PREPARATION, VIBRATIONAL-SPECTRA AND NORM AL-COORDINATE ANALYSIS OF THE B-10 AND B-11 ISOTOPOMERS OF [B-2(NCS)(6)](2-) AND CRYSTAL-STRUCTURE OF (PH(3)P=N=PPH(3))(2)[B-2(NCS)(6)]

By reaction of [B6H6](2-) with a solution of (SCN)(2) in dichlorometha ne in the presence of solid KOH the hexaisothiocyanatodiborate anion [ B-2(NCS)(6)](2-) is formed and can be isolated by fractionated crystal lization. The crystal structure of (PNP)(2)[B-2(NCS)(6)] has been dete rmined by single crystal X-ray diffraction analysis; triclinic space g roup <P(1)over bar> with a=12,282(5), b=12,416(5), c=14,155(5)Angstrom , a=68,36(5), beta=70,59(5), gamma=80,93(5)degrees. The [B-2(NCS)(6)]( 2-) anion reveals als staggered conformation with nearly local D-3d sy mmetry and a B-B bond length of 1,73(2) Angstrom. Using the crystallog raphic data a normal coordinate analysis has been performed. With a se t of 12 force constants (e.g. f(d)(BB)=3,62, f(d)(BN)=4,21 mdyn/Angstr om) a good agreement between observed and calculated frequencies for t he B-10 and B-11 isotopomers has been achieved. The potential energy d istribution on the force constants reveals strong vibrational coupling s within the B2N6 framework.