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1 Solutions 2 Electrochemistry 3 Chemical Kinetics 4 The d-and f-Block Elements 5 Coordination Compounds 6 Haloalkanes and Haloarenes 7 Alcohols, Phenols and Ethers 8 Aldehydes, Ketones and Carboxylic Acids 9 Amines 10 Biomolecules

5 Coordination Compounds

5.1 Werner's Theory of Coordination Compounds 5.2 Definitions of Some Important Terms Pertaining to Coordination Compounds 5.3 Nomenclature of Coordination Compounds 5.4 Isomerism in Coordination Compounds 5.5 Bonding in Coordination Compounds 5.6 Bonding in Metal Carbonyls 5.7 Importance and Applications of Coordination Compounds

Isomerism in Coordination Compounds

NCERT Reference: Chapter 5 – Coordination Compounds – Page 126–129

Quick Notes

  • Isomers are compounds with the same molecular formula but different arrangements of atoms.
  • Coordination compounds show two major types of isomerism:
    • Structural isomerism – differs in the bonding arrangement.
    • Stereoisomerism – differs in the spatial arrangement of ligands.
  • Structural isomerism includes:
    • Linkage isomerism
    • Coordination isomerism
    • Ionisation isomerism
    • Solvate (hydrate) isomerism
  • Stereoisomerism includes:
    • Geometrical isomerism
    • Optical isomerism

Full Notes

Isomerism is common in coordination compounds, arising from the different possible arrangements of ligands around a central atom.

Such isomerism falls into two main categories: stereoisomerism (including geometrical isomerism and optical isomerism) and structural isomerism (including Linkage Isomerism, Coordination Isomerism, Ionisation Isomerism and Solvate Isomerism).

Recap - stereoisomers have the same formula and connectivity but differ in the spatial arrangement of their atoms.

Geometrical Isomerism

Occurs due to the different possible positions of ligands around the central metal atom in space.

Square planar complexes:

Ligands can be arranged adjacent (cis) or opposite (trans) to each other.

NCERT Chemistry Class 12 — square planar complex diagram showing relative ligand positions for geometrical isomerism.

Common with Pt(II) and Ni(II) complexes (coordination number 4):

For Example [Pt(NH3)2Cl2]

Cis-isomer: NH3 ligands or Cl− ligands next to each other.

Trans-isomer: NH3 and Cl− ligands opposite each other.

NCERT Chemistry Class 12 — cis and trans square planar isomers of Pt(NH3)2Cl2.

Octahedral complexes:

NCERT Chemistry Class 12 — octahedral complex cis and trans positions for ligands.

Example [Co(NH3)4(H2O)2]2+

NCERT Chemistry Class 12 — octahedral complex [Co(NH3)4(H2O)2]2+ showing cis and trans arrangements.

Cis-isomer: Two H2O molecules adjacent (90° apart).

Trans-isomer: Two H2O molecules opposite (180° apart).

Example [Ni(H2NCH2CH2NH2)2(H2O)2]2+

NCERT Chemistry Class 12 — cis and trans isomers of Ni(en)2(H2O)2 2+ octahedral complex.

Cis = two H2O on same side

Trans = two H2O on opposite sides

Fac-Mer isomerism:

Octahedral complexes with the formula [MA3B3] can show facial and meridional isomers.

If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, it is the facial (fac) isomer. When the positions are around the meridian of the octahedron, it is the meridional (mer) isomer.

NCERT Chemistry Class 12 — fac and mer isomers in an octahedral MA3B3 complex.

Fac-isomer: Three similar ligands occupy one face of the octahedron.

Mer-isomer: Similar ligands occupy a meridional plane (like a meridian line).

Optical Isomerism

Occurs when a compound can exist as non-superimposable mirror images, called enantiomers.

NCERT Chemistry Class 12 — optical isomerism showing non-superimposable mirror images of coordination complexes.

These isomers rotate the plane of polarised light:

Common in:

For Example [Ni(en)3]2+

→ Exists as a pair of optical isomers.

IB Chemistry Class 12 — [Ni(en)3]2+ optical isomers (d- and l-forms).

Note: Square planar complexes do not show optical isomerism due to planar symmetry.

Linkage Isomerism

Occurs when an ambidentate ligand (a ligand that can coordinate through two different atoms) bonds to the metal in different ways.

Ligand Possible donor atoms
NO2− through N (nitro) or O (nitrito)
SCN− through S (thiocyanato-S) or N (isothiocyanato-N)
CN− through C (cyano) or N (isocyano)

Example [Co(NH3)5(NO2)]Cl2

When NO2− is bonded through N = nitro isomer

When NO2− is bonded through O = nitrito isomer

Coordination Isomerism

Occurs when ligands are exchanged between the cationic and anionic coordination entities in a compound.

Requires the presence of both complex cation and complex anion.

Example [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]

Coordination of NH3 and CN− ligands have been swapped between the two metal centres.

Ionisation Isomerism

Occurs when the complex gives different ions in solution, even though the empirical formula is the same.

The exchange of ligands inside and outside the coordination sphere causes this.

Example [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br

[Co(NH3)5Br]SO4 → Gives Br− in solution (SO42− is coordinated)

[Co(NH3)5SO4]Br → Gives SO42− in solution (Br− is coordinated)

Solvate Isomerism

Also called hydrate isomerism when water is involved.

Occurs when solvent molecules (e.g. H2O) are present either inside or outside the coordination sphere.

Example Hydrated chromium(III) complexes

[Cr(H2O)6]Cl3
All 6 H2O ligands are coordinated

[Cr(H2O)5Cl]Cl2·H2O
5 H2O coordinated, 1 outside the coordination sphere

[Cr(H2O)4Cl2]Cl·2H2O
4 H2O coordinated, 2 free water molecules

These three compounds have the same empirical formula but differ in the position of the solvent molecules.

Summary