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Isomerism in Coordination Complexes MCQ - Practice Questions with Answers

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  • Stereoisomerism, Structural Isomerism - 1, Structural Isomerism - 2 is considered one of the most asked concept.

  • 61 Questions around this concept.

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Number of possible isomers for the complex [Co(en)2Cl2] CI will be:    (en = ethylenediamine)

The existence of two different colored complexes with the composition of \left [ Co(NH_{3})_{4}Cl_{2} \right ]^{+} is due to

Which of the following complexes is used as an anti-cancer agent:

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Number of possible isomers for the complex \left [ Co(en)_{2}Cl_{2} \right ]Cl will be (en = ethylenediamine)

Which of the following complexes is used to be as anticancer agent ?

Which one of the following has an optical isomer?

Which of the following molecules is expected to rotate the plane­polarised light?

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Which of the following has an optical isomer ?

The complexes \left [ Co(NH_{3})_{6} \right ] \left [ Cr(CN)_{6} \right ] and \left [ Cr(NH_{3})_{6} \right ] \left [ Co(CN)_{6} \right ]  are examples of which type of isomerism? 

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Which of the following pairs represents linkage isomers ?

Concepts Covered - 4

Stereoisomerism

Geometrical Isomerism
This isomerism is due to ligands occupying different positions around the central metal atom or ion. The ligands occupy positions either adjacent or opposite to one another. This type of isomerism is also known as cis-trans isomerism. When two same ligands are at right angle(90o), the form is cis- form and when they are present diagonally at 180o to each other, the form is termed as trans- from. Geometrical isomerism is very common in coordination number 4 and 6 complexes.

Another type of geometrical isomerism occurs in octahedral coordination entities of the type [Ma_3b_3] like [[Co(NH_3)_3(NO_2)_3]]. If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer. When the positions are around the meridian of the octahedron, we get the meridional (mer) isomer.

Optical Isomerism
Optical isomers 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 in a polarimeter (d rotates to the right, l to the left). Optical isomerism is common in octahedral complexes involving didentate ligands.

Structural Isomerism - 1

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. Jørgensen discovered such behaviour in the complex [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).

Coordination Isomerism
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+.

Structural Isomerism - 2

Ionisation Isomerism
This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. An example is provided by the ionisation isomers [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4.

Solvate Isomerism
This form of isomerism is known as ‘hydrate isomerism’ in case where water is involved as a solvent. This is similar to ionisation isomerism. Solvate isomers 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. An example is provided by the aqua complex [Cr(H2O)6]Cl3 (violet) and its solvate isomer [Cr(H2O)5Cl]Cl2.H2O (grey-green).

Stability of Complexes

The stability of a complex in solution refers to the degree of association between the two species involved in the state of equilibrium. The magnitude of the equilibrium constant (stability or formation) for the association, quantitatively expresses the stability. Thus, if we have a reaction of the type:

\mathrm{M}+4 \mathrm{L} \rightleftharpoons \mathrm{ML}_{4}

then the larger the stability constant, the higher the proportion of ML4 that exists in solution. Free metal ions rarely exist in the solution so that M will usually be surrounded by solvent molecules which will compete with the ligand molecules, L, and be successively replaced by them. For simplicity, we generally ignore these solvent molecules and write four stability constants as follows:

\mathrm{M}+\mathrm{L} \rightleftharpoons \mathrm{ML} \quad K_{1}=[\mathrm{ML}] / \mathrm{[M]} \mathrm{[L]}

\mathrm{ML}+\mathrm{L} \rightleftharpoons \mathrm{ML}_{2} \quad K_{2}=\left[\mathrm{ML}_{2}\right] / \mathrm{[ML]}[\mathrm{L}]

\mathrm{ML}_{2}+\mathrm{L} \rightleftharpoons \mathrm{ML}_{3} \quad K_{3}=\left[\mathrm{ML}_{3}\right] /\left[\mathrm{ML}_{2}\right][\mathrm{L}]

\mathrm{ML}_{(\mathrm{n}-1)}+\mathrm{L} \rightleftharpoons \mathrm{ML}_{\mathrm{n}} \quad K_{\mathrm{4}}=\left[\mathrm{ML}_{\mathrm{n}}\right] /\left[\mathrm{ML}_{\mathrm{(n-1)}}\right][\mathrm{L}]

where K1, K2, ....Kn, etc., are stepwise stability constants. The overall stability constant (β) of the formation of species MLn from M and L can be given as:

\mathrm{M}+\mathrm{nL} \rightleftharpoons \mathrm{ML}_{\mathrm{n}} \quad \beta=\left[\mathrm{ML}_{\mathrm{n}}\right] /[\mathrm{M}] \mathrm{[L]}^{\mathrm{n}}

The stepwise and overall stability constant are therefore related as follows:

\beta_{\mathrm{n}}=K_{1} \times K_{2} \times K_{3} \times K_{4} \ldots \ldots . K_{\mathrm{n}}

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Stereoisomerism
Structural Isomerism - 1
Structural Isomerism - 2

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Reference Books

Stereoisomerism

Chemistry Part I Textbook for Class XII

Page No. : 251

Line : 26

Structural Isomerism - 1

Chemistry Part I Textbook for Class XII

Page No. : 253

Line : 26

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