Can you analyze your reaction mixture via NMR spectroscopy?

As long as your complex does not decompose, TLC should still be possible.  What type of TLC plates have your tried?  Silica gel?  Alumina?  There are also specialty ones available.  What solvents or solvent mixtures have you tried?  My students often use a non-polar solvent to remove the organic material and then follow with a polar solvent to get the complex to move.

There can be other methods like HPLC to do quantification of the metal complex product to know the time required for completion of reaction. However using TLC you can do indirect quantification using a comparative analysis approach. Since you have a solvent system in which you can identify the free Ligand spot on a florescent TLC sheet. However with that solvent system your metal complex spot is not moving that means you can infer the formation of your product from the spot at the base line only so qualitative identification of product is possible.

Now for determining the time required to complete the reaction do indirect approach by focusing on the easily determining ligand spot and not on the baseline metal complex spot. the procedure can be as under:

1. just at the start of reaction, take the TLC to find the ligand spot position (Rf) with either no or weak metal complex spot. preserve the TLC for comparison.

2. After a sufficient time interval say 2 hours, run another TLC and compare the intensity of the spots with the already preserved TLC of the start of reaction. note that the intensity of the free ligand spot would have decreased while the intensity of the product spot would have increased. preserve this TLC also for next comparison.

3. Keep taking TLC after regular intervals of time and compare the intensities of the free ligand spot.  Depending upon the nature of reaction we can have different inferences:

i) if the reaction  goes to completion, then after some time intervals the free ligand spot will vanish or will not be seen in TLC. that much of the time should be the time required for completion of reaction.

ii) In another case, if the reaction stops after some stage due to thermodynamic or kinetic constraints, then in that case the free ligand spot of the same intensity can be seen in the TLC after sufficient time lapse. So in that case you can conclude that the reaction stops at the time after which no change in  the ligand spot intensity  was seen and the reaction does not go to completion.

As a synthetic chemist, i would say TLC is the basic and indispensable bench top tool to get the inferences about the reaction. The synthetic chemist should master the skill of developing and interpreting TLC. His TLC should talk results to him about the reaction like his student.

Not sure why there is a down vote for what I presumed was a legitimate question. I'd appreciate if a reason is offered so I can learn.

@Dr. Rizvi: That was a very well articulated response. I have a question though: Won't the metal complex decompose on the TLC plate? In such a scenario, can the ligand "break free" from the complex and move on the TLC, giving a false positive? Or is this too unlikely a scenario?

There are many cases where in the completion of reactions involving metal complexes were monitored through TLC. But this might not be true in all the cases where in the complexes might not move on a TLC plate even in polar solvent systems. In that case the presence of ligand could be know by using TLC.  Having said that, NMR spectroscopy is also a good technique to use where in you can take out small volume of your reaction mixture, perform a mini work up and monitor for the presence of ligand or any new species using NMR. Both TLC and NMR could be used according to the prevailing situations.