The Crystal and Molecular Structure of Tetraborane

The molecular structure of tetraborane B4H10 has been determined from 616 observed reflections from single crystals. The ordered phase is monoclinic P21/n, with four molecules in a unit cell with parameters, a=8.68, b=10.14, c=5.78A, and β=105.9°. Complete three-dimensional Fourier syntheses, including one from which the boron atoms had been subtracted, were employed to locate unambiguously the positions of all atoms. Molecular dimensions indicate that the isolated molecule has symmetry C2v. The boron atoms form two triangles BIBIBII with a common edge BI–BI the angle between the planes of the triangles being 118.1°. Each of the two BI borons has one attached hydrogen atom, and each BII has two attached hydrogen atoms. The remaining four hydrogen atoms form bridges between the four BI–BII pairs. Molecular parameters are four BI–BII=1.845±0.002A, BI–BI=1.750A, BII–BII=2.786A, six B–H=1.11±0.04A, four BI–H (bridge)=1.21±0.03A, and four BII–H (bridge)=1.37±0.10A. Mean values and their average deviations have been given on the assumptions that the molecular symmetry is C2v and that the six B–H distances are equal. There is an interesting false, incorrect structure which almost satisfies the x-ray diffraction data. A metastable modification, obtained by sudden freezing of the super-cooled liquid, has spherically disordered molecules in hexagonal close packing. There are two molecules in a unit cell of dimensions, a=5.79 and c=9.36A.