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    Got questions on these,

    1) Discuss the possibilities of isomerism in the following:

    Co(acac)3 => i know acac is bidentate, so this should be octahedral complex, this is Co3+, so that should be d6.

    [Co(NH3)5 NO2] 2+

    [Ni(CN)5]3 - => it is Ni2+, so that is d8, but we would have 3 electrons left, how to distribute this?

    [Co(en)2 F2]+ => it is Co3+ d6 so that is fine, i manage to spot inversion at the Co centre, so there shouldn't be any optical isomers. Do correct me if this is wrong.

    CrCl3.6H20 => I am baffled really. Cr shouldn't be able to coordinate 9 ligands, unlike Ln and Ac, so should i only be worried about the Cl ligands? what would be the difference in point group between the non-hydrated and this hydrated compound?

    Thanks so much for any help offered.
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    For Co(acac)3 , the ligands are symmetrical and planar, so the only isomerism I can see is the delta/lambda 'screw' chirality.

    The second complex has the possibility of linkage isomerism, with the nitrite ligand able to coordinate through N or O.

    Pentacyano nickel is a strange thing to be here. Cyano ligands are formally 2 electron donors, so you end up with a nice round 18 electron count. I suppose it could take a square-based pyramid form instead of the more usual trigonal bipyramid.

    [Co(en)2F2]+ can bis cis or trans, and only the trans form has the inversion centre. There'll be delta/lambda isomerism for in the cis form. There's also an issue of 'little delta'/'little lambda' chirality in the way the en ligands coordinate (because they're not planar), but that's not really important.

    You're quite right in that the last one can't coordinate all nine ligands at the same time, but any combination of six ligands is feasible. It can sit as [Cr(H2O)6]3+, with 3 chloride counter-ions, or as [Cr(H2O)5Cl]2+ with two chlorides, and so on. Point groups would obviously vary according to which isomer you've got (and the cis/trans or mer/fac that are associated with them).

    :yy:
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    (Original post by -Kav-)
    For Co(acac)3 , the ligands are symmetrical and planar, so the only isomerism I can see is the delta/lambda 'screw' chirality.

    The second complex has the possibility of linkage isomerism, with the nitrite ligand able to coordinate through N or O.

    Pentacyano nickel is a strange thing to be here. Cyano ligands are formally 2 electron donors, so you end up with a nice round 18 electron count. I suppose it could take a square-based pyramid form instead of the more usual trigonal bipyramid.

    [Co(en)2F2]+ can bis cis or trans, and only the trans form has the inversion centre. There'll be delta/lambda isomerism for in the cis form. There's also an issue of 'little delta'/'little lambda' chirality in the way the en ligands coordinate (because they're not planar), but that's not really important.

    You're quite right in that the last one can't coordinate all nine ligands at the same time, but any combination of six ligands is feasible. It can sit as [Cr(H2O)6]3+, with 3 chloride counter-ions, or as [Cr(H2O)5Cl]2+ with two chlorides, and so on. Point groups would obviously vary according to which isomer you've got (and the cis/trans or mer/fac that are associated with them).

    :yy:
    Thanks, Kav! That helps a lot in explaining most of it, I understand better now.

    Just one more generic type of questions,
    =>How many IR active stretching fundamentals would you predict for the following species?

    ie SiCl4, PF5, SO2

    I am not asking you to like do the whole thing for me, but if you could probably give me a few hints, or point out what I should do to be able to get the answer.


    What I have done:
    - I used the cartesian axes system. I worked out the trace for the cartesian axes(after assigning the point group), then multiply by the number of unshifted atoms, to get trace for total modes of motion.

    - From this, i deducted trace of translation and rotation, that would leave me with trace for vibration which i can use the LOT to find out how many times each irrep appears in the trace(vibration).

    - I have a feeling this method is right, but i really should get whole number for the number of times the irrep appears, but i get fractions, which obviously shows that what i did was wrong.

    - so, what to do?

    Thanks very much!
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    (Original post by shengoc)
    Thanks, Kav! That helps a lot in explaining most of it, I understand better now.

    Just one more generic type of questions,
    =>How many IR active stretching fundamentals would you predict for the following species?

    ie SiCl4, PF5, SO2

    I am not asking you to like do the whole thing for me, but if you could probably give me a few hints, or point out what I should do to be able to get the answer.


