Electron diffraction study of tetraborane(8) carbonyl in the gas phase: structure determination of an endo/exo isomeric mixture

Abstract

The molecular structures of the individual isomers present in a gaseous mixture of arachno-B₄H₈(CO) have been determined by electron diffraction. The sample was known from n.m.r. experiments to consist of 62% of the endo isomer and 38% of the exo isomer and early refinements showed that the distribution in the gas phase was consistent with that found in solution. A satisfactory refinement (R_G= 0.06) was obtained with a model in which the isomers had a ‘butterfly’ B₄ geometry with the ‘hinge’ B(1) carrying either an endo- or exo-CO. The endo and exo isomers were allowed to differ only in the dihedral angles between the planes of the ‘butterfly’ structure [135(4) and 144(2)°, respectively] and in the angles subtended by the carbon atom at the B–B ‘hinge’[125(2) and 109(2)°]. The unbridged B–B distances were 172.7(10)(‘hinge’) and 184.9(4) pm, and the two H-bridged B–B distances were 178.0(6) pm. The BCO angle showed no significant deviation from 180°.