Dative Covalent Bond (Coordinate Bond)

a. A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei. In the formation of a simple covalent bond, each atom supplies one electron to the bond.

b. A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom. “Dative covalent bond” originates from the donation of an electron pair from one atom (or group of atoms) to an empty orbital on a second atom which is electron deficient. Bonds formed are identical to normal covalent bond.

c. The reaction between ammonia and hydrogen chloride

If these colourless gases are allowed to mix, a thick white smoke of solid ammonium chloride is formed.
ammonia_and_HCl_chemcial_eqn.jpg

Ammonium ions, NH4+, are formed by the transfer of a hydrogen ion from the hydrogen chloride to the lone pair of electrons on the ammonia molecule.

ammonia_and_HCl_dot_cross_diag.jpg


When the ammonium ion, NH4+, is formed, the fourth hydrogen is attached by a dative covalent bond, because only the hydrogen's nucleus is transferred from the chlorine to the nitrogen. The hydrogen's electron is left behind on the chlorine to form a negative chloride ion.

Once the ammonium ion has been formed it is impossible to tell any difference between the dative covalent and the ordinary covalent bonds. Although the electrons are shown differently in the diagram, there is no difference between them in reality.

d. Representing co-ordinate bonds

In simple diagrams, a co-ordinate bond is shown by an arrow. The arrow points from the atom donating the lone pair to the atom accepting it.

ammonium_ion.jpg


e. Dissolving hydrogen chloride gas in water to make hydrochloric acid

Something similar happens. A hydrogen ion (H+) is transferred from the chlorine to one of the lone pairs on the oxygen atom.

HCl_dissolve_in_water_dot_cross_diag.jpg


The H3O+ ion is variously called the hydroxonium ion.

hydroxonium_ion.jpg



f. The reaction between ammonia and boron trifluoride, BF3

Boron trifluoride as a compound which does not have a noble gas structure around the boron atom. The boron only has 3 pairs of electrons in its bonding level, whereas there would be room for 4 pairs. BF3 is described as being electron deficient.

The lone pair on the nitrogen of an ammonia molecule can be used to overcome that deficiency, and a compound is formed involving a co-ordinate bond.
ammonia_and_BF3.jpg

g. The structure of aluminium chloride

Aluminium chloride sublimes (turns straight from a solid to a gas) at 178°C. If it contained ions it would have a very high melting and boiling point because of the strong attractions between the positive and negative ions. The implication is that it must be covalent. The dots-and-crosses diagram shows only the outer electrons.

AlCl3_dot_cross_diag.jpg


AlCl3, like BF3, is electron deficient. There is likely to be a similarity, because aluminium and boron are in the same group of the Periodic Table, as are fluorine and chlorine.

Measurements of the relative formula mass of aluminium chloride show that its formula in the solid is not AlCl3, but Al2Cl6. It exists as a dimer (two molecules joined together). The bonding between the two molecules is co-ordinate, using lone pairs on the chlorine atoms.

Al2Cl6_dot_cross_diag.jpg



Quick Checks : Can you draw similar diagrams for the molecules of carbon monoxide, CO and nitric acid, HNO3?