Prepn: Hein, Herzog in Handbook of Preparative Inorganic Chemistry Vol. 2, G. Brauer, Ed. (Academic Press, New York, 2nd ed. 1965) p 1340. Review of chromium halides: Fergusson in Halogen Chemistry Vol. 3, V. Gutmann, Ed. (Academic Press, New York, 1967) pp 227-302.