The Crystal and Molecular Structure of B5H11

Abstract

The molecular structure of B₅H₁₁ has been determined from 299 x‐ray diffraction maxima which have been analyzed by three‐dimensional Fourier and least‐squares methods. There are four molecules in a monoclinic unit cell, in the space group P2₁/n, having parameters a=6.76, b=8.51, c=10.14A, and β=94.3°. Although no molecular symmetry is demanded by the space group, the molecular dimensions indicate C₈ symmetry for the isolated molecule. The boron skeleton is a fragment of the icosahedronlike arrangement in B₁₀H₁₄, and can also be derived from the tetragonal pyramid in B₅H₉ by opening up one of the basal B–B bonds. The apex boron B_I is ``singly'' bonded to two hydrogens, one of which H<sub>VII</sub>, is very weakly bonded to the two outer borons B<sub>III</sub>. Each B<sub>III</sub> is ``singly'' bonded to two hydrogen atoms. Each of the remaining two borons B_II is ``singly bonded'' to one hydrogen atom, and there are three bridge hydrogens, one bridging the B_II–B_II bond and two bridging the B_II–B_III bonds. Molecular parameters are two B_I–B_III=1.86₅, two B_II–B_III=1.75, two B_I–B_II=1.72, one B_II–B_II=1.77A, seven B–H=1.0₇, six B–H (bridge) =1.2₄A, one B_I–H_VII=1.0₀ and two B_III–H_VII=1.6₇A. The B_III–B_I–B_III angle is 107°. Bond angles to hydrogen strongly resemble those in the other known boron hydrides.