Why are some transition metal complexes coloured?

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Ligands coordinated to metal centres effectivily split the metal d-orbitals into different energy levels, some higher and some lower in energy.

When light energy is absorbed by such complexes, electrons are excited from the lower energy levels up into the higher energy levels. 

These excited electrons then relax back down, reemitting light at a particular wavelength. This wavelength corresponds to the energy difference between the ground and excited states.

Different ligands surrounding the metal centre result in different energy splittings and hence give complexes of different colours. 

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