3.1 UV-visible absorption spectroscopy

This section will include:

• Example spectra
• Explanation of key features of the spectra
• Using spectra to calculate values of Δ_o
• Selection rules

Absorption Spectra of Transition Metal Complexes

Solutions of transition metal complexes come in a variety of colours. For example, a solution of [Cu(OH₂)₆]²⁺ is blue, a solution of [MnO₄]^- is an intense purple, and a solution of [Co(OH₂)₆]²⁺ in water is pink. In contrast, [Zn(OH₂)₆]²⁺ is colourless. What is responsible for the difference in colour?

Complexes are coloured because they absorb light in the visible region. The colour observed (or transmitted) is complementary to the colour of the light absorbed. If light from the yellow-green part of the spectrum is absorbed, the colour observed will be white light with yellow-green subtracted out, that is, violet.

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