Crystal Field And Colour

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An aqueous solution of a titanium(III) complex appears purple because the single d electron absorbs a photon and undergoes which electronic transition?

RankGuru · Accepted Answer

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.

Crystal Field And Colour

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