CHEMISTRY COMMUNITY

When naming coordination compounds how is it that we get the roman numeral after the cation name? How do we identify it?
For example if we are given [Co(NH3)5Cl]Cl. 2H20 the name for this compound is Penta Ammine Chloro Cobalt(II) Chloride dihydrate. How is it that we get (II) after cobalt?

The roman numerals after the cation name specifies the charge that the cation (central atom has). In the problem that you provided, since NH₃ has a neutral charge, we are left with two chlorines (one inside the coordination complex, and one outside). These two chlorines amount to a net -2 charge. As the overall compound is neutral, we arrive to the conclusion that the Cobalt must have a +2 charge. This is indicated with the roman numerals (II) after its name, as you have written down.