First, have you diagnosed the AC problem other than "no coolant?"
These systems are sealed, and don't require periodic refilling. If you are low/out of refrigerant, then you have a leak, and that MUST be fixed for your AC to operate properly.
One thing to remember is that even though your compressor has a safety protection to shut it off in the event of low/no coolant, you're rolling the dice on braking something by running a system with a leak.
All late model cars use R134a.
1998 C230 "Black Betty"