Cu Nitrate colour due to its structure

[Da Apprentice]

I've been told that the structure of Cu Nitrate is that of a octrahedral and appears similar to the picture attached. also that the distance at which the two vertical orbitals are away from each other varries the intensity of the blue colour, with the further the orbitals being away from each other the more intese the colour. I was just wondering why is this? and could someone please maybe further explain the images, i kind of know what they represnet but still don't fully understand them, for example are the orbitals pictured actually only the d orbitals and if so where are the others?

Thanks,

 

[Silex7]

i am going to share this thoughts with you..this is maybe incorrect but my guess says: this is due to the change in LFSE (Ligand Field Stabilization Energy) change from a high spin (weak field) configuration to the low spin (strong field) configuration causes electrons to redistributed in different distributin in ''eg'' and ''t2g'' orbitals the new re-distributin of electrons in these orbitals (change from height to low spin) causes to change in color intensity of the complex.

 

[Da Apprentice]

i am going to share this thoughts with you..this is maybe incorrect but my guess says: this is due to the change in LFSE (Ligand Field Stabilization Energy) change from a high spin (weak field) configuration to the low spin (strong field) configuration causes electrons to redistributed in different distributin in ''eg'' and ''t2g'' orbitals the new re-distributin of electrons in these orbitals (change from height to low spin) causes to change in color intensity of the complex.

Ok... so i know that strong field or low spin ligands are able to absorb light at low wavelengths and weak field or high spin ligands are able to absorb light at strong wavelengths. And the colour blue is classifyed as a low wavelength colour, so therefore Cu Nitrate must have a weak field or high spin ligand combination (?because it doesn't absorb the colour blue?) but how is this translated to the octahedral structure diagram?

 

[Borek]

Ok... so i know that strong field or low spin ligands are able to absorb light

It is not ligands that absorb/emit the energy. It is the central atom.

http://en.wikipedia.org/wiki/Crystal_field_theory

http://en.wikipedia.org/wiki/Ligand_field_theory

Also note that copper in water solution is always hydrated (unless water molecules are replaced by other ligands), and that solid copper nitrate is hydrated (most popular crystalline form being hexahydrate), so most likely you should think about water molecules surrounding the central copper ion.

 

[Da Apprentice]

It is not ligands that absorb/emit the energy. It is the central atom.

http://en.wikipedia.org/wiki/Crystal_field_theory

http://en.wikipedia.org/wiki/Ligand_field_theory

Also note that copper in water solution is always hydrated (unless water molecules are replaced by other ligands), and that solid copper nitrate is hydrated (most popular crystalline form being hexahydrate), so most likely you should think about water molecules surrounding the central copper ion.

Color
Crystal field splitting is also used to account for the different colors of the coordinate compounds. Strong field or low spin ligands are able to absorb light at low wavelengths. On the other hand, weak field or high spin ligands are able to absorb light at strong wavelengths. Based on the ligand involved in the coordination compound, the color of that coordination compound can be estimated using the strength and spin of that ligand field.z

http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Crystal_Field_Theory/High_Spin_and_Low_Spin

So this is completely wrong?

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I was told that the extension of the vertical d orbitals of an octehidral structrure (at least for Cu Nitrate) intensifys the blue colour of the Cu Nitrate. I'm struggling to understand how this jump in electrons from lower energy d-orbitals to higher d-orbitals relates to the octehidral diagram. Does this distance represent the strength of the crystal field?

 

[Borek]

So this is completely wrong?

It seems to me like there are small mistakes (or just bad wordings) on the page. Like "Co is a weak-field ligand" - no, Co is not a ligand, water is a ligand and Co is a central atom. Could be this text requires additional work.

Yes, the stronger the field the larger the distance, the higher the energy difference, the shorter the wavelength.

 

[What_Is_X?]

AFAIK pure anhydrous copper sulphate is just a white powder. It's only when you add water that it takes its blue colour. Not sure what bearing that has on the atomic reasons for this.

 

[Da Apprentice]

Another aspect i can't seem to understand with this concept is the effect changing the molarity of a sustance has on the colour produced. For example Cu nitrate at 0.1 mole is brighter than Cu nitrate at 0.01 mole. but why? How does changing the molarity of Cu nitrate influence the octoheral structure of it?

