Coordinate Bonds (DP IB Chemistry): Revision Note
Coordinate bonds
What is a coordinate bond?
In a regular covalent bond, each atom contributes one electron to the shared pair
In a coordinate bond, both electrons in the bond come from the same atom
This typically occurs when:
One atom has a lone pair of electrons
The other atom is electron-deficient, i.e. it has an unfilled outer shell
SL example: the ammonium ion (NH4+)
The hydrogen ion (H+) is electron-deficient and can accept a pair of electrons
The nitrogen atom in ammonia (NH3) has a lone pair that it donates to the H+ ion
This forms a coordinate bond and creates the ammonium ion (NH4+)
HL example: coordinate bonding in transition metal complexes
Coordinate bonding also occurs in some transition metal compounds formed
For example, the aluminium(III) hexaaqua ion ([Al(H2O6]3+) complex
This complex contains six coordinate bonds between water molecules and the Al3+ ion
In each case, the oxygen atom in H2O donates a lone pair to the metal ion
Examiner Tips and Tricks
This aluminium(III) hexaaqua ion ([Al(H2O6]3+) example is officially part of the AHL topic on transition metal complexes
SL students are:
Not required to learn the details of these metal complexes
Should recognise that coordinate bonding can involve species beyond simple molecules like NH4+
More examples of coordinate bonding can be found in the section on Lewis structures
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