Some chemical bonds are unique in that both electrons forming the bond come from a single atom. The two atoms are held together, then, by the attraction between the pair of electrons from one atom and the positively charged nucleus of the second atom. Such bonds have been called coordinate covalent bonds.
An example of this kind of bonding is found in the reaction between copper(II) ion and ammonia. The nitrogen atom in ammonia has an unshared pair of electrons that is often used to bond with other atoms. The copper(II) ion is an example of such an anion. It is positively charged and tends to surround itself with four ammonia molecules to form the cupric ammonium ion, Cu(NH3)4 2+ . The bonding in this ion consists of coordinate covalent bonds with all bonding electrons supplied by the nitrogen atom.