289.

Illustrate with an example each of the following terms: (i) Ionization isomerism, (ii) coordination isomerism, (iii) Linkage isomerism, (iv) Geometrical isomerism (v) Optical isomerism.

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i) Ionization isomers: Ionization isomers are identical except for a ligand has exchanging places with an anion or neutral molecule that was originally outside the coordination complex. The central ion and the other ligands are identical. For example  the two compounds, pentaamminebromocobalt sulfate, [CoBr(NH₃)₅]SO₄, and pentaamminesulfatocobalt bromide, [Co(SO₄)(NH3)₅]Br.

ii) Coordination isomers: This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex. An example is provided by [Co(NH₃)₆][Cr(CN)₆], in which the NH₃ ligands are bound to Co³⁺ and the CN– ligands to Cr³⁺. In its coordination isomer [Cr(NH₃)₆][Co(CN)₆], the NH₃ ligands are bound to Cr³⁺ and the CN^– ligands to Co³⁺.

iii) Linkage isomerism: Linkage isomerism arises in a coordination compound containing ambidentate ligand. A simple example is provided by complexes containing the thiocyanate ligand, NCS–, which may bind through the nitrogen to give M–NCS or through sulphur to give M–SCN. For example  [Co(NH₃)₅(NO₂)]Cl₂, which is obtained as the red form, in which the nitrite ligand is bound through oxygen (–ONO), and as the yellow form, in which the nitrite ligand is bound through nitrogen (–NO₂).

iv) Geometrical Isomerism:  Geometrical isomerism arises in heteroleptic complexes due to different possible geometric arrangements of the ligands. Important examples of this behaviour are found with coordination numbers 4 and 6. In a square planar complex of formula [MX₂L₂] (X and L are unidentate), the two ligands X may be arranged adjacent to each other in a cis isomer, or opposite to each other in a trans. For example Cis -trans isomer of [Co(NH₃)₄Cl₂]⁺.



v) Optical isomerism: Optical isomerism  are mirror images that cannot be superimposed on one another. These are called as enantiomers. The molecules or ions that cannot be superimposed are called chiral. The two forms are called dextro (d) and laevo (l) depending upon the direction they rotate the plane of polarised light. For example [Co(en)₃]³⁺.