Molecular structure of 1-(dichloroboryl)pentaborane(9), in the gas phase as determined by electron diffraction and supported by theoretical calculations

Abstract

The structure of gaseous 1-(dichloroboryl)pentaborane(9), 1-(Cl₂B)B₅H₈, has been determined by electron diffraction. The results confirm that the molecule consists of a pentaborane(9) cage substituted at the apical boron atom, B(1), by a dichloroboryl group; the BCl₂ moiety is essentially free to rotate about the exo B–B bond. Salient experimental structural parameters (r_a) are r(B–B)(base–base) 182.1(14), r(B–B)(base-apex) 173.9(26), r(B–B)(apex–exo) 170.1(18), r(B–Cl) 172.0(15), r(B–H_t) 126.2(22), and r(B–H_b) 134.6(16) pm (H_t= terminal hydrogen, H_b= bridging hydrogen); Cl–B–Cl 121.7(28)°, B–H_t‘rise’(above basal-boron plane) 15.1(26)°, and B–H_b‘dip’(below basal-boron plane) 62.0(39)°. These conclusions are supported by ab initio(MP2/6–31G* or MP2/DZP level) optimisations of the molecular geometry, and by comparison of the calculated ¹¹B NMR chemical shifts {individual gauge for localised orbitals (double zeta)//gas-phase electron diffraction level [IGLO(DZ)//GED level]} with the experimental NMR data.