PARAMAGNETIC CLUSTER IONS [B(6)HAL(N)HAL'(6-N)](CENTER-DOT-) (HAL, HAL' = CL, BR, I) - EPR EVIDENCE FOR RADICAL STABILIZATION THROUGH ELECTRONIC EFFECTS OF THE HALOGEN SUBSTITUENTS

Monoanionic hexaborate cluster radicals [B(6)Hal(n)Hal'(6-n)](.-) with mixed halogen substitution were prepared from oxidizable dianionic pr ecursors and were characterized by vibrational and UV-vis spectroscopy . EPR studies of these and the structurally established homoleptic spe cies [B(6)Hal(6)](.-) (Hal = Cl, Br, I) reveal strongly increasing g a nisotropy and relaxation rate on replacing Cl by Br and especially I s ubstituents; the very stable B6I6.- ion (g(1) = 2.04, g(2) = 1.66, g(3 ) = 1.15) thus exhibits an EPR spectrum only at 4 K. The extent of the se effects is attributed to the Jahn-Teller situation in [B6X6](.-) wi th only partial occupancy of a degenerate MO. Both the absence of B-H bonds and the evidently strong participation of the halogen substituen ts in the singly occupied MO contribute to the extraordinary stability of these cluster radicals.