An aqueous solution of the complex ion [Cr(H2O)6]3+ is observed to be emerald green
CHEMISTRY · Multi-part · A Level · Chemistry · Transition metals and their chemistry · Transition metals and their chemistry
Problem
An aqueous solution of the complex ion [Cr(H2O)6]3+ is observed to be emerald green. A simplified UV–visible absorbance spectrum of the solution is provided below in textual form.
Wavelength (nm) : 400 420 440 460 480 500 520 540 560 580 600 620
Absorbance (a.u.): 0.65 0.80 0.60 0.40 0.20 0.15 0.12 0.20 0.35 0.55 0.70 0.60
Answer the following questions about the colour of the solution.
(a) Using the data in the spectrum, identify the wavelength region where the complex absorbs most strongly. Explain how this absorption leads to the observed green colour of the solution.
(b) Transition‐metal complexes often exhibit absorption bands that correspond to d→d electronic transitions. Describe, in terms of crystal-field splitting of the Cr(III) d orbitals, why these transitions fall in the visible region of the electromagnetic spectrum for [Cr(H2O)6]3+.
(c) Predict qualitatively how replacing the six water ligands with six chloride ligands to form [CrCl6]3− would be expected to change (i) the position of the absorption maximum and (ii) the colour of the solution. Justify each prediction.