Question

The colour of $CoC{l_3} \cdot 5N{H_3} \cdot {H_2}O$ is:
(A) Red
(B) Orange
(C) Orange-Yellow
(D) Pink

Answer 1

The colour of the complex depends upon the wavelength the complex absorbs. The actual colour of the complex is a complimentary colour of the wavelength absorbed by the complex.

Complete step by step solution:
We are given a complex of cobalt metal and we need to give its colour.
- We know that transition metals show different colours when they bind with different ligands and form complexes.
- So, we can say that some part of the visible light is removed when it passes through the same and that is the reason that the white light is no longer white. The colour of the complex is complementary to the colour absorbed by the complex.
- Thus, we can say that the complimentary colour is the colour that is absorbed by the complex. i.e. If green coloured light is absorbed by the complex, then the complex appears red in colour.
- The colour of the coordination complex can be explained by crystal field theory. It states that the colour of the coordination compound is due to the d-d transition of the electron.
- Colour of the complex depends upon things like oxidation state of the metal and ligand field.
- Given complex $CoC{l_3} \cdot 5N{H_3} \cdot {H_2}O$ shows a pink colour.

Thus, we can conclude that the correct answer is (D).

Note: Note that when we remove the ligands from the metal complex, then crystal splitting does not occur and thus, the complex does not have any colour. e.g. $CuS{O_4} \cdot 5{H_2}O$ is blue in colour but as we remove water from it, the compound $CuS{O_4}$ is colourless.