Some of the selenium oxyhalides, such as selenium oxyfluoride (SeOF2) and selenium oxychloride (SeOCl2) have been used as specialty solvents. Selenides
Analogous to the behavior of other chalcogens, selenium forms hydrogen selenide, H2Se. It is a strongly odiferous, toxic, and colorless gas. It is more acidic than H2S. In solution it ionizes to HSe−. The selenide dianion Se2− forms a variety of compounds, including the minerals from which selenium is obtained commercially. Illustrative selenides include mercury selenide (HgSe), lead selenide (PbSe), zinc selenide (ZnSe), and copper indium gallium diselenide (Cu(Ga,In)Se2). These materials are semiconductors.
Selenium dioxide is formed by the reaction of elemental selenium with oxygen:

  
    
      
        
          
            Se
            
              8
            
            
              
            
          
          +
          8
          
          
            O
            
              2
            
            
              
            
          
          ⟶
          8
          
          
            SeO
            
              2
            
            
              
            
          
        
      
    
    {\displaystyle {\ce {Se8 + 8 O2 -> 8 SeO2}}}
  It is a polymeric solid that forms monomeric SeO2 molecules in the gas phase.