When is a Roman numeral most likely needed in the name of an ionic compound

When the metal of the ionic compound has multiple oxidation states.

Explanation:

Nickel (Ni - At No 28) has multiple oxidation states* => −2, −1, 0, +1,[2] +2, +3, +4[3] (a mildly basic oxide)*. When naming compound of nickel on needs to include a roman numeral in parenthesis after the symbol of the element in order to identify which oxidation state is relevant.

examples

Nickel Halide derivatives include

NiCl => nickel(I) chloride

NiCl₂ => nickel(II) chloride

NiCl₃ => nickel (III) chloride

Such is important in that each of the compounds have different physical and chemical properties due to variations in oxidation states of nickel.

https://en.wikipedia.org/wiki/Nickel

I thought this was unanswered; it is true about the multiple oxidation states, Fe [I]. [II], [III] and so on; generally it evokes the idea of The Transition Metals.

Explanation:

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