(also coordination bond), a term denoting one of the ways in which a chemical covalent bond is formed. The ordinary covalent bond between two atoms is due to the interaction of two electrons, one from each atom. The donor-acceptor bond is formed by a pair of electrons from one atom (the donor) and a free (unfilled) orbital from another (the acceptor). The difference can be expressed schematically as
| Covalent bond | Donor-acceptor bond |
| A + B->A:B | A: + B->A:B |
In both cases the electrons become common to two atoms. A typical example is the formation of an ammonium ion by reaction of ammonia with a hydrion (proton):
In the ammonia molecule the nitrogen atom has an unshared pair of electrons, and in the hydrogen ion the Is orbital is free. If the NH3 molecule and the H+ ion approach sufficiently closely, the two-electron cloud of the nitrogen enters the sphere of attraction of the hydrion and becomes common to the nitrogen and hydrogen ions—that is, a fourth covalent N—H bond forms. All the N—H bonds in the ion acquire the same value and become identical. Another important example is the formation of the oxonium ion:
In this case the donor is the water molecule, and a proton is the acceptor.
This means of formation of covalent bonds plays an important role in the chemistry of complex compounds.
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