[mossbreaker]

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).  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!