The possibilities to produce quaternary Mg-based transition-metal complex hydrides have been explored. Mg2Mn1--xFex (x = 0.5, 0) elemental powder mixtures were ball milled in a reactive D2 atmosphere (about 5 MPa). The results were compared with the formation of Mg2(FeD6)0.5(CoD5)0.5 from Mg-Fe-Co powders. The changes of D2 pressure were monitored during milling and deuterium absorption was detected within the first 15 h of milling. Powder X-ray and neutron diffraction analysis for the as-milled Mg2Mn0.5Fe0.5 powder showed a large fraction of unreacted elemental Mn, as well as the formation of a cubic K2PtCl6-type phase (a = 6.452 A). The latter could be identified as Mg2FeD6. Infrared spectroscopy confirmed the presence of [FeD6]4- anions (stretching band at 1261 cm(-1)) and ruled out the formation of [MnH6]5- complexes. As for the as-milled Mg2Mn, powder diffraction analysis showed the formation of MgD2 as the main reaction product. A small fraction of a cubic K2PtCl6-type phase (a = 6.526 A) was also observed. This was identified as a Mg2FeD6-type compound. It was likely formed as a result of Fe impurities originating from the milling media and detected by EDX analysis. Infrared spectroscopy confirmed the presence of the [FeD6]4- complex (stretching band at 1259 cm(-1)) and showed an additional band at 1313 cm(-1). The likelihood of this band arising from Mn-D bonds is discussed. Finally, deuterium desorption upon heating, was investigated for all systems by differential scanning calorimetry.