5.3.4 Coordinate Covalent Bond

This topic explains the coordinate covalent bond (also called a dative bond) — a special type of covalent bond where both shared electrons come from one atom.

Definition

A coordinate covalent bond is a type of covalent bond where the shared electron pair comes from a single atom (called the donor).

Simple explanation: In a normal covalent bond, each atom contributes one electron. In a coordinate covalent bond, one atom contributes both electrons.

Main Idea

Atoms are held together because both nuclei attract the shared pair of electrons. Once a coordinate covalent bond is formed, it is impossible to distinguish the origin of the electrons — it behaves like a normal covalent bond.

This type of bonding is usually observed when metal ions bind to ligands. However, non-metals can also participate in this bond. The reaction between a Lewis acid and a base is a coordinate covalent bond.

Simple explanation: One atom "lends" both electrons for the bond. After the bond forms, you can't tell which atom gave what — it looks just like a normal covalent bond.

Core Concept

Detailed Explanation

Example 1: Ammonium Ion (NH₄⁺)

The ammonium ion is formed from the reaction of ammonia (NH₃) gas with hydrogen chloride (HCl) gas.

• Only the hydrogen's nucleus is transferred from the chlorine to the nitrogen.
• The hydrogen's electron is left behind on the chlorine to form a negative chloride (Cl⁻) ion.

Equation:

NH₃ + H⁺ → NH₄⁺

• Nitrogen in NH₃ has a lone pair of electrons.
• H⁺ (from HCl) has no electrons — it is just a proton.
• The lone pair on nitrogen is donated to form a bond with H⁺.
• Both electrons in the N–H bond come from nitrogen.

Simple explanation: Nitrogen shares its lone pair with the hydrogen ion, forming a coordinate covalent bond.

Example 2: Hydronium Ion (H₃O⁺)

When hydrogen chloride (HCl) gas dissolves in water to make hydrochloric acid (HCl aq.), a coordinate covalent bond is formed in the hydronium ion.

The hydrogen (H) nucleus is transferred to the water (H₂O) molecule, which has a lone pair of electrons to form hydronium. So, H⁺ does not contribute any electrons to the bond.

Equation:

H₂O + H⁺ → H₃O⁺

• Oxygen in H₂O has two lone pairs.
• H⁺ (from HCl) has no electrons.
• One lone pair on oxygen is donated to form a bond with H⁺.
• Both electrons in the O–H bond come from oxygen.

Simple explanation: Oxygen shares one of its lone pairs with the hydrogen ion.

Example 3: Ammonia Boron Trifluoride (NH₃–BF₃)

Boron trifluoride (BF₃) is a compound that does not have a noble gas structure around the boron (B) atom. The boron only has three pairs of electrons in its valence shell and requires a pair to complete the orbital. Hence, BF₃ is electron deficient.

The lone pair on the nitrogen (N) of the ammonia (NH₃) molecule is used to overcome that deficiency, and a complex compound forms through a coordinate covalent bond.

Equation:

NH₃ + BF₃ → NH₃BF₃

• Nitrogen in NH₃ has a lone pair.
• Boron in BF₃ has only 6 electrons — it is electron deficient.
• Nitrogen donates its lone pair to boron.
• Both electrons in the N–B bond come from nitrogen.

Simple explanation: Nitrogen "helps out" boron by sharing its lone pair, forming a coordinate covalent bond.

Intermediate Level

How to Identify a Coordinate Covalent Bond

• An atom with a lone pair (the donor).
• An atom that is electron deficient or has no electrons (the acceptor).
• A bond that forms where both electrons come from the same atom.

Formation of NH₄⁺ — In Detail

Step 1: Ammonia (NH₃) has a lone pair on nitrogen.

Step 2: HCl provides H⁺ (a proton with no electrons).

Step 3: Nitrogen donates its lone pair to H⁺.

Step 4: NH₄⁺ forms. The N–H bond is a coordinate covalent bond.

Note: The Cl⁻ ion is left behind from HCl.

Formation of H₃O⁺ — In Detail

Step 1: Water (H₂O) has two lone pairs on oxygen.

Step 2: HCl provides H⁺ (a proton with no electrons).

Step 3: Oxygen donates one lone pair to H⁺.

Step 4: H₃O⁺ forms. The new O–H bond is a coordinate covalent bond.

Formation of NH₃BF₃ — In Detail

Step 1: Ammonia (NH₃) has a lone pair on nitrogen.

Step 2: BF₃ has only 6 electrons around boron — it is electron deficient.

Step 3: Nitrogen donates its lone pair to boron.

Step 4: NH₃BF₃ forms. The N–B bond is a coordinate covalent bond.

Comparison with Normal Covalent Bond

Important: Once formed, a coordinate covalent bond is indistinguishable from a normal covalent bond.

Important Points

• - Coordinate covalent bond: shared electron pair comes from a single atom (donor).
• - Also called a dative bond.
• - The donor provides both electrons.
• - The acceptor receives the electron pair.
• - After formation, it is indistinguishable from a normal covalent bond.
• - Examples:
• - NH₄⁺ (ammonium ion)
• - H₃O⁺ (hydronium ion)
• - NH₃BF₃ (ammonia boron trifluoride)
• - Usually observed when metal ions bind to ligands.
• - Reaction between a Lewis acid and a base is a coordinate covalent bond.

Remember

• - Coordinate covalent bond = one atom provides both electrons.
• - Donor = provides both electrons.
• - Acceptor = receives the electron pair.
• - After formation, identical to a normal covalent bond.
• - Examples:
• - NH₃ + H⁺ → NH₄⁺
• - H₂O + H⁺ → H₃O⁺
• - NH₃ + BF₃ → NH₃BF₃
• - Also called a dative bond.

Common Mistakes

• 1. Thinking coordinate bonds are completely different from normal covalent bonds. After formation, they are identical.
• 2. Thinking both atoms contribute electrons. Only one atom (the donor) contributes both.
• 3. Forgetting the role of lone pairs. The donor must have a lone pair to donate.
• 4. Confusing donor and acceptor. Donor provides electrons; acceptor receives them.
• 5. Thinking coordinate bonds only form with metals. Non-metals can also form them (e.g., NH₄⁺, H₃O⁺, NH₃BF₃).
• 6. Forgetting the examples. NH₄⁺, H₃O⁺, and NH₃BF₃ are the key examples.

Quick Revision

• Coordinate covalent bond: a type of covalent bond where the shared electron pair comes from a single atom (called the donor).
• The donor provides both electrons; the acceptor receives the electron pair.
• Once formed, it is impossible to distinguish the origin of the electrons — it behaves like a normal covalent bond.
• Examples:
• Ammonium ion (NH₄⁺): formed from NH₃ (donor) and H⁺ (acceptor).
• Hydronium ion (H₃O⁺): formed from H₂O (donor) and H⁺ (acceptor).
• Ammonia boron trifluoride (NH₃BF₃): formed from NH₃ (donor) and BF₃ (acceptor).
• Usually observed when metal ions bind to ligands.
• The reaction between a Lewis acid and a base is a coordinate covalent bond.

60-Second Revision

• Coordinate covalent bond = one atom provides both electrons.
• Donor = provides electrons; acceptor = receives.
• After formation, identical to normal covalent bond.
• Examples: NH₄⁺, H₃O⁺, NH₃BF₃.
• Warning: only one atom provides both electrons in a coordinate bond.