The Roman numerals found in the names of some chemical compounds indicate that the cation has a variable charge. The Roman numeral nomenclature helps in identifying which particular oxidation number of the cation was used in the compounds.
Roman numerals in chemical names can aid in identifying the exact chemical formula of the compound in question. The Roman numeral comes from analyzing not only the oxidation number of the cation but also that of the anion. To do this, the general rule on neutrality of compounds must be followed, in which the sum of charges of the cation and the anion in a single compound is equal to zero. For anions with an oxidation number of -1, the Roman numeral translates to the positive oxidation number of the cation. For example, the compound cobalt (III) bromide is composed of the bromide anion and the cobalt cation with an oxidation number of three. Simply put, the cation is Co3+. Note that cobalt comes in two oxidation states, one with a charge of +3 and the other +2.
Some cations have varying charges because their source elements exist in nature with more than one oxidation number. Some of the common elements with multiple oxidation numbers include titanium, chromium, manganese, iron and lead.