Question

Why do coordination compounds have color?

Answer 1

Hint: We know that coordination compounds are those which contain a central metal atom that is surrounded by the non-metal atoms or groups which are called ligands. The metal, especially those of transition metals i.e. d- block elements form coordination complexes.

Complete answer:
A coordination complex consists of a central atom or ion surrounded by the non-metal atoms or groups. The central atom or ion is called coordination centers and the atoms or groups which surround the coordination center are called ligand or complexing agents. In other words, these are the metals containing compounds. Coordination complexes are formed mainly by transition metal i.e. d- block metal.
Different ligands show different effects on the central metal atom or ion and on its energy. More energy is required to promote an electron from lower energy orbitals to higher energy orbitals. Higher energy refers to the shorter wavelength in terms of absorbance of the color of light.
Mostly transition metals form coordination complexes, and we know that transition metals have d- orbitals which are unfilled or half filled. According to the crystal field theory, it describes the splitting of d- orbitals into higher and lower orbital. Now, whenever the electrons of transition metals jump from lower to higher energy. Hence, there will be d-d transition of electrons. The color will be seen and it is a complementary color of the wavelength absorbed.

Note:
A very famous coordination complex of platinum (II) with two chloride and two ammonia group attached i.e. Cisplatin \[[Pt\,C{l_2}{(N{H_3})_2}]\] is used as one of the successful anticancer drugs. Some other examples of coordination complexes such as vitamin B12, hemoglobin and chlorophyll, dyes and catalyst for the synthesis of organic compounds.