It is possible to form complexes with two NTA ligands, but I'm not certain if it has been reported with iron(II). I don't see why not. Iron loves nitrogen containing ligands.

For more information I refer you to the following:

http://pac.iupac.org/publications/pac/pdf/1982/pdf/5412x2693.pdf

http://www.dojindo.com/Images/Product%20Photo/Chelate_Table_of_Stability_Constants.pdf

Perhaps these are not the answers that you're looking for, but you may find them interesting.

In Fe(II)-EDTA complex, iron atom is coordinated with 4 COOH groups and two N atoms. In Fe-NTA, with 3 COOH and one N. This is the main reason for lower stability. It is possible to imagine a NTA-Fe_NTA complex, however I am not sure that it would be stable (sufficiently stable to avoid disproportionation to NTA-Fe and NTA).

Another difference is that while (Fe)-NTA is biodegradable EDTA and its complexes are not (or least)

Dr. Dubey and Dr. Carraher

Thank you for the responses. Very helpful. 

I am just not sure about the conditions (pH etc.) under which formation of NTA-Fe-NTA would occur and remain stable.

If we take 'equimolar' amounts of Fe2+ and NTA in a solution at around pH 2-3, could it be possible that half of the Fe2+ bind to all the NTA molecules while half of the Fe2+ remain free !!

Thanks !! 

Dr. Tóth

Yes !! Since NTA is a less complex,low molecular weight molecule and also the stability of Fe with EDTA is stronger than with NTA, microbes can probably degrade Fe-NTA complex more easily. 

But do you have any idea if NTA degrdation can occur under anerobic conditions also? Is ammonium (NH4+) the end product of NTA degrdation ?

By the way NTA is potentially known for its severe nephrotoxicity and carcinogenecity in humans and animals !

Thanks !!