Reinvestigation of the molecular structure of gaseous beryllium borohydride BeB2H8 by electron diffraction

The structure of beryllium borohydride has been reinvestigated by gaseous electron diffraction at a nozzle temperature of 40 °C. The experiments were characterized by unusual photographic problems which were overcome with difficulty. The structure analysis led to the unexpected result that the molecule, or at least the major part of the free molecules, has a linear heavy-atom structure, in agreement with conclusions based on an early electron-diffraction investigation but in disagreement with those from more recent electron-diffraction and spectroscopic studies which have generally favored triangular configurations. Our diffraction data were satisfactorily interpreted in terms both of molecules with configuration B–Be–B and with configuration Be–B–B. The data require the peripheral atoms of the molecule, or the principal species of the molecule, to be bonded to the central one through three hydrogen bridges giving the central atom sixfold coordination; the successful models have molecular symmetries D3d and C3v for the B–Be–B configuration and C3v symmetry for the Be–B–B. The discovered Be–B–B models seem rather unlikely on several grounds, but it cannot be ruled out that more plausible ones exist. The B–Be–B models appear to fall in one broad region of parameter space roughly defined by the following values of the principal geometric parameters chosen as averages and differences of bond distances and bond angles: r̄(Be–B) = 1.790 Å(0.015), 0 ≤ Δr(Be–B) ≤ 0.10 Å, r̄(B–Hb) = 1.303 Å(0.012), 0 ≤ Δr̄ (B–Hb) ≤ 0.12 Å, r̄(B–Ht) = 1.16 Å (0.04), ∠HbBHt = 117.5° (1.2), and 0 ≤ ∠HbBHt ≤ 10°; the parenthesized quantities are estimates of 2σ. The possibility that gaseous beryllium borohydride may comprise more than one species is discussed.