Isomerism in Coordination Compounds

Compounds that have the same molecular formula but differ in their properties due to a difference in their structure are called Isomers.

Isomerism in Coordination Compounds refers to the phenomenon where coordination compounds have the same chemical formula but differ in the arrangement of atoms or groups around the central metal atom or ion.

1.0Types of Isomerism in Coordination Compounds

Isomerism is an important concept in coordination chemistry because different isomers can exhibit distinct chemical, physical, and biological properties. Isomerism in coordination compounds can be broadly classified into two main types: structural isomerism and stereoisomerism.

2.0Structural Isomerism

Structural isomerism arises when isomers have the same molecular formula but differ in the connectivity of atoms. In coordination compounds, structural isomerism can be further divided into several types:

a. Ionization Isomerism:

• Ionization isomers occur when the exchange of ligands between the inner coordination sphere and the outer ionic sphere results in different compounds that produce different ions in solution.
• Example: [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br. In the first compound, bromide (Br⁻) is within the coordination sphere, and sulfate (SO₄²⁻) is outside. In the second compound, sulfate is inside, and bromide is outside.

b. Coordination Isomerism:

• Definition: Coordination isomers arise in compounds containing both cationic and anionic coordination complexes when the ligands are exchanged between the cation and anion.
• Example: [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆]. In the first complex, cobalt is coordinated with ammonia, and chromium is coordinated with cyanide. In the second, the metals swap their ligands.

c. Linkage Isomerism:

• Definition: Linkage isomers occur when a ligand can coordinate to the metal center through different atoms. This is seen in ligands that have multiple donor atoms.
• Example: The ligand nitrite (NO₂⁻) can bind through nitrogen, forming a nitro complex ([Co(NO₂)(NH₃)₅]²⁺), or through oxygen, forming a nitrito complex ([Co(ONO)(NH₃)₅]²⁺).

d. Coordination Position Isomerism:

• Definition: This type of isomerism is observed in polynuclear coordination compounds where the composition of ligands attached to different metal atoms is interchanged.
• Example: [CrCl₂(NH₃)₄]⁺ and [Cr(NH₃)₄Cl₂]⁺ are coordination position isomers with the same ligands but in different arrangements around the central metal ions.

3.0Stereoisomerism

Stereoisomerism arises when isomers have the same chemical bonds but differ in the spatial arrangement of ligands around the central metal ion. Stereoisomerism can be categorized into two main types:

Geometrical Isomerism

Geometrical isomers differ in the spatial arrangement of ligands around the central metal atom or ion, which leads to different physical and chemical properties.

Types:

• Cis-Trans Isomerism: Observed in square planar and octahedral complexes. In cis isomers, similar ligands are adjacent to each other, while in trans isomers, similar ligands are opposite each other.

Example (Square Planar): 

[Pt(NH₃)₂Cl₂] can exist as cis- [Pt(NH₃)₂Cl₂] (where both NH₃ ligands are next to each other) or trans- [Pt(NH₃)₂Cl₂] (where NH₃ ligands are opposite each other).

Example (Octahedral): [Co(NH₃)₄Cl₂]⁺ can exist in cis form, where both chloride ligands are adjacent, or trans form, where chloride ligands are opposite each other.

• Facial (fac) and Meridional (mer) Isomerism: Observed in octahedral complexes with three ligands of one type and three of another. In facial (fac) isomerism, three identical ligands occupy one face of the octahedron, while in meridional (mer) isomerism, three identical ligands occupy a meridian of the octahedron.

Example: [Co(NH₃)₃(NO₂)₃]. The fac isomer has the three NO₂ groups on the same face, and the mer isomer has them arranged along a meridian.

Square Planar Complexes (Coordination Number = 4)

1. Ma₂b₂ Complexes: Can have cis (adjacent identical ligands) and trans (opposite identical ligands) isomers, 

e.g., [Pt(NH₃)₂Cl₂].

2. Ma₂bc Complexes: Also exhibit cis and trans isomers with two identical and two different ligands, 

e.g., [Pt(NH₃)₂ClBr].

3. M_abcd Complexes: Have three possible isomers due to all four ligands being different, 

e.g., [Pt(NH₃)(Py)ClBr].

4. [M(AB)₂] Complexes: With unsymmetrical bidentate ligands (e.g., glycine), have two isomers based on ligand orientation,

e.g., [Pt(gly)₂].

Octahedral Complexes (Coordination Number = 6)

1. [Ma₄b₂] Complexes: Two isomers (cis and trans) based on the arrangement of two identical ligands.

e.g., [Fe(NH₃)₄Cl₂].

2. [Ma₄bc] Complexes: Also have cis and trans forms with four identical and two different ligands.

e.g., [Fe(NH₃)₄ClCO₃].

3. [Ma₃b₃] Complexes: Exhibit facial (fac) and meridional (mer) isomers, depending on whether three identical ligands form a triangular face or are aligned in a plane.

e.g., [Cr(NH₃)₃(H₂O)3]⁺³.

Optical Isomerism

• Optical isomers (enantiomers) are non-superimposable mirror images of each other, similar to how left and right hands are mirror images but cannot be perfectly overlaid on each other.
• These isomers have identical physical and chemical properties, except for their ability to rotate plane-polarized light in opposite directions. One isomer will rotate light clockwise (dextrorotatory, or "d"), and the other will rotate light counterclockwise (levorotatory, or "l").
• Requirements: Optical isomerism is observed in complexes that lack a plane of symmetry. It is commonly seen in tetrahedral complexes with four different ligands or octahedral complexes with bidentate ligands arranged asymmetrically.
• Example: [Cr(ox)₃]³⁻, where "ox" is the oxalate ligand (C₂O₄²⁻), can exist as two non-superimposable mirror images, each rotating polarized light in opposite directions.

Optical Isomerism in Octahedral Complexes with Coordination Number = 6

For an octahedral complex to exhibit optical isomerism, its structure must lack a plane of symmetry, a center of symmetry, or an improper axis of rotation. This is usually possible when the ligands are arranged in such a way that a mirror image of the complex cannot be superimposed onto the original.

1. Octahedral Complex with the General Formula [Ma_2​b₂​c₂​]ⁿ⁺ 

Example: [Pt(py)₂(NH₃)₂Cl₂]²⁺

2. Octahedral Complex with the General Formula [M_abcdef]

Example: [Pt(py)(NH₃)(NO₂)ClBrI]