Advanced level inorganic chemistry: UV and visible light absorption spectroscopy of cobalt complex ions

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

[Author ©  Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11-12 & AP honors chemistry courses: The uv-visible absorption spectroscopy of some cobalt(II) complexes [uv-visible spectra page updated May 25th 2026 **]

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The chemistry of cobalt

The uv and visible absorption spectra of some cobalt complex ions

(a) The hexaaquacobalt(II) and the tetrachlorocobaltate(II) complex ions and compounds

explaining diagram of uv-visible absorption spectra of cobalt complex ions hexaaquacobalt(II) tetrachlorocobaltate(II) complex ions spectra cobalt complexes

Image adapted from https://web.iitd.ac.in/~sdeep/Elias_Inorg_lec_4.pdf (failed link now?)

diagram image of the visualisation of the wavelengths of visible light in nanometres nm

pink hexaaquacobalt(II) complex ion structure coordination number 6 octahedral shape water ligandThe pink, octahedral, hexaaquacobalt(II) ion, [Co(H2O)6]2+, has a λmax of ~ 540 nm, weakly absorbing in the blue to yellow region, violet and red transmitted giving a pink colour.

and the blue tetrahedral tetrachlorocobaltate(II) ion, [CoCl4]2-,  has a λmax of ~  720 nm, strongly absorbing in the yellow to red region, resulting in the deep blue colour.

(There are two other significant peaks of lesser absorbances).

blue tetrachlorocobalt(II) complex ion structure coordination number 4 tetrahedral shape chloride ion ligandYou readily convert the pink ion to the blue ion by adding hydrochloric acid.

[Co(H2O)6]2+(aq) + 4Cl–(aq) rev [CoCl4]2–(aq) + 6H2O(l) 

Changing both the ligand, coordination number (6 to 4) and shape, produce a completely different complex ion colour.

The Co oxidation state remains at +2.

For more details see the chemistry of cobalt

Electronic explanation of the spectra of two ions above

This is a  pre-university website so my approach to explaining the colour of the complex ion(s) of cobalt is VERY simplistic and does not involve ligand field molecular orbital theory! For explaining the absorption spectra of the complex ions of cobalt I've presented a simple view of crystal field theory on a separate page. Although not shown, for the 3d electronic quantum levels, the principal field splitting is shown as a 3 lower : 2 higher for octahedral and square planar complexes and a 2 lower : 3 upper for tetrahedral complexes. BUT, in all cases, there are more sub-splittings, but this is well beyond the scope of this website for the complex ions of cobalt.

3d orbitals field splitting electronic diagram for a Co2+ octahedral complex ion of pink hexaaquacobalt(II) ion cobalt complex ions cobalt(II) complexes

Both these ions are coloured because:

(i) the ligand field cause a splitting of the 3d electronic energy levels of the cobalt(II) ion,

(meaning the orbitals are no longer of the same quantum electronic energy level),

(ii) it is possible for an electron to be promoted from a ground state to an excited state in the Co2+ ion,

(This electron goes into a sub-level not shown in the diagrams)

(iii) The splitting energy, ∆E, is in the visible region of photon energy, therefore a colour is perceived as some visible light, as well as absorption, will be transmitted through a solution of the cobalt(II) complex ion, producing the colour from the resulting absorption spectrum in the visible region - in this case the pink of the hydrated cobalt(II) ion..

The electronic diagrams for a cobalt(II) Co2+ ion octahedral complex is shown above, pink [Co(H2O)6]2+, and that for a tetrahedral complex ion of cobalt(II), blue [CoCl4]2-, shown below.

A weakly binding ligand like water gives a high spin complex ion of cobalt(II), where the maximum number of unpaired electrons in the 3d orbitals is allowed.

3d orbitals field splitting electronic diagram for a Co2+ tetrahedral complex ion of cobalt tetrachlorocobaltate(II) ion complexes of cobalt(II)

 

(b) Complexes formed with the ligand ammonia - which displaces water from the hexaaqua Co2+ ion

 uv-visible absorption spectra of cobalt comlex ions hexaamminecobalt(III) ion spectra

yellow-orange hexaamminecobalt(III) complex ion structure coordination number 6 octahedral shape ammonia ligandThe octahedral complex ion, hexaamminecobalt(III) ion, [Co(NH3)6]3+(aq) absorbs in the blue and has a yellow-orange (light brown) appearance,

The are two λmax peaks at 340 nm (uv) and 475 nm (visible).

The hexaamminecobalt(II) ion, [Co(NH3)6]3+ is formed by adding aqueous ammonia to the pink hexaaquacobalt(II) ion solution, giving a brown coloured complex.

brown hexaamminecobalt(II) complex ion structure coordination number 6 octahedral shape ammonia ligand

[Co(H2O)6]2+(aq) + 6NH3(aq) ===> [Co(NH3)6]2+(aq) + 6H2O(l) 

This can be converted to the more redox stable hexaamminecobalt(III) ion, [Co(NH3)6]2+ by bubbling in oxygen or adding hydrogen peroxide solution, giving a yellow-orange coloured complex ion.

(iii) 4[Co(NH3)6]2+(aq) + O2(g) + 4H+(aq) ===> 4[Co(NH3)6]3+(aq) + 2H2O(l)

(iv) 2[Co(NH3)6]2+(aq) + H2O2(aq) + 2H+(aq) ===> 2[Co(NH3)6]3+(aq) + 2H2O(l)

In both cases, changing the ligand from water to ammonia, changes the colour of the octahedral complex ion.

Here the change in oxidation state causes the change in colour, since both ammine complexes have the same ligand, shape and coordination number of 6.

For more details see the chemistry of cobalt

Electronic explanation of the spectra of the cobalt(III) hexaammine ion

3d orbitals field splitting electronic diagram for a Co3+ octahedral complex ion of brown hexaamminecobalt(III) ion cobalt complex ions cobalt(III) complexes  

One of the unpaired electrons is promoted to a higher level, again ∆E is provided by visible light photons

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Associated links

The chemistry of cobalt

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

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

UV and visible spectroscopy index

SPECTROSCOPY INDEXES

All Advanced A Level Inorganic Chemistry Notes

GCSE/IGCSE revision notes on Transition Metals

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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

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university/college advanced level inorganic chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These chemistry revision notes on spectroscopy on the uv and visible absorption spectra of selected cobalt complex ions are suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry. Examples of uv and visible light absorption spectra described and explained at Doc Brown's Chemistry

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