The molecular structure of tetraborane(10) in the gas phase as determined by a joint analysis of electron-diffraction and microwave data

Abstract

The structure of the gaseous tetraborane(10) molecule, B₄H₁₀, has been redetermined by electron diffraction. The new analysis confirms not only that the structure comprises a folded diamond of boron atoms bridged by four hydrogen atoms, but also (i) that the B–H–B bridges are unsymmetrical, the H₂B–H_b distance being 17 pm longer than the HB–H_b distance, and (ii) that the bridging hydrogen atoms are situated 5.6 pm above the plane defined by the three boron atoms of each H₂B(HB)₂ moiety to fall within the fold of the tetraboron framework. Other salient parameters are (distances correspond for r_a; figures in parentheses are the estimated standard deviations of the last digits) : r[B(1)–B(2)] 185.6(0.4), r[B(1)–B(3)] 170.5(1.2), r[B(1)–H_t] 122.1 (1.4), r[B(2)–H_t] 119.4(0.7), r[B(1)–H_b] 131.5(0.9), r[B(2)–H_b] 148.4(0.9) pm; H_t–B(2)–H_t′ 122.7(3.5), H_t″–B(1)–B(3) 111.2(3.5), and the dihedral angle between the two planes B(1)B(2)B(3) and B(1)B(3)B(4) 117.1(0.7)°(t = terminal, b = bridging).