Coordinate covalent bonds are also known as dative covalent bonds and just like the ordinary covalent bonds, there is the sharing of electrons. However, unlike the ordinary covalent bond in which the shared pair of electron is contributed equally by the two participating atoms, the pair in the coordinate covalent bond is donated by only one of the participant. This pair of electrons is known as a lone pair. This means that one of the reactants in a coordinate covalent bond or combination must have a lone pair.
Ammonia and water molecules posses’ lone pairs and so readily enter into coordinate covalent combinations. When ammonia reacts with hydrochloric acid, the hydrogen ion from the acid accepts the lone pair from the ammonia molecule that results in a coordinate covalent bond. This bond helps the hydrogen ion to acquire a stable duplet structure of helium, the nearest rare gas, and at the same time, allows the nitrogen atom of the ammonia molecule to maintain the stable octet configuration. The positive charge on the hydrogen ion is carried over to give the positively charged ammonium ion, NH4+.
Instructively, the coordinate covalent bond is denoted or represented conventionally by an arrow pointing from the donor atom to the acceptor atom. The oxonium ion is formed in a similar way where the hydration of copper(II) ion is also due to the formation of coordinate covalent bonds. The copper(II) ion has no electrons in its outermost shell. It accepts four lone pairs of electrons from four water molecules(that is, one lone pair from each water molecule). The blue hydrated ion that results is [Cu(H2O)4]2+. If excess aqueous ammonia is added to a solution of copper(II) ions, the ammonia molecules will replace the water molecules to form [Cu(NH3)4]2+.
Other compounds that exhibit coordinate covalent bonds are the hexacyanoferrate(II) and the hexacyanoferrate(III) ions. These compounds have a central iron(II) or iron(III) that are surrounded by six cyano ions (CN–). Four of the cyano ions are on the same plane as the iron(II) or iron(III) ion. One of the cyano ions is above and the other below the iron ion. In silver diammine ion, [Ag(NH3)2]+, the lone pair of electrons in the ammonia molecule from a coordinate covalent bond is shown below
H3N → Ag+, NH3→ Ag+
Compounds containing coordinate covalent bonds and covalent bonds have properties which are very similar to covalent compounds. The presence of a coordinate covalent bond tends to make a compound less volatile.