The coordinate covalent bond: one atom gives the pair

A donor and an acceptor

A coordinate covalent bond is a covalent bond in which both shared electrons come from one atom. It forms when one species has an electron pair to give to another.

The species that gives the pair is the donor; the one that receives it is the acceptor. The bond is drawn as an arrow pointing to the acceptor.

H⁺ has an empty shell. It accepts one of the two lone pairs on oxygen in water, giving the hydroxonium ion, H₃O⁺. Ammonia donates its pair to boron in BF₃ in the same way.

Once formed, it cannot be told apart: all three O–H bonds in H₃O⁺, and all four N–H bonds in NH₄⁺, behave alike. When it breaks, both electrons return to the donor.

The only difference from an ordinary covalent bond is who supplies the electrons while it forms.

Key terms

Coordinate covalent bond

Joining a species with a spare pair to one with room for it.

Donor

The atom with a lone pair.

Acceptor

The atom or ion with an empty space in its outer shell.

Hydroxonium ion

The ion formed when water takes H⁺.

Short questions with model answers

1. Q1. How does water form the hydroxonium ion? (Fig 3.7, p. 40)

   • H₂O: oxygen has two lone pairs
   • H⁺: empty shell
   • H₂O: → H⁺ gives H₃O⁺

   Oxygen is the donor, H⁺ the acceptor; H₃O⁺ has three identical O–H bonds.

2. Q2. Ammonia reacts with boron trifluoride. Which is the donor? (Fig 3.8, p. 40)

   • NH₃: nitrogen has one lone pair
   • BF₃: boron has only six outer electrons
   • H₃N→BF₃

   Nitrogen of NH₃ donates; boron of BF₃ accepts.

Common mistakes

• ✗ In NH₄⁺, the coordinate N–H bond is weaker than the other three.

  ✓ All four N–H bonds behave exactly alike; only the way of forming differs.

• ✗ The arrow points from the acceptor to the donor.

  ✓ It points towards the acceptor, the way the pair moves.

• ✗ When the bond breaks, the acceptor keeps the electrons.

  ✓ Both electrons go back to the donor (p. 41).

MCQs

1. 1. In a coordinate covalent bond, the shared pair comes from:

   1. (a) Both atoms, one electron each
   2. (b) One atom only
   3. (c) A metal
   4. (d) The acceptor
   Show answer

   (b) p. 39: the shared electron pair is donated by one atom only.

2. 2. Which ion forms when water accepts H⁺?

   1. (a) OH⁻
   2. (b) NH₄⁺
   3. (c) H₃O⁺
   4. (d) H⁻
   Show answer

   (c) p. 40: water and H⁺ give the hydroxonium ion, H₃O⁺.

3. 3. In H₃N→BF₃, which is the acceptor?

   1. (a) Nitrogen
   2. (b) Hydrogen
   3. (c) Fluorine
   4. (d) Boron
   Show answer

   (d) p. 40: ammonia's lone pair goes to boron; the arrow points to boron.

4. 4. How do the four N–H bonds in NH₄⁺ compare?

   1. (a) All four behave exactly alike
   2. (b) One is weaker
   3. (c) One is ionic
   4. (d) Two are coordinate
   Show answer

   (a) p. 41: all four behave exactly alike; the difference is only in how they formed.

5. 5. When a coordinate covalent bond breaks, the two electrons go to:

   1. (a) The acceptor
   2. (b) One to each atom
   3. (c) The donor
   4. (d) They are lost
   Show answer

   (c) p. 41: both bonded electrons go to the donor species.

Quick revision

• In a coordinate covalent bond one atom, the donor, supplies both electrons.
• Water + H⁺ gives H₃O⁺; NH₃ + BF₃ gives H₃N→BF₃.
• After forming it is like any covalent bond; when broken, the donor keeps the pair.