    What I have done:
    - I used the cartesian axes system. I worked out the trace for the cartesian axes(after assigning the point group), then multiply by the number of unshifted atoms, to get trace for total modes of motion.

    - From this, i deducted trace of translation and rotation, that would leave me with trace for vibration which i can use the LOT to find out how many times each irrep appears in the trace(vibration).

    - I have a feeling this method is right, but i really should get whole number for the number of times the irrep appears, but i get fractions, which obviously shows that what i did was wrong.

    - so, what to do?

    Thanks very much!
    Aye, that's the most rigorous way of approaching something like this, and is essential if you want to find all vibrational modes, but it's incredibly unwieldy for something like PF5. The trouble is probably just arithmetical (I'm not sure I've ever managed to get through a problem like this without swapping a minus for a plus somewhere). :o:
    Since they're only asking for the stretches, you can employ the same method you would for, say, CO stretches on a hexacarbonyl complex. For SiCl4, just assign a vector to each Si-Cl bond and see what remains unshifted after each symmetry operation. I'm pretty sure that should work.
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    (Original post by -Kav-)
    Aye, that's the most rigorous way of approaching something like this, and is essential if you want to find all vibrational modes, but it's incredibly unwieldy for something like PF5. The trouble is probably just arithmetical (I'm not sure I've ever managed to get through a problem like this without swapping a minus for a plus somewhere). :o:
    Since they're only asking for the stretches, you can employ the same method you would for, say, CO stretches on a hexacarbonyl complex. For SiCl4, just assign a vector to each Si-Cl bond and see what remains unshifted after each symmetry operation. I'm pretty sure that should work.
    rigorous huh, i would expect nothing less from a real exam question, it is worth 20/3 marks, so i think i'd go for the standard way; after doing a few, i think i can manage it. but how do you determine which of the vibrational modes are stretches or bends?

    I have a hunch that stretches are to do with bond lengths, so they should be represented by the 1d irreps which are A or B

    whereas the bond angles determine the bends, so they would be represented by spatial 2d and 3d irreps which are T(3D) and E(2D)

    but since we have seen that bond length could be determined in 2D provided angle is known, so should i include that as part of stretch?

    ie let say I have trace(vibration) = A + B +E + T

    that means I have 3 IR stretch, because A, B and E are dependent on bond length, so they can give 3 different stretches, am i right in saying that?

    thanks again. i might check the post later in the morning. no rush in replying.
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    (Original post by shengoc)
    rigorous huh, i would expect nothing less from a real exam question, it is worth 20/3 marks, so i think i'd go for the standard way; after doing a few, i think i can manage it. but how do you determine which of the vibrational modes are stretches or bends?

    I have a hunch that stretches are to do with bond lengths, so they should be represented by the 1d irreps which are A or B

    whereas the bond angles determine the bends, so they would be represented by spatial 2d and 3d irreps which are T(3D) and E(2D)

    but since we have seen that bond length could be determined in 2D provided angle is known, so should i include that as part of stretch?

    ie let say I have trace(vibration) = A + B +E + T

    that means I have 3 IR stretch, because A, B and E are dependent on bond length, so they can give 3 different stretches, am i right in saying that?

    thanks again. i might check the post later in the morning. no rush in replying.
    Sadly you can't simply identify each irrep. as corresponding to a bend or a stretch. For example, water has three vibrations, 2A1 + 1B1. Two are stretches and one is a bend, you find that one of the A1 vibrations is the symmetric stretch while one is the bend. The B1 is then the asymmetric stretch. The only way I've come across to determine which are stretches and which are bends is to predetermine a stretching motion, plug the vectors in, and see what irrep. comes up. This is fine for water, but a pain for anything bigger. The best I can do is that an A1 is definitely the totally symmetric stretch in SiCl4. Beyond that, I've no idea what the vibrational motions of a Td molecule even look like.

    The best way to tackle the problem is by using the method I mentioned above, because it only detects stretches. If you want to find specifically stretches and specifically bends, I'd suggest using your Cartesian approach, then using the simple bond vector approach, and subtracting the irreps. of the latter from those of the former. :yy:
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    Cheers Kav, I confirmed with my tutor today, and it was something along those lines you mentioned.
 
 
 
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