PREPARATION, B-11 NMR AND VIBRATIONAL-SPECTRA OF U-DINITROGEN-BIS(PENTAHYDRO-CLOSO-HEXABORATE)(2-), [B6H5(NN)B6H5](2-), AND THE CRYSTAL-STRUCTURE OF [AS(C6H5)(4)](2)[B6H5(NN)B6H5]

By reduction of [B6H5(NO2)](2-) With aluminium powder in aqueous alkal ine solution the orange u-dinitrogen-bis(pentahydro-closo-hexaborate)( 2-), [B6H5(NN)B6H5](2-), is formed, which can be precipitated with tet raphenylarsonium ions. The crystal structure of [As(C6H5)(4)](2)[B6H5( NN)B6H5] has been determined by single crystal X-ray diffraction analy sis; tetragonal, space group P4/n with a = 12.6610(12), c = 14.6790(11 ) Angstrom. The B-6 cages are slightly compressed (2%) in the directio n of the connecting linear N-2 bridge with bond lengths N-N of 1.129 a nd B-N of 1.421 Angstrom. The B-11 NMR spectrum exhibits signals at -2 7.0 (ipso-B), +1.26 (equatorial-B), +3.54 ppm (antipodal-B atoms). The NN stretching vibration is observed in the Raman spectra as a very st rong line at 2065 cm(-1).