A coordinate covalent bond (also known as dative covalent bond) is a special type of covalent bond in which the shared electrons come from one of the atoms only. Once the bond has been formed, its strength is no different than that of a covalent bond.
Coordinate covalent bonds are formed when a Lewis base (an electron donor or giver) donates a pair of electrons to a Lewis acid (an electron accepter) and the resultant compound is then called an adduct.
Coordinate bonds can be found in many different substances, such as in simple molecules like carbon monoxide (CO), which contains one coordinate bond and two normal covalent bonds between the carbon atom and the oxygen atom, or the ammonium ion (NH4+), where a coordinate bond is formed between a proton (a H+ ion) and the nitrogen atom. Coordinate bonds are also formed in electron deficient compounds, such as in solid beryllium chloride (BeCl2), in which every beryllium atom is bonded to four chlorine atoms, two with normal covalent bonding, and the other two with coordinate bonds, which will give it a stable octet of electrons.
Coordinate bonding can also be found in coordination complexes involving metal ions, especially if they are transition metal ions. In such complexes, substances in a solution act as Lewis bases and donate their free pairs of electrons to the metal ion, which acts as a Lewis acid and accepts the electrons. Coordinate bonds form and the resulting compound is called a coordination complex, while the electron donors are called ligands. There are many chemicals with atoms that have lone pairs of electrons, often oxygen, sulfur, nitrogen and halogens or halide ions, which, in solution, can donate their electron pairs to become ligands. A common ligand is water (H2O), which will form coordination complexes with any hydrated metal ions, like Cu2+, which will form [Cu(H2O)6]2+ in aqueous solution. Other common simple ligands are ammonia (NH3), fluoride ions (F-), chloride ions (Cl-) and cyanide ions (CN-).
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