Chemical bond
Coordinate covalent bond (dipolar bond)

A coordinate covalent bond is a covalent bond in which the two shared bonding electrons are from the same one of the atoms involved in the bond. For example, boron trifluoride (BF) and ammonia (NH) form an adduct or coordination complex F B←NH with a B–N bond in which a lone pair of electrons on N is shared with an empty atomic orbital on B. BF with an empty orbital is described as an electron pair acceptor or Lewis acid, while NH with a lone pair that can be shared is described as an electron-pair donor or Lewis base. The electrons are shared roughly equally between the atoms in contrast to ionic bonding. Such bonding is shown by an arrow pointing to the Lewis acid. (In the Figure, solid lines are bonds in the plane of the diagram, wedged bonds point towards the observer, and dashed bonds point away from the observer.)

Transition metal complexes are generally bound by coordinate covalent bonds. For example, the ion Ag reacts as a Lewis acid with two molecules of the Lewis base NH to form the complex ion Ag(NH), which has two Ag←N coordinate covalent bonds.
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