Question

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

Answer 1

Well to put it simply, its needed when the cation is a transition metal.

Answer 2

AN ionic compound with more than one charge uses a roman numeral. For example copper (III) sulfate and copper (II) sulfate nomenclature is used because copper has 2 charges +2 and +3. Among other metals are mercury, lead, tin, etc.