Which of the following compounds has coordinate bonds in its structure?

Analyze the question: A coordinate (dative covalent) bond is a covalent bond in which both shared electrons come from one atom. We need the compound that contains such a bond.

Concept:

• Ionic compounds (MgCl₂, NaCl, AgCl) are held together by electrostatic attraction between ions. They have no coordinate bonds in the usual WASSCE treatment.
• Coordinate bonds form when an atom with a lone pair donates it to an atom with an empty orbital (an electron-deficient atom).

Solution:

1. Aluminium chloride (AlCl₃) is covalent. In AlCl₃, Al has only 6 electrons in its outer shell, so it is electron-deficient and has an empty orbital.
2. Chlorine atoms have lone pairs. In the dimer, a chlorine atom on one AlCl₃ unit donates a lone pair to the Al atom of another AlCl₃ unit.
3. This forms two coordinate (dative) bonds, which gives the dimer Al₂Cl₆. Each Al then has a full octet.

Structure (simplified):

 Cl     Cl     Cl
   \   /  \   /
    Al      Al
   /   \  /   \
 Cl     Cl     Cl

The two bridging Cl atoms each form one normal covalent bond and one coordinate bond.

4. MgCl₂, NaCl and AgCl are ionic lattices (Mg²⁺/Cl⁻, Na⁺/Cl⁻, Ag⁺/Cl⁻), so they do not have coordinate bonds.

Answer: A. Al₂Cl₆

Tip: Think of coordinate bonds in these familiar examples: NH₄⁺, H₃O⁺, Al₂Cl₆ and CO. If you see an electron-deficient atom (Al, B) or a lone-pair donor (N, O) bonding with H⁺ or another electron-deficient species, expect a dative bond.