Chromatography Forum

Dear All,
I would like to know if someone has any experience on the HPLC analysis of coordination complex of metals ? Do you have any suggestion on the analytical conditions to be used to have good resolution factor between this type of substance : X(ligand)4 and X(ligand)3 ? (X=metal).
Are there literature references on this topic ?
Thank you.

I have avoided that type of analysis. If the ligand exchange rates are significant on the time scale of the separation, then the chromatography will be a mess.

Here is an interesting paper related to the topic:

Identification and Characterization of Isomeric Intermediates in a Catalyst Formation Reaction by Means of Speciation Analysis Using HPLC-ICPMS and HPLC-ESI-MS

Q.Tu, T.Wang, C.Welch, P.Wang, X.Jia, C.Raab, X.Bu, D.Bykowski, B. Hohenstaufen, M.Doyle

Anal. Chem., 78 (4), 1282 -1289, 2006

http://pubs.acs.org/cgi-bin/abstract.cg ... 1679y.html

Quite a while ago, I worked on a cobalt complex. The cobalt was in the center of a structure somewhat like a porphyrin ring with upper and lower imidazole derived ligands. I found that the free ligand was available in pure form and the assay was meant to be for the complex, so I ran the whole thing down a silica column using a MeOH-Water based MP doped with a very small quantity of the free ligand.

I know, it sounds wierd, but it really did work.

Without the "doping" with the ligand it didn´t work?

Multiple peaks ensued w/o ligand in the MP - non, mono, and di-ligand complexes were evident under those conditinons. Basically, I forced the 2-ligand configuration and that worked well for what we needed.

Interesting, with the added ligands you are apparently pushing the equilibrium far toward the 2-ligand complex, very fast, yet in the ligand´s absence you see individual intermediates (similar to the study mentioned in the Anal Chem ref from above). This is somewhat like adjusting the pH to "select" an advantageous acid or base species, except there the reaction between different entities (HA and H+ A-, etc.) is so fast that you may have a broad peak at the wrong pH.
I am having trouble (initially with TLC) with HDP, a 99mTc complex with a bisphosphonate, which breaks down on exposure to air (reforms 99mTcO4-, which was reduced with Sn++ to form the complex). Maybe the equilibrium can be shifted sufficiently toward the complex by adding a reducing agent to the mobile phase. (Excess SnCl2 is kept in the preparation and coinjected into patients, actually a case where sloppy work "saves the day". This SnCl2 is not enough to prevent oxidation in the diluted samples used in TLC and especially after application).