One of the most distinctive properties of transition metal complexes is their wide range of colours. This means that some of the visible spectrum is being removed from white light as it passes through the sample, so the light that emerges is no longer white. The colour of the complex is complementary to that which is absorbed. The complementary colour is the colour generated from the wavelength left over. If green light is absorbed by the complex, it appears red. Table given below, gives the relationship between different wavelengths of the light absorbed and the colour observed. (a) What is spectrochemical series ? Write the relationship between $\Delta_{\text{t}}$ and $\Delta_{\text{o}}$. (b) (i) Arrange $[\text{Co}(\text{CN})_6]^{3-}$, $[\text{Co}(\text{NH}_3)_6]^{3+}$ and $[\text{Cu}(\text{H}_2\text{O})_4]^{2+}$ in increasing order of wavelength of light absorbed. OR (b) (ii) On the basis of crystal field theory, write the electronic configuration of $d^5$ ion if $\Delta_{\text{o}} < \text{P}$. (c) Define crystal field splitting energy.