When is it appropriate to use parentheses in the naming of an ionic compound?

When the compound contains an ion with more than one atom in each ion, and the number of such units in a formula unit of the ionic compound is at least 2, parentheses are needed in the formula of the compound. In more conventional naming, parentheses containing a Roman number are often used after the name of a cation to denote its oxidation state, particular for atoms that form more than one stable cation.