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While searching for iron carbonyl complexes, I met the compound Potassium tetracarbonyliron hydride. I am wondering the oxidation state of iron as well as whether the anion [HFe(CO)4]- obeys the 18-electron rule.

As the wikipedia suggests, the name for the compound include "1-" at the end. It is either the oxidation state for iron, or the charge of the whole anion. If it were the oxidation state of the iron atom, then the compound is expected to show paramagnetism, so I was searching for the magnetism of the complex, but no result of it. Is there any publication indicating the magnetism of the complex?

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We may regard the compound as formally a combination of the following:

$\ce{HFe(CO)_4^-} = \ce{H^+ + 4 CO +Fe^{2-}}$

Following the observed acidity of metal carbonyl hydrides, the hydrogen is regarded as $\ce{H^+}$ and, to balance charges, the iron is regarded as having an oxidation state of $-2$ (not $-1$). The iron(-2) then has ten valence electrons, and each $\ce{CO}$ ligand will share two more with the iron so the latter has eighteen valence electrons. But that will fill all of the valence $4s, 3d, 4p$ orbitals with all electrons paired into the bonds, rendering the complex diamagnetic in its ground state.

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  • $\begingroup$ Then, the Fe-H bond is formed by the electron from Fe to H+, isn't it? $\endgroup$
    – Shira
    Commented Sep 4 at 14:38
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    $\begingroup$ Yes, but we do not count any electrons shared from $\ce{H^+}$ to iron. Both electrons in that sigma bond would come from the iron. $\endgroup$
    – Oscar Lanzi
    Commented Sep 4 at 14:41

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