 – 6, 3 – 6, 4 – 6, 5 – 6, 2 – 3, 3 – 4, 4 – 5, 5 – 2 etc.
trans-positions : 1-6, 2-4, 3-5 etc.
→ Complexes with general formula Ma4 b2 (where a and bare monodentate ligands) can show :
Example : [Pt(NH3)4 Cl2]2+, [Co(H2O), Cl2]
→ Complexes with general formula Ma, b, (where a and bare monodentate ligands) can show:
Example: [Pt(NH3)3 Cl3]+, [Co(H2O)3 Cl3]
→ In facial isomer, all similar ligands occupy positions on a plane while not in meridianal isomer.
→ Complexes with general formula M(aa)2 b2 or M(aa)2 bc can also exist as cis and trans-isomer.
Example : [Pt(en)2Cl2]+2, [Co(ox)2 (NH3)H2O]+3
→ Complexes with general formula Ma2b2c2 Ma2 b2 cd, M(aa)b2 c2, M(ab)2 etc. can also show this isomerism.
- Octahedral complexes of general formula Mabcdef can have 15 geometrical isomers.
- The complexes with general formula Ma6, Ma5b, do not show geometrical isomerism.
→ Optical Isomerism : When the coordination compound have same formula but differ in their ability to rotate direction of the plane polarised light are said to exhibit optical isomerism.
→ Isomer which rotates the plane polarised light to the right is called dextrorotatory and which rotates the plane polarised light to the left is called laevo rotatory. The optically active isomers are collectively called enantiomers. Enantiomers are non-superimposable mirror images to each other.
→ The essential requirement for a substance to be optically active is that the substance should not have an element of symmetry in its structure.
→ Optical isomerism rarely occur in square planar complexes on account of the presence of axis of symmetry.
→ Optical isomerism is expressed in tetrahedral complexes of the type Mabcd but no optical isomer has been isolated until now. However, compounds containing two unsymmetrical bidentate ligands have been dissolved in two isomers. The presence of bidentate ligand for slowing optical isomerism is compulsary. But some exceptionsare also there.
Example : Bis-benzoylacetonato-beryllium (II)
→ Optical isomerism is very common in octahedral complex containing poly dentate ligands.
Octahedral complexes of general formula M(aa)3, M(aa)2b2, M(aa)2ab, M(aa)b2c2, Ma2b2c2, Mabcdef show optical isomers,
→ Example : Ma2b2c2 type:
→ Example : Mabcdef type
→ Example : M(aa)3 type
→ Example: M(aa)2 b2 type
→ Transform of M(aa)2 b2 does not show optical isomerism.
Example: M(aa)2 bc type
→ Example : M(ab)3 type
→ Example : M(aa)b2 c2 type