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explain various isomerism shown by complex compounds with example.

1.Isomerism.

The compounds which have the same chemical composition but have different structural arrangement of their atoms are called isomers  and the phenomenon is called isomerism.

2.Structural isomerism

1. Ionisation isomerism: This arises when the coordination compounds with same molecular formula give different ions in solution. For examples, violet coloured pentaamminebromidocobalt (III) sulphate is an ionisation isomer of red coloured pentaamminesulphatocobalt (III) bromide.
   [Co(NH₃)₅Br] SO₄ (Violet) pentaamminebromidocobalt(III) sulphate
   [Co(NH₃)₅SO₄]Br(Red)   
   pentaamminesulphatocobalt (III) bromide
2. Linkage isomerism : It arises in complexes containing monodentate ligands with more than one donor atoms. example: NO₂^- may be bonded to metal through nitrogen (nitrito-N) or through oxygen (- O - N = O). Such ligands are also known as ambidentate ligands. For example,
   [Co(NH₃)₅NO₂]Cl₂     pentaamminenitrito -N cobalt (III) chloride (Yellow)
   [Co(NH₃)₅ONO]Cl₂    pentaamminenitrito -O cobalt (III) chloride (Red)
   Other common ambidentate ligands are CN^-, CNS^- etc.
3. Coordination isomerism : It involves exchange of ligands between complex cation and complex anion that is, between coordination spheres in a compound. For example,
   [Cr(NH₃)₆] [Co(CN)₆]     hexaamminechromium (III) hexacyanocobaltate (III)
   [Co(NH₃)₆] [Cr(CN)₆]     hexaamminecobalt (III) hexacyanochromate (III)
4. Hydrate isomerism : It arises in complexes due to different number of water molecules present in the coordination spheres. For example,
   [Cr(H₂O)₆]Cl₃ (violet)     hexaaquachromium (III) chloride
   [Cr(H₂O)₅Cl] Cl₂.H₂O (grey green) pentaaquachloridochromium (III) chloride monohydrate

3.Stereo isomerism

When two compounds contain the same ligands coordinated to the same central ion, but the special arrangement of ligands in space is different, the two compounds are said to be stereoisomers and the type of isomerism is called stereoisomerism. It is of two types:

1. Geometrical isomerism or cis-trans isomerism and
2. Optical isomerism.

1. Geometrical isomerism
   Geometrical isomerism arises when ligands occupy different positions around the central ion. The ligands may occupy positions either adjacent to one another or opposite to one another. These are referred to as cis- form and trans form respectively. This type of isomerism is, therefore, referred to as cis-trans isomerism.
   Geometrical isomerism of compounds with coordination numbers 4 and 6 is most important.
   (a) Square planar complexes (coordination no.4) with formula Ma₂b₂ and Ma₂bc show cis-trans isomerism.
   Example: [Pt(NH₃)₂Cl₂]
                                         
   (b) Octahedral complexes(coordination no.6) with formula Ma₄b₂ and Ma₄bc, show cis trans isomerism. Example: [Co(NH₃)₄Cl₂]⁺, [Co(en)₂Cl₂]⁺
                          
   Another type of geometrical isomerism occurs in octahedral complexes of the type [Ma₃ b₃] like [Co(NO₂)₃(NH₃)₃]. If each trio of donor atoms occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer and when the positions are around the meridian of the octahedron, we get the meridional (mer) isomer.
                                 
2. Optical isomerism
   Optical activity is the ability of a compound to rotate the plane polarised light. An optically active compound can be obtained in d- and l- isomeric forms (enantiomers), the d-isomer will rotate the plane of polarized light to the right (dextro) and the l-isomer to the left (laevo). Optical isomerism is shown by the chelates like [Co(en)₃]Br₃. This compound has no plane of symmetry and it gives rise to non-superimposable mirror images.
   
   The optical isomers have identical, physical and chemical properties. They differ only in the direction in which they rotate the plane of plane polarised light.
              
   Note: In coordination entity of the type [PtCl₂(en)₂]²⁺, only the cis isomer shows optical isomerism.