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Along a period the ionization energy should increase because the atomic number is increasing, but there is negligible increase in shielding. However, $\mathrm{IE}_\ce{Mn} < \mathrm{IE}_\ce{Fe} > \mathrm{IE}_\ce{Co} > \mathrm{IE}_\ce{Cu}$, so why does iron have such a high ionization energy?

I've considered that iron has a $\mathrm{4s^2 3d^6}$ configuration that goes to a $\mathrm{4s^1 3d^6}$ configuration upon ionization, while $\ce{Co}$ has a $\mathrm{4s^2 3d^7}$ that goes to $\mathrm{4s^0 3d^8}$ configuration, $\ce{Ni}$ has a $\mathrm{4s^2 3d^8}$ that goes to a $\mathrm{4s^0 3d^9}$ configuration and $\ce{Cu}$ has a $\mathrm{4s^2 3d^9}$ that goes to a $\mathrm{4s^0 3d^{10}}$ configuration (Lang, J. Chem. Ed., 2003). In the Long paper, the authors explain the increase of IE at iron is due to "electronic structure" and I would like to know exactly what about the electronic structure it is that causes iron to have a high IE than expected based on a general periodic trend.

I was thinking that adding an electron to the $\mathrm{3d}$ orbitals might be energetically favorable enough to lower the ionization energy of $\ce{Ni}$, $\ce{Co}$ and $\ce{Cu}$, so rather than iron having a high ionization energy the surround metals just have a low ionization energy because the ion is stabilized by having another $\mathrm{d}$-electron added? Does that sound reasonable?

Another thing I was thinking was that iron is the first transition metal to have a paired $\mathrm{d}$-electron, could that effect the shielding and increase the ionization energy?

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    $\begingroup$ The first ionization energy of cobalt is only 0.3% lower than that of iron. So I wouldn't say that iron has an abnormal high ionization energy. $\endgroup$
    – aventurin
    Commented Aug 26, 2017 at 21:00
  • $\begingroup$ @aventurin I mean "abnormal" in that it defies a general periodic trend, the magnitude itself is not abnormal $\endgroup$
    – Mecury-197
    Commented Aug 27, 2017 at 13:41
  • $\begingroup$ The increase in effective nuclear charge pulls the d orbitals closer; the increase in shielding may be subtle but is not negligible. The empirical data is trying to explain the interplay. $\endgroup$
    – jimchmst
    Commented Jun 13 at 21:07
  • $\begingroup$ Similar thing is apparent for gadolinium. $\endgroup$
    – Paul Kolk
    Commented Jun 14 at 9:09

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