Pentafluoroethylseleninic acid is oxidized by KMnO4 in slightly alk. aq. soln. to give K[C2F5SeO3]. With HClO4 the salt is converted into the free acid. Neutralization of C2F5SeO3H with NaOH, Ag2O, NH3 + H2O provides the corresponding salts. The Et ester is prepd. from Ag[C2F5SeO3] and C2H5I. Oxidn. of C6F5TeC6F5 with concd. HNO3 yields C6F5Te(OH)ONO2 which on controlled hydrolyses gives (C6F5)2Te(OH)2. Strong acids like HClO4 or H2SO4 form mixed anhydrides like (C6F5)2Te(OH)ClO4 or (C6F5)2Te(OH)OS(O)2OTe(OH)(C6F5)2. For the latter compd. a cyclic or noncyclic polycondensate form cannot be ruled out. Acidic 35% H2O2 soln. oxidizes (C6F5)2Te(OH)2 at 40 Deg (48 h) to give (C6F5)2TeO2. This anhydride dissolves only in CF3SO3H forming (C6F5)2Te(OSO2CF3)4. It is sol. in CH3CN and reacts in soln. with water to yield (C6F5)2Te(OH)2(OSO2CF3)2. Oxidn. of C6F5TeTeC6F5 with concd. HNO3 provides oligomeric [C6F5Te(O)OH]x which still contains some HNO3. A single-crystal x-ray structure detn. carried out on [(C6H5)4As][C2F5SeO3] shows convincingly that the central selenium atom is coordinated by three equiv. oxygen atoms with d(Se-O) = 1.614(3) .ANG., proving that Se has the oxidn. state VI.