Color of complex Chromium (III) solution?

[Cr(H₂O)₆]³⁺ violet

[Cr(NH₃)₆]³⁺ yellow

[Cr(H₂O)₄Cl₂]⁺ green

The absorption bands of these complexes move to lower energies in the order of the spectrochemical series for the ligands:

NO⁺ > CO ≥ PF₃ ≥ CN⁻ > NO₂ > NH₃ > H₂O > OH⁻ > F⁻ > S²⁻ > Cl ⁻ > Br ⁻ > I⁻

It is a matter of you seeing the colors that aren’t absorbed. The position of the absorption peaks can be understood. The answers above are correct, but I’m an inorganic chemistry professor who teaches spectroscopy of these kinds of compounds and the truth is that the colors aren’t as easy to predict from these as textbooks pretend they are. Like Roland said, he ᴘᴇᴇked. (Well, he would have said that if he’d hadn’t made a mistake with a homonym.) If I hadn’t seen these complexes before, I would have had to ᴘᴇᴇk too.

EDIT: Roland, we’re very close in age, I suspect. This is a just another question that can’t really be answered. Don’t feel bad about the homonym – my correction was meant to be good-natured. They’re embarrassing errors I find that I make more often now than I ever did in ninth grade…sigh…

Hi john: I’ll save you from Ms redhead619!

You need to know that [Cr(H2O)6]3+ is violet (Miss R is correct). (Oh! didn’t read last line so you can do it with info provided.) In other words it is letting through red and some blue so it is absorbing green. The energy of the absorption can be related to Δoct that the H2O ligand produces. Now NH3 is further along in the spectrochemical series and gives a slightly larger Δoct (cf adding NH3 to Cu2+aq). This will move the absorption to higher E in [Cr(NH3)6]3+ and take out all the blue to give a yellow soln ([Cr(NH3)6]3+ is yellow: I ᴘᴇᴇked!). On the other hand, Cl- is a weaker field ligand than H2O which will shift the absorption to lower E and take out the red and the cmplx is predicted to be yellow (trans-[Cr(H2O)4Cl2]+ is yellow: ᴘᴇᴇked again).

Hope this helps: it should! dr p

PS Thank you for pointing out the typo Timothy. I am also a professor of inorganic chemistry (full professor since 1990). At this level we can’t get into RS coupling and spectroscopic states.

For the best answers, search on this site shorturl.im/avW77

These are Cr(III) cmpds: [Ar] 3d3. In the six-coordinate arrangement of ligands about the central Cr(III) ion (the octahedral ligand field) the five 3d atomic orbitals do not have the same energy. They split into a low energy set of three d orbitals (labelled t2g) and a higher energy set of two d orbitals (the eg set) this is called crystal field or more correctly ligand field splitting. The three d electrons in Cr(III) occupy the t2g set and (in accordance with Hund’s rule) are unpaired. The t2g → eg energy gap is in the visible region and absorption of a photon of a specific wavelength (actually due to vibrations of the ligands it is a band of wavelengths) causes the following electronic transition t2g3 eg0 → hν → t2g2 eg1 and an observer sees the complementary color to the light being absorbed. In [Cr(H2O)6]^3+ the absorption is in the green/yellow region of the visible spectrum and red/blue is transmitted and reflected and hence the solution is purple. Changing the ligand environment changes the ligand field splitting and it is found that the chloride ion causes a smaller splitting (google spectrochemical series). In [Cr(H2O)5(Cl)]^2+ the splitting is smaller than in the pure aqua complex and absorbs in the lower-energy red region of the visible spectrum. Consequently an observer sees the complementary color to red namely blue.

Cr Nh3 6

purple