It is a special case of covalent bond the formation of which was postulated by Perkins (1921). It is formed by mutual sharing of electrons between the two atoms but the shared pair of electrons is contributed only by one of the two atoms, the other atom simply participates in sharing. The atom which donates an electron pair for sharing is called donor and it must have already completed its octet. On the other hand, the atom which accepts the electron pair in order to complete its octet is called acceptor. The bond is represented by an arrow pointing from the donor towards the acceptor. Let us consider the formation of ozone molecule. A molecule of, -oxygen contains two oxygen atoms which share four electrons and complete their octets. Now, if an atom of oxygen having six valence electrons comes close to oxygen molecule, it shares a lone pair of electrons with one of the oxygen of the molecule. It can be represented as follows:
It is important to note that co-ordinate bond one formed, cannot be distinguished from covalent bond.
Some more examples of molecules/polyatomic ions having co-ordinate bond are as follows:
(i) SO2 molecule
(ii) Ammonium ion
(iii) Hydronium ion
(iv) Carbon (II) oxide. In this molecule carbon and oxygen atoms contribute two electrons forming pure covalent bonds. At the same time oxygen also act as donor atom to form co-ordinate covalent bond.
LIMITATIONS OF OCTET RULE
Although octet rule is successful in explaining valencies of large number of elements, yet it fails to many of the structures. Some of its limitations are:
1. Formation of Compounds with Electron Deficient A
On the basis of octet rule, the elements of group 1, 2 13 should not form covalent bonds because they have than four electrons in their valence shell. Therefore, cannot achieve octet by mutual sharing of electrons. But of the elements of these groups do form covalent compounds,
LiCl, BeF 2 and BC13 are. common examples.
In LiCl, Li atom has only 2 electrons; in Bef2, Be atom has four electrons and .in BC13 the boron atom has only six electrons in its outer shell.
Formation of Super Octet Molecules Like PCI5, SF 6 and IF7
Violation of octet rule is also observed in case of compounds like PF5 and SF6 and IF7 where the central atoms have more than eight electrons in their valence shell as shown below:
3. formation of Compounds of Xepon Octet rule is primarily based upon the fact that noble
gases are inert. But recent researches have shown that some of the noble gases like Xe and Kr do take part in bonding and from compounds with elements like fluorine and oxygen such as XeF2, XeF4, XeOF2, XeOF4, etc.
4. Inability to Predict Energy Changes
In addition to the above exceptions, the octet rule is not to predict energy changes during the bond formation.