📘 IB Definition
A coordinate (dative) bond is a covalent bond in which both electrons in the shared pair are provided by one atom (the donor). The other atom (acceptor) has an empty orbital.
Once formed, a coordinate bond is identical in strength and length to an ordinary covalent bond. It is represented by an arrow (→) pointing from the donor to the acceptor.
Key Examples
NH₄⁺ (Ammonium)
NH₃ has a lone pair on nitrogen. When H⁺ (empty 1s orbital) approaches, nitrogen donates both electrons to form the 4th N–H bond. All four bonds become equivalent.
CO (Carbon Monoxide)
Carbon and oxygen share a triple bond. Two of the three bonds are conventional; the third is a coordinate bond where oxygen donates a lone pair to carbon's empty orbital.
H₃O⁺ (Hydronium)
Water has two lone pairs on oxygen. When H⁺ bonds to water, oxygen donates one lone pair to form a coordinate bond → H₃O⁺. This is how acids protonate water.
⚠️ Examiner Trap
In Lewis diagrams, all four N–H bonds in NH₄⁺ are indistinguishable. The coordinate bond is only different in how it was formed, not in its final properties. Don't mark one bond as "special" unless specifically asked about formation.