Chapter 3: – Chemical Bonding
B.sc 1st year Book
(Page 6)

Coordinate Bond

Sidgwick proposed that a coordinate bond is formed when an atom has the capability to donate a lone pair of electrons and another atom has the capability to accept the donated electrons. The atom which donates the tone pair of electrons is called the ‘donor’ and the atom which accepts them is called the ‘acceptor’. This type of bond is also called a coordinate covalent bond or dative bond’ and the compounds containing such bonds are called coordinate compounds. According to G.A. Parkins (1921), coordinate bonds are formed by the one-sided sharing of electron pairs between the two atoms. The donation of lone pair of electrons to the acceptor fulfills the need for the acceptor to complete its octet. Such a type of bond is represented by an arrow (→ ) pointing towards donor to acceptors Coordinate bonds are extremely important in the case of transitional elements when they form complex compounds. Example:
This reaction causes the formation of an adduct i.e. an addition compound between two different molecules of independent existence. In this mechanism since the contribution of lone pair of electrons is one-sided, therefore, a slight polarity is developed in the molecule. Due to this reason, Sugden called it a ‘semipolar bond’.

Since A donates a lone pair of electrons to B due to this, A is regarded as Lewis base and a positive charge is developed on it whereas B accepts the lone pair donated by A is regarded as Lewis acid, and a negative charge developed on it. Some other examples of coordinate compounds are.Characteristics of coordinate compounds :

1- They do not ionize in water and hence act as a poor conductor of electricity.
2- These compounds are sparingly soluble in water but soluble in organic solvents.
3- They have a higher value of melting and boiling points as compared to purely covalent molecules but lower than ionic compounds.
4- Since the coordinate bond is directional hence it exhibits space isomerism.
5- Such type of bond can be easily broken when the donor and acceptor have their independent existence like H3N⟶BF3.
6- Coordinate bond results in semi-polarity in the molecule, the coordinate compounds, therefore, lie between electrovalent and covalent compounds in their volatility.
7-Coordinate bonds are formed only when an anion or bonded atom containing at least one nonbonding electron pair (l.p.) combines with electron-deficient molecules or metallic ions. In other words, they are formed by the combination of Lewis bases with Lewis acids.
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