Crystal Field And Colour
An aqueous solution of a titanium(III) complex appears purple because the single d electron absorbs a photon and undergoes which electronic transition?
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Solution
At2gtoegd−dtransition
The colour of many transition-metal complexes comes from d-d transitions, where an electron absorbs visible light and is promoted between the crystal-field-split d levels. The hexaaqua titanium(III) ion has the configuration d^1, so its lone electron resides in the lower t_{2g} set in the ground state. Absorbing a photon of the right energy lifts this electron to the higher e_g set, a t_{2g} to e_g transition. The absorbed wavelength lies in the green-yellow region, so the transmitted light makes the solution appear purple. An e_g to t_{2g} jump is the reverse, downhill process and would emit, not absorb. Ligand-to-ligand charge transfer is not a d-d transition and does not apply to this simple aqua complex. A transition between two s orbitals is not relevant to d-block colour. This single-electron example is the standard NCERT demonstration that splitting energy corresponds to absorbed visible light. Plausibility check: the energy gap of the absorbed green-yellow photon matches the small octahedral splitting expected for a weak-field aqua ligand, consistent with the observed purple colour.
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About This Question
- Subject
- chemistry
- Chapter
- coordination compounds
- Topic
- crystal field and colour
- Difficulty
- Hard
- Year
- 2025
Solution
Correct Answer:
At2gtoegd−dtransition
The colour of many transition-metal complexes comes from d-d transitions, where an electron absorbs visible light and is promoted between the crystal-field-split d levels. The hexaaqua titanium(III) ion has the configuration d^1, so its lone electron resides in the lower t_{2g} set in the ground state. Absorbing a photon of the right energy lifts this electron to the higher e_g set, a t_{2g} to e_g transition. The absorbed wavelength lies in the green-yellow region, so the transmitted light makes the solution appear purple. An e_g to t_{2g} jump is the reverse, downhill process and would emit, not absorb. Ligand-to-ligand charge transfer is not a d-d transition and does not apply to this simple aqua complex. A transition between two s orbitals is not relevant to d-block colour. This single-electron example is the standard NCERT demonstration that splitting energy corresponds to absorbed visible light. Plausibility check: the energy gap of the absorbed green-yellow photon matches the small octahedral splitting expected for a weak-field aqua ligand, consistent with the observed purple colour.
This hard difficulty chemistry question is from the chapter coordination compounds, covering the topic of crystal field and colour. It appeared in the 2025 exam.
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