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Topic: Crystal Field Theory  (Read 9958 times)

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Offline zain

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Crystal Field Theory
« on: January 01, 2008, 10:15:43 AM »
Hey guys... i don't post here often but i'm absolutely stuck..
Its to do with crystal field theory. I generally understand the concepts but now i've got to this Jahn Teller stuff and i just can't see any light!

The problem is i have a question to do...
i have to sketch the crystal field splitting diagram for an octahedral low spin d5 and low spin d7 complex.
I can do everything apart from decide what is Low and high spin. and also what type of distortion will be in the d5 complex and how to find out.
If anyone can help i would be very grateful!
Many Thanks, Zain

Offline Alpha-Omega

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Re: Crystal Field Theory
« Reply #1 on: January 01, 2008, 01:08:35 PM »
Hey guys... I don't post here often but i'm absolutely stuck..
Its to do with crystal field theory. I generally understand the concepts but now i've got to this Jahn Teller stuff and i just can't see any light!

The problem is i have a question to do...
i have to sketch the crystal field splitting diagram for an octahedral low spin d5 and low spin d7 complex.
I can do everything apart from decide what is Low and high spin. and also what type of distortion will be in the d5 complex and how to find out.

If anyone can help i would be very grateful!
Many Thanks, Zain

The distinction of high spin/low spin only applies to octahedral molecules with d4, d5, d6, and d7 electronic configurations.

For configurations d4 to d7, there are two ways in which the electrons can be placed into the t2g and eg orbitals which depend on the magnitude of o (the gap between the two levels).

In the weak field case (also called high spin of spin free), the electrons are distributed one at a time, with parallel spins before pairing as if the d-orbitals were still degenerate. In the strong field case (also known as low spin or spin paired), the electrons first fill the t2g set. For configurations d1 to d3 and d8 to d10 there is no difference.

With d4-d7 metals, there is at least one electron pair for strong field ligands in the t2g energy level, and at least one empty orbital in the eg energy level.

This allows for a decrease in pairing with a weak field ligand, and thus differering paramagnetism between weak field and strong field ligands, e.g. for d5 metals:

WEAK FIELD

eg  two spin up unpaired       ---    ----

tg  three spin up unpaired  ----   ----  ----

STRONG FIELD

eg  empty                                ----     ----




tg   all five fill this level (four paired and one unpaired)     ----   ----   ----
                       

Jahn Teller Effect:


NUMBER OF ELECTRONS:   1     2    3   4    5   6    7    8    9   10
HIGH SPIN                       W   W        S        W   W         S
LOW SPIN                        W   W       W   W        S          S


W: weak Jahn-Teller effect (t2g orbitals unevenly occupied)
S: strong Jahn-Teller effect expected (eg orbitals unevenly occupied),
blank: no Jahn-Teller effect expected.

Offline zain

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Re: Crystal Field Theory
« Reply #2 on: January 01, 2008, 02:00:21 PM »
thank you! that was very helpful.
but just one more Q
how do you know whether a complex will distort by..

1. Compression
2. Elongation...

is that what you meant by weak and strong distortion?

many thanks

Offline Alpha-Omega

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Re: Crystal Field Theory
« Reply #3 on: January 01, 2008, 10:53:02 PM »
Axis of the degeneracy  axial or equitorial....where is the bonding....which molecular axes are being affected.....it is all in the geometry...

Basically, any non-linear molecule with a degenerate electronic ground state will undergo a geometrical distortion that removes that degeneracy. This also has the effect of lowering the overall energy of the complex.

For example: 

In a d9 electronic configuration like a Cu (II) octahedral complexes the distortion normally takes the form of elongating the bonds to the ligands lying along the z axis, but occasionally occurs as a shortening of these bonds instead (the Jahn-Teller theorem does not predict the direction of the distortion, only the presence of an unstable geometry).

When such an elongation occurs, the effect is to lower the electrostatic repulsion between the electron-pair on the Lewis basic ligand and any electrons in orbitals with a z component, thus lowering the energy of the complex.

For octahedral coordination, susceptible species are d4, d9 and low spin d7 in which 1 or 3 electrons occupy eg. The effect is small when the degeneracy is in the d2g group.

For tetrahedral coordination, susceptible species are high spin d2, d5, d7 and low spin d4.

The Jahn-Teller Theorem does NOT say how large a distortion should occur.

Addressing your specific question:

A non linear system with an orbitally degnerate will distort as to remove the degeneracy and lower the total energy of the system.

The effect is commonly associated with transition metal coordination complexes. In practice only degeneracies in the eg orbitals cause distortions as these are antibonding while the t2g set is largely non-bonding.

The mechanism is a mixing of the dz2 and the 4s orbital resulting in a hybrid with extended amplitude in the z direction. The distortion is therefore always an axial elongation rather than a compression. The magnitude of the energy gain may be greater or smaller than kT giving rise to the static and dynamic effects.

Tetrahedral complexes can become distorted also. [Cu(Hal)4]2- complexes are Cu(II) d9 e4t5 and are flattened tetrahedra.

The square planar geometry of Ni(II) complexes does not result from a Jahn-Teller distortion. The octahedral d8 configuration is orbitally non-degenerate.
 




Offline zain

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Re: Crystal Field Theory
« Reply #4 on: January 02, 2008, 01:09:13 PM »
thanks this was very helpful.

just one more...
is there any way to mentally figure out whether there is weak or strong distortion other than remembering which they are.

Offline Alpha-Omega

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Re: Crystal Field Theory
« Reply #5 on: January 02, 2008, 08:39:43 PM »
Yes...you will have to picture the geometry in your head....that is how you would approach that.

Offline Alpha-Omega

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Re: Crystal Field Theory
« Reply #6 on: January 04, 2008, 01:59:49 AM »
One more thing.  I found this visual-it is an actual animation...remember The Jahn-Teller Theorem does NOT say how large a distortion should occur. ....I think this visual...may help you out..


wwwchem.uwimona.edu.jm:1104/courses/JahnTeller.html

Just be patient until the applets load...You can use your mose to click on the trans metal and the coordinating atoms...pretty decent....

Offline zain

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Re: Crystal Field Theory
« Reply #7 on: January 12, 2008, 06:55:34 AM »
thank you very much

Offline Alpha-Omega

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Re: Crystal Field Theory
« Reply #8 on: January 12, 2008, 09:22:06 AM »
You are welcome!!!!

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