Explain the violet colour of the complex [Ti(H2O)6]3+ on the basis of crystal field theory.
The complex [Ti(H₂O)₆]³⁺ is an octahedral complex where Ti³⁺ is a 3d¹ system with a single electron. According to crystal field theory, in the ground state, this electron occupies the lower energy t₂g level.
When light corresponding to the blue-green region is absorbed by the complex, the electron gets excited from the t₂g level to the higher energy e₉ level (t₂g¹ e₉⁰ → t₂g⁰ e₉¹). This d-d transition causes the complex to absorb blue-green light.
Since the blue-green component is absorbed, the complementary colour is transmitted, making the complex appear violet in colour. Thus, crystal field theory attributes the colour of coordination compounds to d-d transitions.
Explanation
The textbook explains that [Ti(H₂O)₆]³⁺ is violet because the single d-electron in Ti³⁺ (3d¹) undergoes a d-d transition from t₂g to e₉ level upon absorbing blue-green light. The absorbed wavelength corresponds to the energy difference between t₂g and e₉ orbitals, and the transmitted complementary colour (violet) is what we observe.
Solution Steps
Step 1: Identify Ti³⁺ as 3d¹ system with one electron in t₂g level (ground state)
Step 2: Light absorption excites electron from t₂g to e₉ level (d-d transition)
Step 3: Blue-green light absorbed, so violet colour transmitted/observed