I don't completely understand it's use. Here's an example I don't understand:

Shortly after discussing $\ce{[CuCl4]^2-}$, an exam paper asked a question about copper(II) chloride. I understand that copper(II) chloride could refer to $\ce{CuCl2}$, calling it copper(II) chloride, but couldn't it also refer to $\ce{[CuCl4]^2-}$? I would understand the question in reference to $\ce{CuCl2}$, but it got confusing when I realised it could equally be referring to $\ce{[CuCl4]^2-}$, especially after it just mentioned it.


No, those are two distinctive species with different names:

  • $\ce{CuCl2}$: copper(II) chloride;
  • $\ce{[CuCl4]^2−}$: tetrachloridocuprate(II).

Oxidation number denoted with the Roman numerals in parentheses refers to the element it's placed after and doesn't carry any additional information besides the fact that copper exists as $\ce{Cu^2+}$ in both compounds.

Nomenclature rules were designed that way so that a name would unambiguously define a single compound only (I'd suggest the Red Book's Nomenclature by IUPAC [1] as a reference).


  1. IUPAC “Red Book” Nomenclature of Inorganic Chemistry, 1st ed.; Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; IUPAC Recommendations; Royal Society of Chemistry: Cambridge, UK, 2005. ISBN 978-0-85404-438-2. (PDF)

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