Iron (II) disulfide. Iron pyrite. Fool's gold to some. But what bond does it fall under? It could possibly be used for both?

This would give a deeper understanding into other like examples, where it is a fine line between the determination of an ionic compound or a covalent compound.


1 Answer 1


Why do you feel a need to classify it under one or another? Chemistry is not about having a list of ionic compounds and another list of covalent compounds.

There are "ionic" bonds (see below) between $\ce{Fe^2+}$ and $\ce{S2^2-}$, and there is a $\ce{S-S}$ covalent bond in $\ce{S2^2-}$. That's all there is to it.

If I absolutely had to pick one, I would say it is an ionic compound. $\ce{S2^2-}$ is a covalently bonded species; $\ce{FeS2}$ is an ionic crystalline solid. The fact that it melts at $1193~^\circ\mathrm{C}$ supports this "classification".

As @Mith notes in the comments, all ionic bonds are not 100% ionic, and a lot of ionic bonds are in fact covalent to a large degree. The fact that $\ce{Fe^2+}$ is a transition metal ion (meaning it has empty 3d and 4s orbitals available for overlap) and also that sulfur is a rather polarisable ion (because of its larger size) means that there is likely to be significant covalency in the $\ce{Fe-S}$ bonds.

What does this mean? It just shows that trying to classify something as "ionic" or "covalent" is futile. $\ce{FeS2}$ is likely somewhere right in the middle of the spectrum.

  • $\begingroup$ Are there not more ways to go about categorizing FeS2 though? Per se, could iron (II) disulfide instead be an iron (VI) disulfide, and then the bonds are singularly covalent? Would it still be classified at FeS2, for it technically would be, granted, or would it acquire a different name, with the alternate properties of the transition metal in the equation? $\endgroup$ Commented Dec 23, 2015 at 22:26
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    $\begingroup$ Actually S and Fe are bonded covalently, only with slight polarisation. It's popular misunderstanding that all so called salts are ionic; most of them have stronger covalent component. $\endgroup$
    – Mithoron
    Commented Dec 24, 2015 at 0:05
  • $\begingroup$ @Mith I assumed that it was mostly ionic - however given that $\ce{Fe^2+}$ is a transition metal ion and sulfur being rather polarisable I can see why there would be Fe-S covalency. Well, for the purposes of this question, I can probably call it "ionic", but I'll make a note. $\endgroup$ Commented Dec 24, 2015 at 4:03
  • $\begingroup$ @Navysubgirl the Roman numeral indicates oxidation state and not the valency of the compound - for iron(VI) disulfide to be electrically neutral, you would need two $\ce{S^3−}$ ions - I don't think that is happening. The truth is that if you have a molecular covalent compound with the formula $\ce{FeS2}$, it would extremely quickly rearrange to give you the pyrite structure, just like how if you tried to prepare an ionic lattice of $\ce{H+}$ and $\ce{O^2−}$ ions it would say no thank you and go back to being ordinary water. $\endgroup$ Commented Dec 24, 2015 at 4:09
  • $\begingroup$ @Mith (and orthocresol) Thank you. I somewhat understand the ideas of preferential bonding, as in the case of H2O and suchlike. Thus, as a clarification, compounds following similar characteristics and properties to FeS2 would prefer covalency to being to alternate ions, or in this case, an ion and covalent bonding between two sulfurs, yes? $\endgroup$ Commented Dec 24, 2015 at 10:11

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