Colour of complex compounds and crystal field theory

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  • #1
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Hi
It is know that the colour exhibited by coordinate compound is because of d-d electron transitions.
These d-d transition lead to absorption of photons of a paticular frequence. AND when a
electron become exited due to absorption of photon it should radiate electromagnetic wave of frequency
corresponding to energy difference between exited and ground state, isn,t it? It would mean that photon
absorbed by electron would be radiate by it at some point of time or instantaneously? But it is said that
colour of coordinate compound which we percive is complementary to the colour absorbed, HOW?
IS it because of the reason that photon that will be randomly radiated by exited electron in all directions
and because of this rate at which that particular photon comes out of coordinate susbstance is very low compared
to the photons of complementary colour which are just simple reflected off from coordinate substance? I am not
sure. I need help !
 

Answers and Replies

  • #2
chemisttree
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The color you percieve an object to be are those colors reflected (not absorbed) from the object. If something absorbs blue light it reflects everything else and it appears yellowish red.

Apply this to your problem.
 
  • #3
DrDu
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The color you percieve an object to be are those colors reflected (not absorbed) from the object. If something absorbs blue light it reflects everything else and it appears yellowish red.

Apply this to your problem.
No, that's not true. In fact, a coloured substance always reflects best (at least for specular reflection) the colour it absorbes. You can observe this easily writing with a transparency marker on a black piece of paper to eliminate the reflection from the paper below.
It is also seen in crystals of strongly coloured substances like potassium permanganate.
 
  • #4
DrDu
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Hi
It is know that the colour exhibited by coordinate compound is because of d-d electron transitions.
These d-d transition lead to absorption of photons of a paticular frequence. AND when a
electron become exited due to absorption of photon it should radiate electromagnetic wave of frequency
corresponding to energy difference between exited and ground state, isn,t it? It would mean that photon
absorbed by electron would be radiate by it at some point of time or instantaneously? But it is said that
colour of coordinate compound which we percive is complementary to the colour absorbed, HOW?
IS it because of the reason that photon that will be randomly radiated by exited electron in all directions
and because of this rate at which that particular photon comes out of coordinate susbstance is very low compared
to the photons of complementary colour which are just simple reflected off from coordinate substance? I am not
sure. I need help !
You are mixing up two things here. One is absorption. When looking through a solution of e.g. Copper sulfate, the copper ions will absorb the yellow part of the spectrum leaving only the complementary blueish color components of the incident white light.
The second thing is that some atoms may re-emit radiation after absorption. This process is called fluorescence. Usually, the fluorescent light has a lower frequency than the absorbed light. Furthermore, it is emitted in all directions, not only in the direction of the incident light. However most substances, especially if they contain water (like copper sulfate) do not fluoresce as the excitation energy of the atoms is not re-emitted but is transformed into vibration energy (that is, at the end, into heat).
 
  • #5
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Hello Drdu.
When a coordination entity say [Cu(H2O)4]2+ absorb a photon and goes to exited state then what will happen to the next
photon which is incidented on same coordination complex which it has absorbed earlier? Will it be reflected back because
there is no electron available for d-d transtion? OR By the time next photon will be incidented on coordinate complex it
would have come to the ground state? How a coordination entity deals with the energy of photon it had absorbed? Is it radiated
back in form of electromagnetic wave or it gets incorporated in to molecular motional energy?
 
  • #6
DrDu
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At usual light intensities (i.e not in a strong laser) it is very improbable that two photons hit the same atom one shortly after the other. In the case of the copper complex, the energy of the first photon will be rapidly be converted into heat and the electronic system will return to its ground state.
 
  • #7
chemisttree
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No, that's not true.
It's absolutely true!

In fact, a coloured substance always reflects best (at least for specular reflection) the colour it absorbes.
This is such nonsense! The OP is asking about the color of complexes. These are determined in solution! Care to discuss specular reflection of a dissolved substance?

Are you seriously implying that a complex that absorbs in the blue appears blue because that is where it best reflects blue?

Nonsense.
 
  • #8
DrDu
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The color of complexes is mostly determined in solution.true. But not the light reflected from the surface of the solution but the color of the light transmitted. At least that is how it is quantified in photometers. The light you call "reflected" is due to reflections at the backside of the solution so that in effect they are also due to absorption in transmition.
In case of a dilute solution, the differences in reflection from the surface are minute and the reflected light appears white.
 

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