[just.curious]

Thank you so much for clicking in.

Hi, this is my first time posting here and I'll try to be as specific as I can with my problem =( These questions were attached to a practical paper but since it doesn't use any data from the practical results itself, I've decided to put it in this section of the forum instead, do forgive me if I'm wrong.

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The purpose of the experiment was to investigate the electronic effects of axial ligand field strength in tetragonal nickel (II) complexes. Using N,N-diethylethylenediamine as the equatorial ligands and introducing two other ligands (either NO₃, Br, Cl, I and NCS) as the axial ligands.

These are the questions that came with it and what I think about the question or what might be the answer.. either I'm not sure or I don't know =(.

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1.


Which other isomers of the complex shown is possible? How might the variation of the N-susbtituents promote the exclusive formation of the trans-isomer?

>> Well, I think the possible isomer would be the cis-version of this complex where the one of the X substituents would be in the equatorial position. (is this right??) Where the N-substituent is a huge molecule eg. N,N-diethylethylenediamine in terms of it's relative size, it clusters around the equatorial position, creating a steric hindrance for the X ligands to be cis isomer without being pushed together. The X ligands would preferably be in trans position as far apart from each other. (not sure about this part)

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2.


Partial energy diagram is shown in Fig.1. Explain the relative energies in terms of Crystal Field Theory. Why are the orbitals labelled as gerade in all the complexes?

>> What I do understand about CFT's concept is that there is in fact an interaction between the positively charged metal centre and the negatively charged ligands surrounding it. Taking into account that the ligands acts as point charges around the metal centre that originally has five degenerate d-orbitals. When the ligand approaches the degenerate orbitals, some of the electrons from the ligands are closer to the orbitals and further away from others, making the orbitals to no longer be degenerate. The orbitals closer to the ligands will have higher energy than those further away and hence, causes a split in energy.

(Is this correct? If it is, do I stop here or do I explain it in terms of octahedral complexes too? I really don't know if I even have the right answer here)
 
As for gerade, I don't understand what it's about =(


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3.
Give the electronic configuration for the tetragonal Ni(II) complexes with:
i)weakly coordinating axial ligands and
ii) strongly coordinated axial ligands.
Predict the behaviour of each species.

>> I'm not really sure how to answer this question. Does it have anything to do with the ligand stretching? if it does, .. the weakly coordinating would have longer bonds as compared to the stronger one ?? As for the behavior, I really do not know what it's about.

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4.Show all the possible metal binding modes for NCS^- and NO3^-

>> I have no idea at all

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5.
Assign the anions used in this experiment (X=NO₃, Cl, Br, I, NCS) as weakly or strongly coordinating in the tetragonal nickel complexes. In each case, briefly justify your answer with reference to the experimental data.

>> As for this part of the question, we had a clue that it has something to do with the color of the product but I don't know how to explain which is strongly or weakly coordinating compound in the complexes. I have IR spectra data but I don't know if it would be useful in this part of the question.

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Sorry for taking your time but I do appreciate that you are willing to help me.

Thank you.

[Francopper]

Hey, I'll see if i can help a little with your doubts.
1. Right, the cis-isomer version of the complex would be the other posibility, buy as the (en) are quite large they're likely to occupy the ecuatorial positions leaving X to the axial positions (trans-isomer).
2. That's is correct and, as far as I know, it is already explained in terms of octahedral complexes. They experience the same changes in the d-orbitals that you have already described. What you sould point out is the other splitting, from already separated d orbitals (t2g and eg) to eg, b1g, b2g and a1g.That is produced due to the stabilization of d-orbitals involving the z axe (dz^2, from the eg orbitals, and dxz and dyz from the t2g orbitals), causing such orbitals to go down in energy as they are stabilized beacuse of the fact that ligands located in z axe are further away from the metal than ligands located in the x and y axes. I would suggest that you really make yourself clear in the explanation as to why this is caused since it's a key factor in tetragonal complexes description.
Gerade, indicated by the letter g in the orbital name, just means the orbital has simmetry relative to an inversion centre located in the nucleus, meaning that every point that you step around the nucleus, when reflected to the other side of the nucleus (as if locating the mirror image of such point) you find the same sign of wavefunction. (To be continued)

[Francopper]

2. All d-orbitals have the property described in the previous post, so all of them must be gerade (analize each one bu yourself, and if you still don't understand what gerade means look for it in an inorganic chemistry book. I don't think i was clear enough with my explanation).
3.  I think that this is just writing the electronic configuration in the cases that:
i) The d-orbitals have splitted twice, as in the diagram of point 2, so you have to write the electronic configuration in terms of eg, b1g, b2g and a1g orbitals (weekly coordinated axial ligands).
ii) The d-orbitals have splitted once, so the electronic configuration must be writen in terms of tsg and eg orbitals (strongly coordinated axial ligands).
This should be clear when you completely understand point 2.
4. NCS- can bond with S electrons (S-thiocyanide) or with N electrons (N-thiocyanide)
NO3- can only bond with O electrons beacuse N does not hace solitary electron pairs.
(I guess that's what's asked, not really sure)
5. Here UV-Visible spectra would be useful as it has to do with colours, but that's only experimental data confirmation of the theorical analysis. (To be continued)

[Francopper]

5. You should first analyse which ligands are strongly or weakly coordinated using the spectrochemical series. Then try to undestand that the ones strongly coordinated cause a different energy gap than the weakly coordinated, identify which ones cause the biggest gaps and, therefore, which ones absorbe radiation at a higher frequency (more frequency, more energy (E=hv), less wave length).
Hope it helps to make thinks a little clear to you, you may need to read a bit on complexes to fully understand this though. It would take forever to explain every single here.
Cheers.