Final answer:
MgCl2 features a magnesium ion with a fixed +2 charge and does not have a variable charge. Therefore, it does not require a Roman numeral designation in its name.
Step-by-step explanation:
No, MgCl2 does not contain a metal ion with variable charge. The formula MgCl2 indicates that the compound is composed of one magnesium ion, Mg2+, and two chloride ions, Cl-. Magnesium is an alkaline earth metal, and it typically forms a +2 charge when it becomes an ion.
Unlike transition metals which often have multiple possible charges, the charge on magnesium in MgCl2 is always +2. The subscript '2' in the formula MgCl2 is there to show that two chloride ions are needed to balance the +2 charge of the single magnesium ion.