Question

What is a `"coordinate covalent bond"`?

Answer 1

A co-ordinate covalent bond is a 2-centre, 2-electron bond in which the 2 electrons formally derive from the same atom.

Explanation:

This is best illustrated with an example: consider the basicity of ammonia:

`NH_3(aq) + H_2O(l) rightleftharpoons NH_4^+ + HO^-`

In the forward equilibrium, ammonia has bound a proton to form the ammonium ion. In `NH_4^+` all the hydrogen atoms are equivalent, yet the 2 electrons that form one of the `N-H` derive from the lone pair on the nitrogen. Coordination chemistry is replete with examples of `"coordinate covalent bonds"`, or `"donative"` or `"dative"` bonds, i.e. the hexaqua transition metal complexes, `[M(OH_2)_6]^(2+)`, in which a lone pair on the oxygen donor forms a covalent bond with the metal centre.