Solid metal ammine complexes are applied for safe and high-density storage of ammonia to be released for use as reducing agent in selective catalytic reduction of NOx in exhaust gases. The compositional formula of the metal ammine complexes is M(NH3)nXz, where MZ+ represents one or more metal ions capable of binding ammonia, X represents one or more anions, n is the coordination number (from 2 to 12), and z the valency of the metal ion (and thus the total number of compensating anion charges). Ammonia is released by generating a reduced gas-phase pressure of ammonia inside the container, which is below the equilibrium desorption pressure of ammonia at the operating temperature of the storage material thus enabling the unit to be operated is a safe manner with lower operating temperature and pressure.