Coordinate covalent bonds, also known as coordinate links or semipolar bonds, are different from normal covalent bonds because both of the electrons that are shared by the bonded atoms originally come from the same atom. This contrasts with normal covalent bonds, in which each atom gives up one of the two electrons that form the shared electron pair.
Coordinate covalent bonds occur between two types of substances: Lewis acids and Lewis bases. When a coordinate covalent bond is formed, a pair of electrons from a Lewis base are shared between the base and a Lewis acid. The acid does not give up any electrons when this bond is formed. Often, this type of bonding is found in complex chemical compounds. When an acid and a base are bound with a coordinate covalent bond, they are referred to as a coordination complex.
This type of bonding can also occur between Lewis bases and other substances. Lewis bases have free pairs of electrons, which makes them easier to transfer and share with other substances. Coordinate covalent bonding is also common in complexes that involve metals. In these instances, Lewis bases donate their electron pairs to metals, which fill the role of Lewis acids.