The nitrogen atom in an ammonia molecule has one lone pair. A hydrogen ion, \mathrm{H^+}, has no electrons at all, but it can bond to this lone pair.
The new bond is made from a pair of electrons that both came from the nitrogen atom. A covalent bond formed in this way is called a coordinate bond, or dative covalent bond, and the result is the ammonium ion, \mathrm{NH_4^+}.
Once it has formed, the coordinate bond cannot be told apart from the other three \mathrm{N{-}H} bonds. All four bonds are identical, and the ion has a regular tetrahedral shape.
The oxygen atom in a water molecule has two lone pairs. When one of them bonds to a hydrogen ion, the oxonium ion, \mathrm{H_3O^+}, forms.
In water and in acidic solutions, hydrogen ions do not exist on their own but are bonded to water molecules in this way.
The oxonium ion still has one lone pair left on its oxygen atom.
Ions of transition metals can accept lone pairs from the molecules or ions around them. The particles that bond to a metal ion in this way are called ligands.
When excess ammonia solution is added to a solution of copper(II) ions, four ammonia molecules bond to each \mathrm{Cu^{2+}} ion through the lone pair on their nitrogen atom.
An ion made of a metal ion and its ligands is called a complex ion. This one is the tetraamminecopper(II) ion, [\mathrm{Cu(NH_3)_4}]^{2+}, which gives the solution its deep blue colour.
A ligand is a molecule or ion with at least one lone pair. Common ligands include ammonia \mathrm{NH_3}, water \mathrm{H_2O}, hydroxide ions \mathrm{OH^-}, cyanide ions \mathrm{CN^-} and chloride ions \mathrm{Cl^-}.
The atom that provides the lone pair is called the donor atom: nitrogen in ammonia, oxygen in water and in hydroxide ions, carbon in cyanide ions and chlorine in chloride ions.
The number of ligands bonded to the central metal ion is called the coordination number. It is 4 in [\mathrm{Cu(NH_3)_4}]^{2+} and 6 in [\mathrm{Fe(CN)_6}]^{4-}.
The ligands of a complex ion are arranged around the central metal ion in a definite shape, which depends on the coordination number and on the metal ion.
The diamminesilver(I) ion, [\mathrm{Ag(NH_3)_2}]^+, is linear, and the tetraamminezinc(II) ion, [\mathrm{Zn(NH_3)_4}]^{2+}, is tetrahedral.
The tetraamminecopper(II) ion, [\mathrm{Cu(NH_3)_4}]^{2+}, is square planar, and the hexacyanidoferrate(II) ion, [\mathrm{Fe(CN)_6}]^{4-}, is octahedral.