TheStoryteller
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I was reading through some past paper questions and I found this: Why does [Mn(H2O)6]2+ have a pale pink colour whilst [Cr(H2O)6]2+ is strongly coloured?

Is it because of the special stabilities of a half full d-subshell in the case of Mn2+? I understand that colour comes from the promotion of electrons from the lower d orbital to the upper d orbital but I can't really put my finger on why it transmits a paler colour :curious:
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QuantumSuicide
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The reasons for the colours are more than just the fact that d-orbitals are partially filled.

If this is for ocr, don't worry about it, you don't need to know why.

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TheStoryteller
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(Original post by QuantumSuicide)
The reasons for the colours are more than just the fact that d-orbitals are partially filled.

If this is for ocr, don't worry about it, you don't need to know why.

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Oh no (unfortunately) I'm from outside the UK and our examination board literally examines us on the smallest possible details :rolleyes: I read somewhere that it's due to spin-allowed transitions but I can't seem to find anything about that in any of my notes/books
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jacksonmeg
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(Original post by TheStoryteller)
Oh no (unfortunately) I'm from outside the UK and our examination board literally examines us on the smallest possible details :rolleyes: I read somewhere that it's due to spin-allowed transitions but I can't seem to find anything about that in any of my notes/books
I remember its something to do with 3d orbital splitting into two levels and electrons pass between the lower 3d orbital to the high 3d orbital and changing something about how light is absorbed. I don't know much about it
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Peteyology
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(Original post by TheStoryteller)
I was reading through some past paper questions and I found this: Why does [Mn(H2O)6]2+ have a pale pink colour whilst [Cr(H2O)6]2+ is strongly coloured?

Is it because of the special stabilities of a half full d-subshell in the case of Mn2+? I understand that colour comes from the promotion of electrons from the lower d orbital to the upper d orbital but I can't really put my finger on why it transmits a paler colour :curious:
Depends on the energy gap between the split levels. This jump of the electron to the higher energy level reflects a colour..the different ligands have a different energy gap and therefore reflect a different colour.
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TheStoryteller
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(Original post by Peteyology)
Depends on the energy gap between the split levels. This jump of the electron to the higher energy level reflects a colour..the different ligands have a different energy gap and therefore reflect a different colour.
Indeed But apparently there's a reason why the colour is paler than normal hmm. I think it might have something to do with repulsion; since Mn2+ has one electron in each box when drawing the electrons-in-boxes, then promoting an electron to a higher level will cause two electrons to pair up creating repulsion but I can't see how that would explain why there would be a paler colour since the wavelength of light would have already been absorbed to promote the electron so technically the colour transmitted should simply be a pink. :confused:
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langlitz
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It is to do with something called the crystal field stabilisation energy (CFSE). Cr2+ had a dn configuration of d4, Mn2+ has a dn configuration of d5. Water is a weak field ligand and thus the complexes are high spin. This means that the CFSE for the Cr2+ complex is -3/5Δo where as it is 0Δo for the Mn2+. Different Δo=different colours
Absorption of light of correct frequency promotes electrons from the ground state to an exited state and is responsible for the colour of the metal complex. This is called a d-d transfer (transition) and can be measured
by absorption spectra
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charco
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(Original post by Peteyology)
Depends on the energy gap between the split levels. This jump of the electron to the higher energy level reflects a colour..the different ligands have a different energy gap and therefore reflect a different colour.
There is NO reflection.

The process involves absorption of energy, and the energy that is not absorbed is transmitted through the solution and observed. This is the complementary colour to the absorbed colour.
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Borek
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I suppose "reflect" was intended to mean "embody or represent (something) in a faithful or appropriate way".
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TheStoryteller
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(Original post by charco)
There is NO reflection.

The process involves absorption of energy, and the energy that is not absorbed is transmitted through the solution and observed. This is the complementary colour to the absorbed colour.
I actually didn't realise the post said reflected but yep it's definitely the absorption of energy Thanks for pointing it out
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