The Roman numerals are used to show the oxidation number of transition metals because some elements have more than one possible oxidation state.
Some metals have the ability to form differently charged ions. For example, iron can form
A Roman numeral is to indicate the charge of the iron.
So, iron (
(The oxide ion has a
(Here you need to find the common multiple of 6, so two iron ions with a
Here is a video with added information about how to use Roman numerals in names of compounds.