Introduction to Coordination Compounds

The Introduction to coordination compounds explains how a central metal atom or ion binds to ligands (molecules or ions that donate electron pairs) to form coordinate covalent bonds; this overview uses IUPAC-friendly terminology.

Table of Contents

1. Key Features
2. Examples
3. Importance & Applications
4. Quiz (interactive)
5. MCQs (with answers)
6. FAQ
7. References & Links

Key Features of Coordination Compounds

• Central Metal Ion: Usually a transition metal which acts as a Lewis acid (electron-pair acceptor).
• Ligands: Atoms, ions, or molecules with lone pairs (Lewis bases) that donate to the metal.
• Coordinate Bond: Covalent bond where both electrons originate from the ligand.
• Coordination Number: Number of ligand donor atoms attached to the metal.
• Complex Ion: A coordination entity carrying a net charge (cationic or anionic).

Examples

• [Cu(NH₃)₄]²⁺ – Tetraamminecopper(II) ion
• [Fe(CN)₆]³⁻ – Hexacyanoferrate(III) ion
• [Co(H₂O)₆]³⁺ – Hexaaquacobalt(III) ion

Importance & Applications

Coordination compounds are vital across chemistry and industry:

• Catalysis — e.g., Wilkinson’s catalyst for hydrogenation.
• Medicine — e.g., cisplatin (a platinum coordination compound) used in chemotherapy.
• Biological systems — e.g., hemoglobin contains an iron coordination complex (heme).
• Industrial uses — dyes, pigments, analytical reagents and extraction processes.

Quiz: Test Your Knowledge (interactive)

Q1: What is a ligand in coordination chemistry?

Answer: An atom, ion or molecule that donates an electron pair to the central metal ion.
Explanation: Ligands act as Lewis bases; they supply both electrons for the coordinate bond.

Q2: Define coordination number.

Answer: The number of ligand donor atoms directly bonded to the central metal.
Explanation: A bidentate ligand contributes two donor atoms to the coordination number.

Q3: Name a coordination compound in biological systems.

Answer: Hemoglobin (iron-centered heme complex).
Explanation: The heme group coordinates Fe2+ with nitrogen donors from the porphyrin ring.

Multiple Choice Questions (MCQs)

1. Which of the following best describes a ligand?
   a) Electron pair acceptor
   b) Electron pair donor
   c) Free radical
   d) Metal ion
   Explanation: Ligands donate an electron pair to the metal, so they are electron-pair donors (Lewis bases).
2. Coordination number refers to:
   a) Number of ligand donor atoms bonded to the metal
   b) Number of electrons in metal
   c) Charge on the complex
   d) Number of atoms in ligand
   Explanation: Coordination number counts donor atoms attached directly to the metal centre.
3. What kind of bond is formed in coordination compounds?
   a) Ionic bond
   b) Coordinate covalent bond
   c) Metallic bond
   d) Hydrogen bond
   Explanation: The shared pair of electrons both come from the ligand — a coordinate covalent (dative) bond.
4. Which metal ion is commonly found as the central atom in coordination compounds?
   a) Alkali metals
   b) Transition metals
   c) Noble gases
   d) Halogens
   Explanation: Transition metals have variable oxidation states and available d-orbitals suited to coordination.
5. Hemoglobin contains which central metal in its coordination complex?
   a) Copper
   b) Zinc
   c) Iron
   d) Magnesium
   Explanation: Hemoglobin’s heme group contains iron (Fe), which coordinates to the porphyrin ring and oxygen.

MCQ Answers

1. b) Electron pair donor
2. a) Number of ligand donor atoms bonded to the metal
3. b) Coordinate covalent bond
4. b) Transition metals
5. c) Iron

Frequently Asked Questions (FAQ)

Q: Why are transition metals common central atoms?

A: Transition metals have variable oxidation states and available d-orbitals, enabling diverse ligand binding and complex geometries.

Q: What is the difference between a coordinate covalent bond and a regular covalent bond?

A: In a coordinate covalent bond both bonding electrons originate from the same atom (the ligand); in typical covalent bonds electrons are shared with contributions from both atoms.

Q: Where can I study official nomenclature rules?

A: See the IUPAC recommendations and the IUPAC Nomenclature of Inorganic Chemistry for authoritative rules. (IUPAC)

• Official IUPAC guidance — IUPAC (external)
• CHEMASH: Nomenclature of Coordination Compounds (internal)