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Advanced level inorganic chemistry: UV and visible light absorption spectroscopy of chromium complexes

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Doc Brown's Advanced Chemistry: PART 15.5 uv and visible absorption spectra of chromium complex ions - transition metal absorption spectroscopy - chromium compounds

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses Spectroscopic methods of analysis and molecular structure determination investigating chromium complexes

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15.5.1 The origin of colour, the wavelengths of visible light, our perception!

15.5.2 uv-visible spectroscopy theory, spectrometer, examples of absorption & reflectance spectra explained

15.5.3 uv-visible absorption spectra - index of examples: uses, applications, more on the chemistry of colour

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The uv and visible absorption spectra of chromium complex ions and compounds

(a) The uv-visible light absorption spectrum of the hexaaquachromium(III) ion

For the [Cr(H2O)6]3+(aq) ion, data from an old textbook quotes a λmax of 400 nm in the violet region and a λmax of 580 nm centred around the yellow orange region.

uv-visible absorption spectra of purple hexaaquachromium(III) complex ion aqueous Co3+ ion wavelength peaks of maximum absorption

structure and shape of hexaaquachromium(III) complex ion octahedral Co3+(aq)aqueous ion coordination number 6 oxidation state +3The colour of the [Cr(H2O)6]3+(aq) hexaaquachromium(III) ion is a sort of violet-blue-grey, but tends to be greenish in the presence of chloride ions due to the formation of aqua-chloro complexes e.g. [Cr(H2O)4Cl2]+(aq) which is dark green.

[Cr(H2O)5Cl]2+(aq) is green and note how to work out the overall charge on these three chromium(III) complex ions.

Remember a change of ligand usually results in a change of 3d orbital electronic levels, and the ligand field splitting, hence a change in absorption spectrum and resulting colour.

diagram showing the 3d orbital ligand/crystal field splitting effect in the hexaaquachromium(III) complex ion [Cr(H2O)6]3+(aq) Cr3+

A diagram showing the 3d orbital ligand/crystal field splitting effect in the hexaaquachromium(III) complex ion and the excitation by a visible light photon of an electron from a lower to a higher 3d orbital level.

There must be at least two possible excitations to produce two absorbance peaks in the visible region.

For more see the chemistry of chromium

 

(b) The colour of the hexaamminechromium(III) ion

structure and shape of hexaamminechromium(III) ion octahedral complex ion coordination number 6 oxidation state +3If you change the ligand, you usually change the ∆Eelec , in this case excitation of an electron from the lower 3d orbital to a higher 3d orbital in the hexaamminechromium(III) complex ion.

The change of ligand is from water to ammonia - both lone pair donors to form dative covalent bonds with the central metal ion, the result is the dark green coloured hexaamminechromium(III) ion, formed a simple ligand exchange reaction.

[Cr(H2O)6]3+(aq) + 6NH3(aq) ===> [Cr(NH3)6]3+(aq) + 6H2O(l)

diagram showing the 3d orbital ligand/crystal field splitting effect in the hexaamminechromium(III) complex ion [Cr(NH3)6]3+(aq)

A diagram showing the 3d orbital ligand/crystal field splitting effect in the hexaamminechromium(III) complex ion and the excitation by a visible light photon of an electron from a lower to a higher 3d orbital level.

A different ligand gives a different ∆Eelec, hence different absorption spectra and different coloured complex ion.

For more see the chemistry of chromium

 

(c) The colour of the hexaaquachromium(II) ion

structure and shape of hexaaquachromium(II) ion octahedral Co2+(aq)aqueous ion coordination number 6 oxidation state +2The blue hexaaquachromium(II) ion, [Cr(H2O)6]2+(aq)

The change in oxidation state changes the electronic environment of the theoretical central Cr2+ ion in the complex compared to the Cr3+ ion.

This in turn changes the ∆Eelec energy needed to excite an electron from the lower 3d orbital to a higher one from the field splitting effect of the six ligands surrounding the central Cr2+ ion.

diagram showing the 3d orbital ligand/crystal field splitting effect in the hexaaquachromium(II) complex ion Cr2+(aq)[Cr(H2O)6]2+(aq)  

The ligand field electronic splitting diagram for the aqueous Cr2+ ion.

For more see the chemistry of chromium

 

(d) The uv-visible light absorption spectra of the dichromate(VI) ion and chromate(VI) ion

The yellow chromate(VI) ion and orange dichromate(VI) ion form an equilibrium, one species dominates depending on the pH of the solution.

2CrO42(aq) +  2H+(aq) Cr2O72(aq)  +   H2O(l) 

There is no change in oxidation state, but the electronic environment of the central chromium 'ion' is changed so they have different uv-visible absorption spectra and the different field splitting ∆Eelec values result in the two different colours.

the uv-visible absorption spectra of the yellow chromate(VI) ion CrO42- and the orange dichromate(VI) ion Cr2O72- wavelength peaks of maximum absorption

The chromate(VI) ion strongly absorbs in the violet region and gives a yellow colour.

The dichromate(VI) ion absorbs in the blue and produces an orange colour.

For more see the chemistry of chromium

 


Key words & phrases: interpreting the uv-visible absorption spectra of chromium compounds, identifying the maximum absorption peaks in the uv-visible absorption spectra of chromium compounds, explaining the uv-visible absorption spectra of chromium compounds, how to use the visible absorption spectra of chromium compounds to explain the different colours of chromium compounds, applications of the uv-visible absorption spectra of chromium compounds interpreting the uv-visible absorption spectra of chromium complex ions, identifying the maximum absorption peaks in the uv-visible absorption spectra of chromium complex ions, explaining the uv-visible absorption spectra of chromium complex ions, how to use the visible absorption spectra of chromium complex ions to explain the different colours of chromium complex ions, applications of the uv-visible absorption spectra of chromium complex ions interpreting the uv-visible absorption spectra of hexaaquachromium(III) ion, identifying the maximum absorption peaks in the uv-visible absorption spectra of hexaaquachromium(III) ion, explaining the uv-visible absorption spectra of hexaaquachromium(III) ion, how to use the visible absorption spectra of hexaaquachromium(III) ion to explain the different colours of hexaaquachromium(III) ion, applications of the uv-visible absorption spectra of hexaaquachromium(III) ion interpreting the uv-visible absorption spectra of chromate(VI) ion dichromate(VI) ion comparison, identifying the maximum absorption peaks in the uv-visible absorption spectra of chromate(VI) ion dichromate(VI) ion comparison, explaining the uv-visible absorption spectra of chromate(VI) ion dichromate(VI) ion comparison, how to use the visible absorption spectra of chromate(VI) ion dichromate(VI) ion comparison to explain the different colours of chromate(VI) ion dichromate(VI) ion comparison, applications of the uv-visible absorption spectra of chromate(VI) ion dichromate(VI) ion comparison


Associated links

UV and visible spectroscopy index

General introduction to electron configuration of transition metal ions and colour theory

The chemistry of chromium

Index of Advanced A level Notes on the 3d block and Transition Metals

SPECTROSCOPY INDEXES

All Advanced Inorganic Chemistry Notes

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