 

[DrDu]

The color of copper complexes is due to both various d-d transitions and charge transfer transitions from the ligands to the copper atom. I wouldn't dare to discuss the overall colour of copper complexes on the basis of only a qualtitative crystal field splitting diagram.
See, e.g.
www.wag.caltech.edu/publications/sup/pdf/204.pdf

 

[Da Apprentice]

The color of copper complexes is due to both various d-d transitions and charge transfer transitions from the ligands to the copper atom. I wouldn't dare to discuss the overall colour of copper complexes on the basis of only a qualtitative crystal field splitting diagram. See, e.g.
www.wag.caltech.edu/publications/sup/pdf/204.pdf

I'm sorry I had a lot of trouble trying to understand the document on the Url and still don't.

d-d transitions. An electron jumps from one d-orbital to another. In complexes of the transition metals the d orbitals do not all have the same energy. The pattern of splitting of the d orbitals can be calculated using crystal field theory. The extent of the splitting depends on the particular metal, its oxidation state and the nature of the ligands. The actual energy levels are shown on Tanabe-Sugano diagrams."

http://en.wikipedia.org/wiki/Transition_metal#Coloured_compounds

Also

http://en.wikipedia.org/wiki/Crystal_field#Explaining_the_colors_of_transition_metal_complexes

remembering: "the higher the energy difference, the shorter the wavelength"

using this information I'm lead to believe that d-d trasitions occur when "an electron jumps from one d-orbital to another.", now doesn't this also happen when "the molecule absorbs a photon of visible light"? So by extension isn't the energy displaced by d-d transitions the measure of the energy difference of the 2 d-orbitals involved? which relates directly to the wavelengths of light and the distance that separates the two vertical orbitals of the diagram?

I know this statement is wrong but where and why?

 

[DrDu]

No, this statement is correct. The point I am wanting to make is that d-d transitions are rather weak as they are electric dipole forbidden. The overall color of complexes is determined by various transitions, usually not only the d-d transitions, but also e.g. charge transfer transitions from the ligand to the central metal.
As a second point, the intensity of the d-d transitions ( in contrast to their position) depends not so much on the electronic splitting but more on the vibronic coupling to the ligand vibrations which have to provide the dipole moment for the transition. This may have some relation to the deviation from a perfect octahedral coordination, i.e. the Jahn-Teller effect.

 

[DrDu]

I had a closer look on the article I was citing. Apparently the d-d transition which is closest to the visible (in the extreme red) is the transition from d_x^2-y^2 to the T_2g orbitals. This is shifted to shorter wavelength when the splitting between the d_x^2-y^2 and the d_z^2 increases (for more elongated complexes) and also for increasing 10Dq.

 

[Da Apprentice]

The overall color of complexes is determined by various transitions, usually not only the d-d transitions, but also e.g. charge transfer transitions from the ligand to the central metal.

Would some other examples of these various transitions relating to overall colour be "the particular metal, its oxidation state and the nature of the ligands." or does this relate only to the extent of the splitting within the d-orbitals?

As a second point, the intensity of the d-d transitions ( in contrast to their position) depends not so much on the electronic splitting but more on the vibronic coupling to the ligand vibrations which have to provide the dipole moment for the transition. This may have some relation to the deviation from a perfect octahedral coordination, i.e. the Jahn-Teller effect.

http://en.wikipedia.org/wiki/Vibronic_coupling][/PLAIN]
vibronic coupling terms are proportional to the interaction between electronic and nuclear motions of molecules"

so isn't vibronic coupling the measure of the intesity of the d-d transitions? I don't understand this concept.
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I'm not quite sure how deviation from a perfect octeahedral coordination influences the colour radiated by the substance. Does this deviation relate to the location of the vertical d-orbitals specifically or all of the d-obitals within the octahedral stricture? or other?

 

[DrDu]

Fig 2. of the article I cited shows how the energy of the d-orbitals changes when the complex is deformed from octahedral to elongated octahedral shape. Compare this to the simplified pictures for octahedral and planar square coordination in
http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Crystal_Field_Theory/High_Spin_and_Low_Spin
which you were referring to.

 

[Da Apprentice]

Fig 2. of the article I cited shows how the energy of the d-orbitals changes when the complex is deformed from octahedral to elongated octahedral shape. Compare this to the simplified pictures for octahedral and planar square coordination in
http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Crystal_Field_Theory/High_Spin_and_Low_Spin
which you were referring to.

Please read the text on the attached picture.

So basically what i gathered from this diagram was that either I'm miss interpreting it (most likely) or my teacher's wrong. So are the statements i made correct?

 

[DrDu]

No, negative values of Q_theta mean that the octahedron gets crushed while positive values signify increasing elongation (with a square planar coordination as the limit Q = infinity). I think that symmetrized displacement refers here to the fact that they assumed that at least the four fold symmetry of rotation around the elongation axis is retained (which is not exactly the case in the compound they are studying).

You are right that the energy difference you painted in read is the one which is responsible for the absorption in the red part of the copper spectrum (=blue colour of the solution). However, strictly speaking, the designation as E_g and T_2g only refer to the ideal octahedral complex.

The intensity of the colour perceived depends in the case of copper complexes critically on how near the center of the shortest wavelength transition (which is still in the infrared) is to the visible part of the spectrum so that at least some absorption takes place in the visible part of the spectrum due to the finite width of the absorption line.

Refering to the second picture, what must be meant is obviously the energetic distance of the d-orbitals not any spatial distance.