Coordination Compounds

Concepts:

1. Werner’s Theory of Coordination Compounds:

• Werner proposed that in a coordination compound, a central metal ion is surrounded by a fixed number of ligands/ions.
• The ligands/ions are bonded to the metal ion through coordinate covalent bonds.
• The coordination sphere refers to the central metal ion and the ligands/ions that are directly bonded to it.

2. IUPAC nomenclature of coordination compounds:

• Cation’s name followed by anion’s name.
• In cation’s name,
  • write metal’s name
  • use appropriate oxidation state, if necessary
  • write name of ligands preceded by appropriate prefixes.
  • in case a ligand with an attached anionic group is present, the name of the metal can be suffixed with ‘ate’.
• In anion’s name,
  • Use prefix ‘aquo’ for water ligand.
  • Use ‘ammine’ or ‘amine’ for ammonia.
  • When the anion is a complex anion, add ‘ate’ to the name.

3. Isomerism in Coordination Compounds: Structural Isomerism

• Ionization isomerism: Same molecular formula but different ions produced in solution.
• Linkage isomerism: Occurs in coordination compounds containing ambidentate ligands (ligand that can bind through two or more donor atoms). This kind of isomerism results from the attachment of ligands to the metal ion through different donor atoms.
• Coordination isomerism: Ligand can bind to metal in two different ways. -Monodentate ligand to chelating ligand.
• Chelating ligands to monodentate ligand.
• Chelating ligand to chelating ligands with different donor atoms.
• Polymerization isomerism: Occurs when one complex acts as ligand (bridging ligand) to one or more metal complexes.

Stereoisomerism

• Occurs when coordination entities have the same composition and sequence but differ in spatial arrangements.
• It is possible in octahedral and square planar complexes. -Optical isomerism: Enantiomers are mirror images. -Geometrical isomerism: cis and trans-isomers.

4. Bonding in Coordination Compounds

• Crystal Field Theory: Treats the metal-ligand bond as purely ionic. The ligands are considered to be point charges that create an electrostatic field around the metal ion.
• Valence Bond Theory: Treats the metal-ligand bond as covalent, involving the overlap of atomic orbitals.

5. Magnetic Properties of Coordination Compounds

• Coordination compounds can exhibit different magnetic behaviors, depending on the number and arrangement of unpaired electrons in their metal ions.
• Paramagnetism: Compounds with unpaired electrons are attracted to magnetic fields, so exhibit paramagnetic behavior.
• Diamagnetism: Substances with all electrons paired exhibit Diamagnetism.

6. Applications of Coordination Compounds

• Catalysts: Widely used as homogeneous and heterogeneous catalysts in various industrial processes.
• Drugs: Many drugs are coordination compounds containing metal ions like platinum (cisplatin), cobalt (vitamin B12), iron (hemoglobin), etc.
• Pigments: Coordination compounds are responsible for the colors of several minerals and pigments. Examples include Prussian blue, chromium yellow, and many more.