Halogen compounds
Iodides of selenium are not well known. The only stable chloride is selenium monochloride (Se2Cl2), which might be better known as selenium(I) chloride; the corresponding bromide is also known. These species are structurally analogous to the corresponding disulfur dichloride. Selenium dichloride is an important reagent in the preparation of selenium compounds (e.g. the preparation of Se7). It is prepared by treating selenium with sulfuryl chloride (SO2Cl2).
Selenium reacts with fluorine to form selenium hexafluoride:

  
    
      
        
          
            Se
            
              8
            
            
              
            
          
          +
          24
          
          
            F
            
              2
            
            
              
            
          
          ⟶
          8
          
          
            SeF
            
              6
            
            
              
            
          
        
      
    
    {\displaystyle {\ce {Se8 + 24 F2 -> 8 SeF6}}}
  In comparison with its sulfur counterpart (sulfur hexafluoride), selenium hexafluoride (SeF6) is more reactive and is a toxic pulmonary irritant.