Co-ordinate Covalent bond
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, SO42-
** 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 SO42- 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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