📘 IB Definition
A coordinate (coordination) 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 coordination 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 coordination 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 coordination 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.
Get flashcards and quizzes in ChemEasy, or plan your revision with ChemPlan IB.