Question

Explain the violet colour of the complex [Ti(H₂O)₆]³⁺ on the basis of crystal field theory.

Answer 1

Coordination compounds exhibit colours attributed to the crystal field theory, which correspond to the d-d transitions of the elements. For example, excitation of an electron from vacant state e.g. absorbing light will excite the electron from t_2g level. Now, in case of [Ti(H₂O)₆]³⁺ when light corresponding to the energy of the yellow-green region is absorbed by the complex, it will excite the electron from t_2g level to a level. Thus, the complex will be violet in colour.

Answer 2

1. The electronic configuration of Ti³⁺ is [Ar] 3d¹.
2. The presence of ligands causes the five d-orbitals to be divided into two sets: three orbitals with lower energy and two orbitals with higher energy.
3. So, there occur d-d transitions between these two sets of orbitals. In the ground state, the only d-electron occupies one of the three lower-energy d-orbitals.
4. Upon absorbing light energy in the visible spectrum, this d-electron is promoted to a higher-energy d-orbital. It absorbs light in the green and yellow parts of the visible spectrum, resulting in transmitted light that is red and violet (purple).