Transition metals (and to some extent other metals) form coordination complexes in which the metal or metal ion is associated with ligands.
Examples of complexes showing their overall charge:
[Ag(NH3)2]+, [Cr(OH2)6]3+, [CuCl2]–,
As shown in the examples, ligands may be
This type of reaction where one atom acts as an electron-pair donor and the other as an electron pair acceptor is an example of a Lewis acid-base reaction.
Examples of complexes showing their overall charge:
[Ag(NH3)2]+, [Cr(OH2)6]3+, [CuCl2]–,
[Zn(OH)4]2–
These are also known as complex ions. Complex ions exist in compounds with an appropriate counter-ion, for example:
Na2Zn(OH)4
.As shown in the examples, ligands may be
molecules (H2O, NH3, CO) OR anions (Cl–, OH–, CN–)
The donor atom that becomes coordinated to the metal for all ligands has a non-bonding electron pair that can form a coordinate bond to the metal or metal ion.
As shown below, O is the donor atom in H2O, N is the donor atom in NH3, O is the donor atom in OH–.
The donor atom that becomes coordinated to the metal for all ligands has a non-bonding electron pair that can form a coordinate bond to the metal or metal ion.
As shown below, O is the donor atom in H2O, N is the donor atom in NH3, O is the donor atom in OH–.
 |  |  |  |
As the diagram shows below, the bond between the metal and the ligand results from overlap
of the orbital on the ligand containing the non-bonding electron pair
with an empty orbital on the metal.
with an empty orbital on the metal.
 | + |  |  |
| metal Lewis acid electron deficient | ligand Lewis base nonbonding pair |
 |
| M-L coordinate bond |
This type of reaction where one atom acts as an electron-pair donor and the other as an electron pair acceptor is an example of a Lewis acid-base reaction.