Chemical Formula for osmium (III) chlorate

Question

I put Os3(Cl0)3 as an answer for the chemical formula for osmium (III) chlorate on my review and got it wrong. So all I can think is that it must be  Os(ClO3)3 or Os(ClO4)3 but why?

Answer 1

The chlorate anion is ClO₃ and it has a charge of 1-.  Osmium would be 3+ in this situation, so the correct formula is Os(ClO₃)₃. 

BTW, Os(ClO4)3 is osmium(III) perchlorate.