Dilute nitric acid behaves as a typical acid in its reaction with most metals. Magnesium, manganese, and zinc liberate: Mg + 2 HNO3→ Mg(NH3)2+ H2 (Magnesium nitrate) ... read more
However, nitric acid reacts with very manganese in dilute form (oxidizing property reduced due to dilution) to give hydrogen gas. Mn+HNO3 gives you Mn(NO3) 2+H2. It is also true for magnesium. read more