Co-ordinate covalent bonds

When the shared electron pair is provided by one of the combining atoms, a coordinate bond is formed. The atom, which provides the electron pair is termed as the donor atom, while the other atom, which accepts it, is termed as the acceptor atom. Such a bond is also known as dative bond. An arrow (g) pointing from donor towards the acceptor atom represents a coordinate bond. When a one sided sharing of electrons takes place, the coordinate bond so formed cannot be distinguished from a normal covalent bond.

Formation of Coordinate Bond

The formation of such bonds is illustrated through some examples given below.

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Formation of ammonium (NH+₄) ion

During the formation of ammonium ion, nitrogen is the donor atom, while H⁺ is the acceptor ion as shown below:

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Formation of ozone (O₃)Molecule

A molecule of oxygen contains two oxygen atoms joined by a double covalent bond (the electronic configuration of oxygen atom is 2, 6; it is two electron short of neon configuration). Thus, the two atoms of oxygen share two electrons each and do not require any more electrons because they have already attained stable octet configurations. If an atom of oxygen having six electrons comes closer to the oxygen molecule, the new atom may share a lone pair of electrons from either of these two oxygen atoms, which donates to the third oxygen atom without sharing any of the electrons of the third oxygen atom. As a result, a coordinate bond is formed between one of the oxygen atoms of the oxygen molecule, and the third atom of oxygen. This is shown below:

Formation of a coordinate bond between two molecules

Sometimes, two or more stable molecules combine to form a molecular complex. In a complex molecule, the constituent molecules are held together by a 'coordinate bond'. One typical example involves the molecules of NH₃ and BF₃. The electron dot structures of these molecules are:

The nitrogen atom has a complete octet around it, but boron atom has only six electrons around it. The nitrogen atom therefore donates its lone pair of electrons to boron so that its atoms also acquire the octet. This one-sided sharing between N and B atoms gives rise to a coordinate bond. Coordinate bonds are involved in the formation of transition metal complexes known as coordination compounds.