Coordination compounds can form various isomers, which have the same formula but different structures. These isomers include structural types like ionization and linkage isomers, and stereoisomers like geometrical and optical isomers.

Understanding isomerism in coordination compounds is crucial for predicting their properties and reactivity. This knowledge helps chemists design and synthesize complexes with specific characteristics for applications in catalysis, materials science, and medicine.

Structural vs Stereoisomers in Coordination Compounds

Distinguishing Characteristics

• Structural isomers have the same chemical formula but differ in the connectivity of atoms or arrangement of ligands around the central metal ion
• Stereoisomers have the same connectivity of atoms but differ in the spatial arrangement of ligands around the central metal ion
• Structural isomers have different physical and chemical properties (melting point, solubility, reactivity)
• Stereoisomers have identical chemical properties but may have different physical properties (optical activity, dipole moment)

Examples

• Structural isomers: [Co(NH3)5Cl]2+ and [Co(NH3)4Cl2]+ have the same formula but different ligand arrangements
• Stereoisomers: cis-[PtCl2(NH3)2] and trans-[PtCl2(NH3)2] have the same connectivity but different spatial arrangements of ligands

Types of Structural Isomers

Ionization Isomers

• Differ in the distribution of counter ions between the complex ion and the outer coordination sphere
• Example: [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br are ionization isomers
• Counter ions can be located either within the coordination sphere or outside as free ions

Linkage Isomers

• Differ in the atom of the ligand that is bound to the central metal ion
• Example: [Co(NH3)5(NO2)]2+ and [Co(NH3)5(ONO)]2+ are linkage isomers
• Ambidentate ligands (SCN-, NO2-, ONO-) can bind through different donor atoms

Coordination Isomers

• Differ in the distribution of ligands between two or more metal centers in a complex
• Example: [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6] are coordination isomers
• Ligands can be distributed differently among multiple metal centers

Solvate Isomers

• Differ in whether a solvent molecule is directly bound to the metal center or present as a solvate outside the coordination sphere
• Example: [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O are solvate isomers
• Solvent molecules can be coordinated to the metal or present as lattice molecules

Geometrical vs Optical Isomers

Geometrical Isomers (Cis-Trans Isomers)

• Differ in the spatial arrangement of ligands around the central metal ion
• Example: square planar complexes [MA2B2] can have cis (adjacent) or trans (opposite) arrangements of A and B ligands
• Geometrical isomers have different physical properties (dipole moment, solubility)
• Occur in square planar and octahedral complexes with certain ligand combinations

Optical Isomers (Enantiomers)

• Non-superimposable mirror images of each other
• Have identical physical and chemical properties, except for their interaction with plane-polarized light
• Enantiomers rotate plane-polarized light in opposite directions with equal magnitude
• Separated using techniques like fractional crystallization with optically active salts or chiral chromatography

Diastereomers

• Stereoisomers that are not mirror images of each other
• Have different physical and chemical properties
• Example: cis-[CoCl2(en)2]+ and trans-[CoCl2(en)2]+ are diastereomers
• Can be separated by conventional separation methods (crystallization, chromatography)

Chirality in Coordination Compounds

Definition and Requirements

• Chirality is the property of a molecule being non-superimposable on its mirror image
• Chiral molecules lack an internal plane of symmetry, center of symmetry, or alternating axis of symmetry
• Coordination compounds can be chiral due to the spatial arrangement of ligands around the central metal ion

Examples of Chiral Coordination Compounds

• Octahedral complexes [Ma3b3] with propeller-like configuration (Δ or Λ)
• Complexes with chelating or bidentate ligands (ethylenediamine, oxalate)
• Tetrahedral complexes [Ma2b2] with non-equivalent ligands

Separation of Enantiomers

• Fractional crystallization with optically active salts (sodium tartrate, brucine)
• Chromatography with chiral stationary phases (cellulose, cyclodextrin)
• Enantiomers have identical physical properties, so conventional separation methods are not applicable