OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 2

✅ Correct Answer: D (Dative covalent)

In the complex ion [Fe(H₂O)₆]²⁺, each water molecule acts as a ligand. Oxygen has lone pairs of electrons which it donates entirely to the empty orbitals of the central Fe²⁺ ion, forming a dative covalent (coordinate) bond.

💡 Key Knowledge

• Ligands: Species with at least one lone pair of electrons that can be donated (e.g., H₂O, NH₃, Cl⁻).
• Transition Metal Ions: Provide vacant orbitals (usually d-orbitals) to accept these electron pairs.
• Also known as coordinate bonding.

🧠 Exam Technique

Do not confuse the overall charge of the complex ion with the type of internal bonding. Even though the overall species has a 2+ charge, the interaction holding the H₂O molecules to the Fe²⁺ ion is directional electron-pair donation, not simple electrostatic attraction between oppositely charged macro-ions.

❌ Common Errors

• Choosing B (Ionic): Students often see the 2+ charge and mistakenly assume the bonding between the ligands and the metal is purely ionic.
• Choosing C (Hydrogen): Confusing intermolecular hydrogen bonding (which water molecules do with each other) with the bonding attaching water to the metal center.