A long time ago I was researching the effect of the self regulatory response in Fe and Co. I found that my results made sense based on the idea of the electronegativity of the ions considered. I found a webpage that listed the relationship between the different electronegativities for different oxidation states of Fe and Co.

My problem is I didn't save the webpage and I don't know where I found it. I am not a chemist so I don't know in which kind of books I could find a list of values of electronegativities for different oxidized ions.

It basically said that the relationship between the electronegativities was something like

$$\ce{Co^2+} < \ce{Fe^2+} < \ce{Fe^3+} < \ce{Co^3+}$$

Note: I don't remember it well, so what I wrote could be lies. The important thing was that there was a flip of order in electronegativities for Fe and Co when changing oxidation state.

Does anyone know where I could find that kind of information? I tried googling, but I am not finding the webpage and don't remember the terms I used to find it in the first place.

I never understood the why of the order either, and I have seen chemist webpages that tell you which ion is more electronegative by just looking at it. If someone could explain that too I would appreciate it.


I forgot to add that it was in octahedral complexes. I was able to find this pdf online (page 32) but it doesn't state a reference. enter image description here


1 Answer 1


Pearson conveniently lists cumulative experimental data in the 1988 paper [1], referrring to the earlier work of Moore [2]. Selected values of $I$ (ionization potential), $A$ (electron affinity), $χ$ (absolute electronegativity – probably, that's what you are looking for) and $η$ (absolute hardness) for iron and cobalt cations are:

Table I. Experimental Parameters for Monatomic Cations (eV)

$$ \begin{array}{lcccc} \hline \text{ion} & I & A & χ & η \\ \hline \ce{Fe^2+} & 30.65 & 16.18 & 23.42 & 7.24 \\ \ce{Fe^3+} & 54.8 & 30.65 & 42.73 & 12.08 \\ \ce{Co^2+} & 33.50 & 17.06 & 25.28 & 8.22 \\ \ce{Co^3+} & 51.3 & 33.50 & 42.4 & 8.9 \\ \hline \end{array} $$

So it looks like the relation is a bit different:

$$\ce{Fe^2+} < \ce{Co^2+} < \ce{Co^3+} < \ce{Fe^3+}$$

Complete table as a screenshot:

Table I - complete


  1. Pearson, R. G. Absolute Electronegativity and Hardness: Application to Inorganic Chemistry. Inorganic Chemistry 1988, 27 (4), 734–740. https://doi.org/10.1021/ic00277a030.
  2. Moore, C. E. "Ionization Potentials and Ionization Limits"; Natl. Stand. Ref. Data Ser. (U.S. Natl. Bur. Stand.); 1970, NSRDS-NBS 34. (NIST - PDF)
  • 1
    $\begingroup$ Thank you so much! The odd thing is that it is contrary to what I remember finding. I think because my case was applied to octahedral complexes but I didn't really know if it made a difference so I didn't mention it in the question. I was just able to find a resource online. I will edit the question to add the image. I will accept your answer later today if no one else answers for taking the time to do so. Sorry for not putting all the information straight away. Hope you can clarify maybe with just the origin of the difference. $\endgroup$
    – M.O.
    Commented May 17, 2019 at 21:34
  • $\begingroup$ @M.O. No prob, I'm glad you found the data you were looking for. AFAIK the values for $χ$ are determined spectroscopically, so, yes, the coordination environment may affect the order since the difference in energies for the same oxidation number is relatively small. $\endgroup$
    – andselisk
    Commented May 17, 2019 at 21:41

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