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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?

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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

References:

  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.
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  • $\begingroup$ So it is based on factual data or is there a way to see the structure of ligand and then decide? $\endgroup$ Jul 24 '20 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 '20 at 11:28
  • $\begingroup$ Okay. Can't transition metals show CN 8 with this ligand? $\endgroup$ Jul 24 '20 at 15:23
  • $\begingroup$ They can, but those compounds are unstable, as I mentioned in my answer $\endgroup$ Jul 24 '20 at 16:00

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