1313-04-8

light brown powder(s); unstable in air [MER06]

Magnesium selenide has the molecular formula of MgSe and the molecular weight of 103.2652 g/mol. It is a stable, light gray to brown crystalline powder with a refractive index, πD=2.44. Its density is 4.21 g/cm³ and it decomposes in water:
MgSe+ 2H2O ? Mg(OH)2+H2Se
Thus, it cannot be prepared by bubbling H2Se gas through a soluble Mg²⁺ aqueous solution. MgSe can be prepared by a variety of other methods. Among the easiest are the reaction of magnesium oxide and selenium. Alternately, the two elements can be reacted together to form the selenide:
MgO(solid)+ Se(solid)+ heat ? MgSe(solid)+ SeO2(gas)
Mg(solid)+ Se(solid)+ heat ? MgSe(solid)
In the first reaction, Se melts at 225°C while the MgO is stable. Selenium dioxide sublimes at 320°C. The reaction is carried out in an inert atmosphere at about 550°C. If a tube furnace is used, crystals of SeO2 appear at the cool end of the tube. In the second reaction, an inert atmosphere is mandatory and the reaction temperature is also 550°C (which is below the melting point of Mg metal).
Magnesium selenide has the CAS number of 1313- 04-8. It is not stable in air and reacts with moisture to form H2Se, a foul-smelling and toxic gas:
MgSe+H2O ? Mg(OH)2 +H2Se