Colour change associated with Ligand Exchange reactions: When ligands are formed around a metal ion (e.g. [Fe(H2O)6]) it is known as a complex. The coordination number of a complex denotes how many ligands are attached to the central ion, it is usually 4 or 6. There are 3 types of shapes associated with complexes: - Square Planar (coordination number, 4) - Tetrahedral (coordination number, 4) - Octahedral (coordination number, 6) When complexes are formed the electrons delocalise within the molecule and this allows the complex to absorb certain frequencies of light. The frequency depends upon the type of central ion, the ligands and the shape of the complex. When the light is absorbed the electrons in the D-orbital split into 2 groups. This allows the light to be absorbed. If the frequency of the light absorbed is 450 nm it means that red light is absorbed, if the red light is absorbed it means the blue light is reflected, and this is the colour seen to the human eye. (Edited by Andy Richardson and Harry Alexander)
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