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As I understand, Roman numerals are used for oxidation states, whereas Arabic numerals are used for the charge of full ions, with some ambiguity allowed when a metal centre is all but ionised, such as in hexaaqua compounds.

But I have never seen a Roman numeral with a negative sign in front of it. Would I write the oxidation state of sulphur in hydrogen sulphide (H$_2$S) as (-II)?

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    $\begingroup$ The Romans would surely look at it with raised eyebrows. $\endgroup$ Mar 4, 2023 at 10:15
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    $\begingroup$ @FurrierTransform Can you clarify if you intend to use the Roman numerals in chemical names, such as iron(II) sulfate, iron(III) sulfate? So far, I interpreted your question could be about reaction equations - maybe departing from an erroneous assumption $\endgroup$
    – Buttonwood
    Mar 4, 2023 at 18:26
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    $\begingroup$ @IvanNeretin TIL supercilium for eyebrow. Too sad TLL did not yet advance to letter S, but it lists cilium (lemma 3:1057) mesosupercilium (lemma 8:854). $\endgroup$
    – Buttonwood
    Mar 4, 2023 at 18:41

2 Answers 2

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According to IUPAC guidelines, roman numerals are used to denote oxidation states when used in the name of a compound. This is used when the cationic group can have varying oxidation states. For example, $\ce{FeSO4}$ is written as iron (II) sulphate, and $\ce{Fe2(SO4)3}$ is written as iron (III) sulphate. Although anionic groups can also have variable oxidation states, we omit mentioning the oxidation state in the name of the compound. Instead, we use standard prefixes to identify the oxidation states. For example, potassium oxide ($\ce{K2O}$), potassium peroxide ($\ce{K2O2}$), and potassium superoxide ($\ce{KO2}$) have different oxidation states for oxygen: -2, -1, and -1/2 respectively. But instead of mentioning this explicitly, we denote these with different standard names.

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    $\begingroup$ Have a look on mhchem syntax which one is encouraged to deploy in the body of questions, answers, and comments for chemical formulae and equations. If used well, the results have share the professional appearance (e.g., italic fonts are used, but not on symbols of chemical elements), and a syntax easier to read and maintain. $\endgroup$
    – Buttonwood
    Mar 4, 2023 at 11:11
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    $\begingroup$ It would be good to cite the specific IUPAC guideline to which you refer $\endgroup$
    – Andrew
    Mar 4, 2023 at 12:35
  • $\begingroup$ Wikipedia's list of known (whole-number) oxidation states uses Roman numerals only for positive ones. Zero and negative numbers are represented with Arabic numerals and the negative ones with of course a minus sign. $\endgroup$ Mar 4, 2023 at 12:57
  • $\begingroup$ I certainly only tend to use them for the cation as well. However, I am comparing the various oxidation states of sulphur, with the upper bound, sulphur(IV), as in sulphate, being non-controversial. But I'm not sure how to write the lower bound. It would seem weird to mix notation. But also weird to incorrectly label the sulphur with a +6 charge. $\endgroup$ Mar 4, 2023 at 14:07
  • $\begingroup$ I think that @NilayGhosh has answered my question: You do use negative Roman numerals, it's just less commonly needed and therefore less commonly seen, but would still be correct in situations where Roman numerals are the correct notation. If you post this, I will accept. Pending any further input. $\endgroup$ Mar 9, 2023 at 12:05
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Converting my comment into answer upon OP's request.


Basically the other answer is correct. Metals tends to hold positive charge and they achieve multiple oxidation state. So, in order to differentiate them, we use roman numerical in their names to avoid confusion and simultaneously providing the O.S. of metal which is always a useful information. For e.g iron(II) oxide and iron(III) oxide In few occasion, metal can achieve negative when the entity holds a negative charge, thus becoming an anion. For anions, we use specific suffixes for different oxidation state. So, in this case we don't need roman numbers. For e.g. Disodium tetracarbonylferrate. We don't need to mention (-II) since it is redundant and in theory, roman numerals doesn't exist for 0 and negative numbers. But some publication(example) and textbooks still adopts the writing style for convenience sake(at least in formulae).

Here is a good article on this topic:https://pubs.acs.org/doi/pdf/10.1021/ed2001347

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  • $\begingroup$ Just to clarify, we usually denote negative anion oxidation states with different suffixes, which allows the oxidation state of the relevant atoms, like sulphur in sulphate, to be figured out (VI here). However, if one wished to refer to the oxidation state of a specific atom, where that atom is not just an ion with a charge $\pm n$, like S in H$_2$S, or F in F$_2$O, than a negative Roman numeral would be appropriate, just weird and not commonly seen. $\endgroup$ May 6, 2023 at 3:07

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