I believe the correct play might be 6. a3, a simple counterthreat. Qa4+ is tempting, but 7 ... Nc6 takes care of this and the bishop on b4 so all you've really done is weaken d2 -- an important square at this particular moment.

a3 absolutely, also 6 g3 Qd7 7 Bg2 0-0-0 8 h3 Bf5 9 a3 f6 which leads to some uncertain outcomes. h3 gives black even chances, the other moves are even worse.