Prepd by the action of manganese salts on alkali selenides: Berzelius et al., cited by Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry 10, 798 (1930); by the action of a manganese salt on a soln of hydrogen selenide out of contact with air: Moser, Atynsky, Monatsh. Chem. 45, 235 (1925).