NEW POLYHEDRAL BORANE ANIONS 1955 can calculate Pa by an independent method making use of absolute absorption intensities from infrared spectra of gases.23 Thus, p, is found to be proportional to the absolute absorption intensity of an infrared-active line (which alters the dipole moment of a molecule) and inversely proportional to the square of the frequency, v, of the line. Therefore, molecular vibrations involving low frequencies and involving atoms carrying partial charges (which tend to cause increased intensity) would make the largest contributions to pa. Boron and nitrogen,24 and probably also halogens, carry partial charges. Therefore, contributing modes in B-trihalogenoborazines would be those (23) D. H. Whiffen, Trans. Faraday Soc., 64, 327 (1958); D. A. A. S. (24) R. Hoffman, Advances in Chemistry Series, No. 42, American Chemipi. Rayo, ibid., 69, 43 (1963).cal Society, Washington, D. c., 1964, p 78. involving B-X in-plane and out-of-plane bending and B-N ring in-plane and out-of-plane torsion. Three infrared-active lines for B-trichloroborazine have been calculated by Watanabe and c o -~o r k e r s~~ to fall a t 108, 118, and 388 cm-l. By mass arguments these three lines should fall a t higher frequencies for B-trifluoroborazine and a t lower frequencies for B-tribromoborazine. If they were of similar intensity, pa would reflect the l / v 2 dependence by increasing from B-trifluoro-to B-trichloro-to B-tribromoborazine.Salts of two novel polyhedral borane anions, BgHs2-and B11Hn2-, have been prepared and characterized. The alkali metal salts of B8Hg2-possess a high thermal stability comparable t o that of the salts of B1oHloZ-and B12H1zZ-. At 600°, CsnBl1Hl1 is converted to a mixture of CszBioHlo and Cs2B12H12. The Bg and B11 anions are hydrolytically unstable in acidic media; the Bs anion is more rapidly degraded than the B11 anion. In solution, the BgHg2-ion appears t o be best described as a symmetrically tricapped trigonal prism (Dah) of boron atoms each of which has a terminally attached hydrogen atom.