Upon research, Shadow appears to be a some kind of grey area in regards to Chaos Control. On one hand, it's claimed by the canon of the series, as well as one of the directors of the game during an interview, that Shadow cannot use Chaos Control without a Chaos Emerald. But, in Sonic Battle, Shadow was supposedly able to use Chaos Control without the use of any Chaos Emeralds, with dialogue in the game suggesting it has something to do with his memory of Maria.
Whatever the case, it'd probably be appropriate to assume the former for Metal Sonic.