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Coordination Compounds - Isomerism in coordination compounds

Grade 12ICSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Isomerism in coordination compounds refers to the existence of compounds with the same molecular formula but different structural arrangements or spatial orientations, leading to different physical and chemical properties.

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Structural Isomerism: Arises due to differences in the chemical bonds or the distribution of ligands. Types include:

  1. Ionization Isomerism: Exchange of ions between the coordination sphere and the ionization sphere (e.g., [Co(NH3)5SO4]Br[Co(NH_3)_5SO_4]Br and [Co(NH3)5Br]SO4[Co(NH_3)_5Br]SO_4).
  2. Solvate/Hydrate Isomerism: Variation in the number of solvent molecules (usually H2OH_2O) inside the coordination sphere (e.g., [Cr(H2O)6]Cl3[Cr(H_2O)_6]Cl_3 vs. [Cr(H2O)5Cl]Cl2⋅H2O[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O).
  3. Linkage Isomerism: Occurs with ambidentate ligands like NO2−NO_2^-, SCN−SCN^-, or CN−CN^- which can bind through different atoms (e.g., M−NO2M-NO_2 nitrito-N and M−ONOM-ONO nitrito-O).
  4. Coordination Isomerism: Occurs in compounds containing both cationic and anionic complex ions, where ligands are exchanged between the two centers (e.g., [Co(NH3)6][Cr(CN)6][Co(NH_3)_6][Cr(CN)_6] and [Cr(NH3)6][Co(CN)6][Cr(NH_3)_6][Co(CN)_6]).
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Stereoisomerism: Arises due to different spatial arrangements of ligands around the central metal atom. It is divided into Geometrical and Optical isomerism.

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Geometrical Isomerism: Common in coordination numbers 4 (Square Planar) and 6 (Octahedral).

  • Square Planar: Complexes of type [MA2B2][MA_2B_2] show ciscis (adjacent) and transtrans (opposite) forms. Tetrahedral complexes do not show geometrical isomerism.
  • Octahedral: Complexes of type [MA4B2][MA_4B_2] show cis/transcis/trans. Complexes of type [MA3B3][MA_3B_3] show Facial (facfac) and Meridional (mermer) isomerism.
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Optical Isomerism: Occurs when a molecule and its mirror image are non-superimposable (chiral). They are called enantiomers: dd (dextrorotatory) and ll (laevorotatory). This is most common in octahedral complexes containing chelating bidentate ligands like ethylenediamine (enen).

📐Formulae

[MA2B2] (Square Planar) → cis and trans isomers[MA_2B_2] \text{ (Square Planar) } \rightarrow \text{ cis and trans isomers}

[MA4B2] (Octahedral) → cis and trans isomers[MA_4B_2] \text{ (Octahedral) } \rightarrow \text{ cis and trans isomers}

[MA3B3] (Octahedral) → fac and mer isomers[MA_3B_3] \text{ (Octahedral) } \rightarrow \text{ fac and mer isomers}

[M(AA)3]n± (Octahedral with bidentate ligands) → Optical isomers (d and l)[M(AA)_3]^{n\pm} \text{ (Octahedral with bidentate ligands) } \rightarrow \text{ Optical isomers (d and l)}

💡Examples

Problem 1:

Identify the type of isomerism exhibited by the pair: [Co(NH3)5(NO2)]Cl2[Co(NH_3)_5(NO_2)]Cl_2 and [Co(NH3)5(ONO)]Cl2[Co(NH_3)_5(ONO)]Cl_2.

Solution:

Linkage Isomerism.

Explanation:

The ligand NO2−NO_2^- is an ambidentate ligand. In the first complex, it is coordinated through the Nitrogen atom (M−NO2M-NO_2), and in the second, it is coordinated through the Oxygen atom (M−ONOM-ONO).

Problem 2:

How many geometrical isomers are possible for the octahedral complex [Co(NH3)3(NO2)3][Co(NH_3)_3(NO_2)_3]?

Solution:

2 isomers: Facial (facfac) and Meridional (mermer).

Explanation:

In the facfac-isomer, the three NH3NH_3 ligands occupy the three corners of one octahedral face. In the mermer-isomer, the three NH3NH_3 ligands occupy positions around the meridian of the octahedron (two trans and one cis).

Problem 3:

Why does [Co(en)3]3+[Co(en)_3]^{3+} show optical isomerism while [Co(NH3)6]3+[Co(NH_3)_6]^{3+} does not?

Solution:

[Co(en)3]3+[Co(en)_3]^{3+} is chiral and lacks a plane of symmetry, whereas [Co(NH3)6]3+[Co(NH_3)_6]^{3+} is highly symmetrical.

Explanation:

Optical isomerism requires the absence of a plane of symmetry. Chelating ligands like enen (ethylenediamine) in [Co(en)3]3+[Co(en)_3]^{3+} create a helical arrangement that results in non-superimposable mirror images (dd and ll forms). [Co(NH3)6]3+[Co(NH_3)_6]^{3+} is a homoleptic complex with a plane of symmetry, making it achiral.