8-VERTEX POLYHEDRAL MONOCARBABORANE CHEMISTRY - 3 CLOSO ANIONS [CB7H8](-), [CB7H7I](-) AND [CB7H6I2](-) - PREPARATION AND STRUCTURAL STUDIES

Salts of the eight-vertex [closo-1-CB7H8](-) monoanion can be prepared from arachno nine-vertex ligand adducts exo-6-L-4-(CBH12)-H-8 by base -degradation reactions. iodination of [closo-1-CB7H8](-) with elementa l iodine yields the [1-CB7H7-7-I](-) and [1-CB7H6-7,8-I-2](-) monoanio ns, characterised by single-crystal X-ray diffraction analysis. Crysta ls of [PPh(4)][1-CB7H6-7-8-I-2] were triclinic, space group P $$($) ov er bar 1, with a = 7.4703(7). b = 12.766(2), c = 13.9102(12) Angstrom, alpha = 100.973(10). beta = 92.886(7), gamma = 93.005(10)degrees and Z = 2; structure refined for 3867 data collected at 150 K to wR(2) = 0 .0348. Crystals of [pph(4)][1-CB7H6-7,8-I-2](-) were triclinic, space group P $$($) over bar 1. with a = 7.8389(8), b = 13.4489(11). c = 13. 6337(13) Angstrom, alpha = 98.009(9). beta = 93.060(7), gamma = 92.939 (7)degrees and Z = 2; structure refined for 4284 data collected at 200 K to wR(2) = 0.0504. The {CB7} cluster geometries of the two anions a pproximate closely to regular triangulated dodecahedral, with no sign of any opening. The three anions are fluxional to ca. 180 K; structure s for the possible intermediates are briefly discussed.