Key Definition
A coordination bond (also called a coordination covalent bond) is a covalent bond where both electrons in the shared pair come from the same atom (the Lewis base / electron-pair donor). Once formed, a coordination bond is identical in properties to a regular covalent bond.
How Coordination Bonds Form
When a Lewis base reacts with a Lewis acid, the base donates a lone pair to the acid. This forms a coordination bond. The key point: once the bond is formed, it behaves exactly like any other covalent bond.
Notation
A coordination bond is shown using an arrow (→) pointing from the donor atom to the acceptor atom. For example: F₃B←NH₃ shows that the N atom donates its lone pair to B.
Examples
1. Formation of the Ammonium Ion (NH₄⁺)
NH₃ (Lewis base) + H⁺ (Lewis acid)
NH₃ + H⁺ → NH₄⁺
The lone pair on nitrogen is donated to the empty 1s orbital of H⁺. The resulting NH₄⁺ has four N-H bonds, all identical. You cannot distinguish which one was the coordination bond.
Lewis base: NH₃ (donor) | Lewis acid: H⁺ (acceptor)
2. BF₃ + NH₃ Adduct
NH₃ donates its lone pair to the empty p orbital on B in BF₃.
F₃B + :NH₃ → F₃B←NH₃
Boron goes from 6 electrons to 8 (octet complete). The arrow in the formula shows the direction of electron donation.
3. Diamminesilver(I) Complex
Two NH₃ molecules donate lone pairs to Ag⁺.
Ag⁺ + 2NH₃ → [Ag(NH₃)₂]⁺
Each ammonia molecule acts as a Lewis base (ligand). Silver acts as a Lewis acid. The product is a complex ion held together by coordination bonds.
Drawing Lewis Formulas
When drawing Lewis formulas for coordination bond formation, you must:
- Show the lone pair on the donor atom explicitly
- Draw a curly arrow from the lone pair to the acceptor atom
- In the product, show the coordination bond as either a normal line (since it is identical to a covalent bond) or with an arrow notation
Think About It
In NH₄⁺, all four N-H bonds are the same length and strength. Why?
Once a coordination bond is formed, it is indistinguishable from a regular covalent bond. Both involve a shared pair of electrons between two atoms. The origin of the electrons does not affect the bond's properties.
Common Exam Mistakes
- Saying a coordination bond is "weaker" than a normal covalent bond. Once formed, they are identical.
- Not showing the lone pair on the donor atom in Lewis diagrams.
- Confusing the direction of the arrow. It goes from the donor (Lewis base) to the acceptor (Lewis acid).
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