Kristallstrukturen von [P(C6H5)4]2[B6Cl6](CH3CN)2 und [P(C6H5)4]2[B6Br6](CH3CN)2 sowie Schwingungsspektren und Normalkoordinatenanalyse der isotopomeren closo-Hexahalogenohexaborate [11Bn 10B6-nX6]2-, Χ = Cl, Br; n = 0-6 / Crystal Structures of [P(C6H5)4]2[B6Cl6](CH3CN)2 and [P(C6H5)4]2[B6Br6](CH3CN)2, Vibrational Spectra and Normal Coordinate Analysis of Isotopomeric closo-Hexahalogenohexaborates [11Bn 10B6-nX6]2-, X = Cl, Br; n = 0-6

Abstract

The crystal structures of (TPP)2[B6Cl6](CH3CN)2 and (TPP)2[B6Br6](CH3CN)2 were determined from single crystal X-ray diffraction analyses at 147 K. They are triclinic, space group P1̅, and contain nearly regular octahedral anions with bond lengths d(B-B) 1.716, d(B-Cl) 1.818 for [B6Cl6]2- and d̅(B-B) 1.684, d̅(B-Br) 1.993 Å for [B6Br6]2-. Using these molecular data, normal coordinate analyses for the ten isotopomers [11Bn 10B6-nX6]2-, n = 0-6, X = Cl, Br, including the pairs of geometric isomers for n = 2, 3, 4 have been performed, based on a general valence force field. With a set of 8 force constants for Cs2[B6Br6] (e.g. fd(BB) = 1.39, fd(BBr) = 3.18 mdyn/Å) and 10 for Cs2[B6Cl16] (e. g. fd(BB) = 1.30, fd(BCl) = 4.01 mdyn/Å) the calculated frequencies are in good agreement with observed IR (10 K) and Raman bands (80 K) in natural abundance as well as for 11B- and 10B-enriched derivatives. For the series [B6X6]2-, X = Cl, Br, I the correlation of decreasing bond lengths d(B-B) and increasing force constants fd(BB) along with vibrational coupling between the B6 cage and the substituent sphere is discussed.