It has been observed that coordination compounds exhibit a wide range of colors. The coloration of opaque substances is due to the absorption of a particular wavelength of the visible light falling on their surface by those opaque substances. This means that coordination compounds absorb some of the visible spectra from white light when it falls on its surface. The spectrum of light reflected by them doesn’t contain the color of a wavelength that was absorbed by the substance. Hence, we observe the color of the coordination compound to be complimentary of the color absorbed by it.
The coloration of coordination compounds can be explained with the help of crystal field theory. Generally, d orbitals are degenerate that is all the orbitals have the same energy level in the absence of ligands. But, once the central metal starts bonding with other ligands, due to different symmetries of d orbitals and the inductive effect of the ligands on the electrons, d orbitals split apart and become non-degenerate. Thus, crystal field theory explains the crystal field splitting (splitting of degenerate levels)and asymmetry on the basis of repulsion between electrons in d orbitals of metal and electrons of ligand,d-orbitals split into two energy levels due to these repulsions:
- t2g– set of three orbitals (dxy, dyz and dxz) with lower energy
- eg – set of two orbitals (dx2-y2 and dz2) with higher energy
Form the figure it can be inferred that for an electron to jump from lower energy (t2g energy level) d orbital to higher energy(energy level)d orbital that is the d-d transition, energy is required. This energy is provided by the frequency of light absorbed. For example: [Ti(H2O)6]3+. [Ti (H2O)6]3+ is violet in color. It is an octahedral transition metal complex which has a configuration of t2g1eg0 in the ground state. In case, the frequency of light corresponding to the energy of blue-green region(region of a color complementary to violet) is absorbed by this transition metal complex, then an electron from t2g level is excited to eg giving it a configuration of t2g0eg1. As a result, [Ti (H2O) 6]3+ appears violet in color. Thus, according to crystal field theory d-d transition is responsible for the coloration of coordination compounds.
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