2.2.4Additional lessons
Coordinate Bonds — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 25 of 98
Section 1 of 6What Is a Coordinate Bond
Coordinate bonding is also called dative covalent bonding.
Definition
- A coordinate bond is a covalent bond in which both electrons in the shared pair come from the same atom.
Donor
- The atom that provides the pair has a lone pair of electrons available in its outer shell.
Acceptor
- The other atom or ion has an empty orbital or a space to accept the pair, such as H⁺ or boron in BF₃.
Notation
- The bond is often drawn as an arrow pointing from the donor atom to the acceptor.
Section 2 of 6Important Examples
Important Examples
Ammonium ion
- NH₃ donates its lone pair to H⁺ to form NH₄⁺, which has four N–H bonds.
Hydronium ion
- H₂O donates a lone pair to H⁺ to form H₃O⁺, the species responsible for acidity in water.
Carbon monoxide
- CO has a triple bond made up of two normal covalent bonds and one dative bond from oxygen to carbon.
Ammonia and BF₃
- NH₃ donates its lone pair to the electron-deficient boron in BF₃ to give H₃N→BF₃.
Section 3 of 6Forming NH₄⁺ Step by Step
1
Ammonia, NH₃, has three N–H bonds and one lone pair on the nitrogen atom.
2
A hydrogen ion, H⁺, has no electrons but does have an empty orbital.
3
The lone pair on nitrogen is donated to the hydrogen ion to form a new N–H bond.
4
The new bond is the same length and strength as the other three N–H bonds, so the four bonds are identical in the ion.
5
The ion carries a 1+ charge because the proton brought its positive charge but no electrons to the bond.
| Species | Donor atom | Acceptor |
|---|---|---|
| NH₄⁺ | N in NH₃ | H⁺ |
| H₃O⁺ | O in H₂O | H⁺ |
| H₃N→BF₃ | N in NH₃ | B in BF₃ |
Section 4 of 6Why It Matters
Acids and bases
- Many acid-base ideas rely on a species using a lone pair to bond to a proton.
Identical bonds
- After formation a coordinate bond behaves like any other covalent bond of the same atoms.
Lone pair requirement
- Only species with a free lone pair can act as donors, so CH₄ cannot form a coordinate bond.
Complex ions
- Metal ions also accept lone pairs from other species, forming complex ions in later study.
Section 5 of 6Worked Example
Question Show that ammonium ion NH₄⁺ contains a coordinate bond and state the charge on the ion.
- Ammonia has 5 valence electrons on nitrogen: three form bonds and two are a lone pair.
- A proton, H⁺, supplies no electrons, so the new N–H bond must use the nitrogen lone pair.
- The ion has 1+ overall charge because the hydrogen ion contributed a 1+ charge and no electrons.
Answer
- NH₄⁺ contains one coordinate bond N→H;
- the ion has a 1+ charge, and all four N–H bonds are identical.
Section 6 of 6Exam Checklist & Tips
2.2.4 – Key Points for the Exam
- Coordinate bond A covalent bond in which both shared electrons are supplied by one atom.
- Lone pair A pair of outer electrons not involved in bonding that can be donated.
- Dative Another name for a coordinate bond.
- Electron deficient Having an atom with fewer than eight electrons or an empty orbital that can accept a pair.
Common Mistakes
- Believing the coordinate bond in NH₄⁺ is different from the other N–H bonds.
- Saying electrons are transferred in a coordinate bond; they are shared.
- Drawing the arrow from acceptor to donor instead of donor to acceptor.
- Forgetting that the donor must have a lone pair.
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