Co-ordinate Covalent bond

** In a normal covalent bond, each of the two bonded atoms contributes one electron to make the shared pair.

** In some cases, a covalent bond is formed when both the electrons are supplied entirely by one atom. Such a bond is called co-ordinate covalent or dative bond.

** Covalent bond defined as: a covalent bond in which both electrons of the shared pair come from one of the two atoms (or ions).

** The compounds containing a coordinate bond are called coordinate compounds.

** If an atom A has an unshared pair of electrons (lone pair) and another atom B is short of two electrons than the stable number, coordinate bond is formed. A donates the lone pair to B which accepts it. Thus both A and B achieve the stable 2 or 8 electrons, the lone pair being held in common

** The atom A which donates the lone pair is called the donor, while B which accepts it the acceptor. The bond thus established is indicated by an arrow pointing from A to B. Although the arrow head indicates the origin of the electrons, once the coordinate bond is formed it is in no way different from an ordinary covalent bond.

** The molecule or ion that contains the donor atom is called the ligand.

Some examples of coordinate compounds or ions

Lewis structures of some common molecules or ions containing a coordinate covalent bond are listed below:

Ammonium ion, NH₄⁺

** In ammonia molecule, the central N atom is linked to three H atoms and yet N has an unshared pair of electrons.

** The H⁺ ion furnished by an acid has no electron to contribute and can accept a pair of electrons loaned by N atom.

** Thus, NH₃ donates its unshared electrons to H+ forming ammonium ion.

** All the N–H bonds in NH4+ are identical, once the coordinate bond N→H+ is established

Hydronium ion, H₃O⁺

** The oxygen atom in water molecule is attached to two H atoms by two covalent bonds.

** There are still two unshared pairs of electrons with the O atom. The O atom donates one of these pairs of electrons to H⁺ ion and the hydronium ion is thus formed.

Fluoroborate ion, BF₄^-

It is formed when a boron trifluoride molecule (BF₃) shares a pair of electrons supplied by fluoride ion (F^–).

Addition compound of NH₃ with BCl₃

** The N atom of ammonia molecule (NH₃) has lone pair while B atom in boron trichloride (BCl₃) is short of two electrons than stable octet.

** An addition compound is formed as the N atom donates its lone pair to B atom of BCl₃.

Nitromethane, CH₃NO₂

** The Lewis structure of nitromethane is shown below. Here the N atom has five valence electrons, three of which are used in forming a covalent bond with C atom and two covalent bonds with O atom.

** The N atom is still left with two unshared electrons which are donated to another O atom.

Sulphur dioxide, SO₂, and Sulphur trioxide, SO₃

** Sulphur achieves its octet by forming two covalent bonds with one O atom, giving SO.

** The S atom in SO has two lone pairs, one of which is shared with a second O atom to form sulphur dioxide, SO₂.

** The S atom in SO₂ still has one lone pair which it donates to a third O atom forming the sulphur trioxide (SO₃) molecule.

Aluminium Chloride, Al₂Cl₆

** Aluminium atom has three valence electrons which it shares with three Cl atoms, forming three covalent bonds. Thus the Al atom acquires six electrons in its outer shell.

** Now Cl atom has three lone pairs, one of which is donated to the Al atom of another molecule AlCl₃.

** Thus both Al atoms achieve octet and stable Al₂Cl₆ results.

Sulphate ion, SO₄^2-

** Sulphur has six valence electrons (2, 8, 6) and achieves the octet by gaining two electrons from metal atoms (say two Na atoms).

** The four pairs of electrons around the S atom are then donated to four oxygen atoms each of which has six electrons.

** Thus the Lewis structure for SO₄^2- ion may be written as:

Ozone, O₃

** Oxygen molecule is made of two oxygen atoms joined by two covalent bonds.

** Each O atom in O₂ has two unshared pairs of electrons. When one pair of these is donated to a third O atom which has only six electrons, a coordinate bond is formed.

** Thus the Lewis structure of ozone may be represented as:

Carbon Monoxide, CO

** Carbon atom has four valence electrons while oxygen atom has six. By forming two covalent bonds between them, O atom achieves octet but C atom has only six electrons.

** Therefore O donates an unshared pair of electron to C, and a coordinate covalent bond is established between the two atoms.

** Lewis structure of CO may be written as:

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.

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