Preparation: Ruff et al., Z. Anorg. Allg. Chem. 208, 293 (1932); Balz, Mail"ander, ibid. 217, 166 (1934); Faloon, Kenna, J. Am. Chem. Soc. 73, 2937 (1951); Kwasnik in Handbook of Preparative Inorganic Chemistry vol. 1, G. Brauer, Ed. (Academic Press, New York, 2nd ed., 1963) pp 184-185. Reviews: Hoffman, Neville, Chem. Rev. 62, 1-18 (1962); Kemmitt, Sharp, Adv. Fluorine Chem. 4, 194-195 (1965); Woolf, ibid. 5, 1-30 (1965); Schmutzler, Angew. Chem. Int. Ed. 7, 440-455 (1968).