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Homework Help: Rank ability of ligands to increase or decrease Δ in a square planar compound

  1. Mar 20, 2012 #1
    1. The problem statement, all variables and given/known data
    2. Relevant equations

    I am working on a formal lab report for an inorganic chem course and forget how to construct a spectrochemical series. I have 4 complexes, each has a single absorption band in the UV-Vis spectrum.

    [NiBr(Mes)(PPh3)2] 450nm
    [Ni(Mes)Br(bipy)] 487nm
    [NiBr(Mes)(dppe)] 425nm
    [NiBr2(dppe)] 477nm

    I am told to rank the ability of the ligands (Br-, dppe, bipy, PPh3, Mes-) to increase or decrease Δ in a square planar compound based only on the experimental data (given above).

    3. The attempt at a solution

    I have decided that the each of the above complexes is square planar in geometry, so they can be compared directly to one another in terms of d-d electronic transitions.

    Comparing [NiBr(Mes)(bipy)] and [NiBr(Mes)(dppe)], which differ by only one ligand, it can be deduced that dppe will cause an increase in Δ relative to bipy. Similarly, [NiBr2(dppe)] and [NiBr(Mes)(dppe)] can be compared to show that Mes- will cause an increase in Δ relative to Br-.

    I am now unsure if I am able to make any further comparisons. Concerning [NiBr(Mes)(PPh3)2] in particular, I do not recall if it is possible to compare this complex to any of the others, given the above data. At best, it differs from [Ni(Mes)Br(bipy)] by bipy and (PPh3)2, and a similar comparison can be made to [NiBr(Mes)(dppe)]. However this only allows the comparison of (PPh3)2 and not a single PPh3. I believe that the first PPh3 will have a significantly different influence on the complex compared to the second PPh3, and so the ligand cannot be ranked using this data.

    I haven't had to do this for quite a while so I just wanted to make sure I remember how to do it correctly. I tried to reference it but had trouble finding information on ranking ligands relative to one another, and in particular when they differ buy 1L_1 and 2L_2 as I detailed above. Any help is greatly appreciated. Thanks!
  2. jcsd
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