COORDINATION COMPOUNDS PDF|ISC|CBSE|JEE|NEET

Coordination compounds, also known as complex compounds, are molecules or ions formed by the combination of a central metal atom or ion with a number of ligands. Here’s a note covering key aspects:

1. Definition and Formation:

• A coordination compound consists of a central metal atom or ion bonded to a surrounding array of molecules or ions called ligands.
• These bonds are typically coordinate covalent bonds, where the ligand donates a lone pair of electrons to the metal.

2. Key Components:

• Central Metal Atom/Ion:
  • Usually a transition metal, which can exhibit multiple oxidation states and have vacant d-orbitals available for bonding.
  • Acts as a Lewis acid (electron acceptor).
• Ligands:
  • Molecules or ions that donate electron pairs to the central metal.
  • Act as Lewis bases (electron donors).
  • Can be neutral (e.g., H₂O, NH₃, CO) or charged (e.g., Cl⁻, CN⁻, OH⁻).
  • Denticity: Refers to the number of donor atoms a ligand uses to bind to the central metal.
    • Monodentate: One donor atom (e.g., Cl⁻, NH₃).
    • Bidentate: Two donor atoms (e.g., ethylenediamine (en)).
    • Polydentate: More than two donor atoms (e.g., EDTA).
    • Chelating ligands: Bidentate and Polydentate ligands that form ring like structures with the metal.
• Coordination Sphere:
  • The central metal atom/ion and its attached ligands enclosed in square brackets [ ].
  • The coordination sphere behaves as a single unit.
• Counter Ions:
  • Ions outside the coordination sphere that balance the charge of the complex ion.

3. Coordination Number:

• The number of ligand donor atoms directly bonded to the central metal atom.
• Determines the geometry of the complex.
• Common coordination numbers: 2, 4, and 6.
• Coordination number 2 is usually linear. Coordination number 4 can be tetrahedral or square planar. Coordination number 6 is usually octahedral.

4. Nomenclature:

• Specific rules are used to name coordination compounds, including:
  • Naming the ligands alphabetically before the metal.
  • Indicating the number of ligands using prefixes (di, tri, tetra, etc.).
  • Specifying the oxidation state of the metal using Roman numerals in parentheses.
  • Naming anionic ligands with an -o suffix(Chloro, cyano).
• Example: [Co(NH3​)6​]Cl3​ is named hexaamminecobalt(III) chloride.

5. Isomerism:

• Coordination compounds can exhibit various types of isomerism:
  • Structural Isomerism: Different connectivity.
    • Ionization isomerism.
    • Hydrate isomerism.
    • Linkage isomerism.
    • Coordination isomerism.
  • Stereoisomerism: Same connectivity, different spatial arrangement.
    • Geometric isomerism (cis-trans).
    • Optical isomerism (enantiomers).

6. Bonding Theories:

• Valence Bond Theory (VBT): Explains bonding in terms of hybridization of metal orbitals.
• Crystal Field Theory (CFT): Explains bonding in terms of electrostatic interactions between the metal ion and ligands, and the splitting of d-orbitals.
• Ligand Field Theory (LFT): An advanced theory that combines aspects of VBT and CFT.

7. Applications:

• Catalysis: Many coordination compounds act as catalysts in industrial processes.
• Medicine: Used in chemotherapy (e.g., cisplatin), MRI contrast agents, and treatment of metal poisoning.
• Analytical Chemistry: Used in complexometric titrations and metal ion detection.
• Pigments and Dyes: Many coordination compounds are brightly colored and used as pigments.
• Extraction and purification of metals.
• Photography.