As quoted at http://www.webelements.com/ from these sources: * A.L. Allred, J. Inorg. Nucl. Chem., 1961, 17, 215. * J.E. Huheey, E.A. Keiter, and R.L. Keiter in Inorganic Chemistry : Principles of Structure and Reactivity, 4th edition, HarperCollins, New York, USA, 1993.