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The ligand given above shows different denticities with common transition and inner-transition metal ion i.e. 6 and 8.

The extra two denticities come from the two Nitrogen having 1 lone pair each. I want to know why is inner-transition metal ion able to somehow get nitrogen to donate these lone pairs while a common transition ion fails to do so?


1 Answer 1


I decided to do some more research for this question as I had faced it before. Turns out the proper name of this ligand is diethylenetriaminepentaacetate ($\ce{DTPA^5-}$). It's an expanded version of EDTA and is primarily used in MRI scanning where it forms complexes with the Gadolinium ion. It forms complexes with a coordination number of six and seven with transition metals simply because these complexes are more stable. Here's Fe(III)-DTPA as an example1:

Experimentally Fe(III)-DTPA has been observed to adopt seven- and six-coordinate crystal structures. This agrees with our result where the 8-coordinate Fe(III)-DTPA is $\pu{23 kJ mol^-1}$ higher in energy relative to the most stable 7-coordinate structure

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With inner transition metals, it forms complexes with a denticity of 8, with Pr-DTPA being an example2: enter image description here


  1. Sillanpää, Atte J., et al. “Density Functional Complexation Study of Metal Ions with (Amino) Polycarboxylic Acid Ligands.” Phys. Chem. Chem. Phys., vol. 5, no. 16, 2003, pp. 3382–93. doi:10.1039/B303234P.
  2. Fusaro, Luca, et al. “Insight into the Dynamics of Lanthanide-DTPA Complexes As Revealed by Oxygen-17 NMR.” Inorganic Chemistry, vol. 51, no. 15, 2012, pp. 8455–61. doi:10.1021/ic3010085.
  • $\begingroup$ So it is based on factual data or is there a way to see the structure of ligand and then decide? $\endgroup$
    – R_Squared
    Jul 24, 2020 at 10:33
  • $\begingroup$ inner transition metals prefer to form complexes with high CN as they have open f-orbitals. this ligand has 8 sites which can donate electron pairs, and it will prefer to coordinate with all those sites when it can. That's how I initially went about solving this problem without knowing the fact. $\endgroup$ Jul 24, 2020 at 11:28
  • $\begingroup$ Okay. Can't transition metals show CN 8 with this ligand? $\endgroup$
    – R_Squared
    Jul 24, 2020 at 15:23
  • $\begingroup$ They can, but those compounds are unstable, as I mentioned in my answer $\endgroup$ Jul 24, 2020 at 16:00