Definition of Coordination Bonds
A coordination bond (dative covalent bond) is a covalent bond in which both shared electrons come from the same atom (the Lewis base / ligand).
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.
Dative Arrow Representation
Coordination bonds are represented with an arrow (\(\rightarrow\)) pointing from the donor atom to the acceptor atom (e.g. \(\text{F}_3\text{B} \leftarrow \text{NH}_3\)). Once formed, a coordination bond is identical in length and strength to any ordinary covalent bond.
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
Equivalence of Bonds in Ammonium Ion
In \(\text{NH}_4^+\), all four \(\text{N}-\text{H}\) bonds are identical in length and strength because electrons are completely delocalised and indistinguishable once the covalent bond forms.
Bond Strength and Lone Pair Omission
Never claim that coordination bonds are "weaker" than regular covalent bonds. Always show the non-bonding lone pair on the donor atom when drawing Lewis structures.
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