 Rhenium compounds are known for all the oxidation states between − 3 and + 7 except − 2 . The oxidation states + 7 , + 6 , + 4 , and + 2 are the most common . Rhenium is most available commercially as salts of perrhenate , including sodium and ammonium <unk> . These are white , water @-@ soluble compounds . 
