Notes On Colour in Coordination Compounds and Limitations of Crystal Field Theory - CBSE Class 12 Chemistry
Transition metal atoms (or) ions with one (or) more unpaired electrons and their complexes exhibit colour both in their solid and in solution states. If absorption occurs in the visible region of the spectrum, then the transmitted light bears a colour complementary to the colour of the light absorbed. The relationship between the absorbed and transmitted wavelengths can be readily understood from the following, The origin of the colour of coordination compounds can be readily explained in terms of the crystal field theory. When a ligand approaches the metal ion containing unpaired electrons the five degenerate d-orbitals separate into two distinct energy levels t2g and eg. The colour of coordination compounds arises from the electron transition between the split d- orbital energy levels. When light passes through a solution of the complex, the electron in the lower t2g level is promoted to a higher eg level by absorbing light with energy equal to the energy difference between the t2g and eg levels. Ex: Hexa Aqua Titanium(III) is coloured due presence of unpaired electron. The colour of gem stones like ruby and emerald is due to the d-to-d transition of electrons within the d orbitals of the transition metal ion. Ruby is a pink to blood red coloured gem stone of aluminium oxide. Emerald is a green variety of beryl. The green colour of emerald is due to the presence of chromium three plus ions. In the absence of a ligand, crystal field splitting does not occur and the substance remains colourless. Drawbacks of Crystal field theory: (i) According to crystal field theory anionic ligands are point charges, then anionic ligands should act as strong ligands and should exert the greatest splitting effect. But, anionic ligands are actually found at the low end of the spectro-chemical series. (ii) It does not take into account the covalent character of bonding between the ligand and the central atom.

#### Summary

Transition metal atoms (or) ions with one (or) more unpaired electrons and their complexes exhibit colour both in their solid and in solution states. If absorption occurs in the visible region of the spectrum, then the transmitted light bears a colour complementary to the colour of the light absorbed. The relationship between the absorbed and transmitted wavelengths can be readily understood from the following, The origin of the colour of coordination compounds can be readily explained in terms of the crystal field theory. When a ligand approaches the metal ion containing unpaired electrons the five degenerate d-orbitals separate into two distinct energy levels t2g and eg. The colour of coordination compounds arises from the electron transition between the split d- orbital energy levels. When light passes through a solution of the complex, the electron in the lower t2g level is promoted to a higher eg level by absorbing light with energy equal to the energy difference between the t2g and eg levels. Ex: Hexa Aqua Titanium(III) is coloured due presence of unpaired electron. The colour of gem stones like ruby and emerald is due to the d-to-d transition of electrons within the d orbitals of the transition metal ion. Ruby is a pink to blood red coloured gem stone of aluminium oxide. Emerald is a green variety of beryl. The green colour of emerald is due to the presence of chromium three plus ions. In the absence of a ligand, crystal field splitting does not occur and the substance remains colourless. Drawbacks of Crystal field theory: (i) According to crystal field theory anionic ligands are point charges, then anionic ligands should act as strong ligands and should exert the greatest splitting effect. But, anionic ligands are actually found at the low end of the spectro-chemical series. (ii) It does not take into account the covalent character of bonding between the ligand and the central atom.