Diborane bridging hydrogen atoms

A. Burawoy (Manchester) There is no unambiguous experimental evidence (from infrared spectra or other properties) that the bridge hydrogen atoms in diborane are in the centre of the B—H. .. B bonds formed (cf. Burawoy A. Nature Land. 1946 328 Lord Nielson J. Chem.Phys. 1981 19 1) The criticisms against the interpretation of the diborane bridge as an electrostatic hydrogen bond (Cartnell Fowles Valency and Molecular Structure p. 223 Butterworth London 1956) do not take sufficient account of the exceptionally... 

Bromodihorane, B H Br, has a structure essentially identical with that of diborane except that a bromine atom replaces one of the non-bridging hydrogen atoms." The B Br bond length, 1.934 0.010 A, corresponds to bond number 0.80 (the B—Br single-bond length is... 

The fully condensated dimer ions dominate these are obtained by retaining all boron and bridging hydrogen atoms and eUminating all terminal H atoms. The only reactions which occur with formation of a neutral boron-containing product (BH3) leading to borane ions BsH ( =3,4,5,6) are endothermic and appear in the case of diborane. They can only be detected by double-resonance experiments if the primary ions have sufficient internal energy. 

The structure now universally accepted is that shown in Fig. 1, in which the two bridge hydrogen atoms lie on an axis that is perpendicularly bisected by the plane of the other six atoms. There are three mutually perpendicular twofold axes of symmetry (point group D.,), and the dimensions given are those ob-tained from a recent detailed analysis and conflation (192) of electron-diffraction data (13) with the results of high-resolution infrared spectroscopy (193), as listed in Table I (where the electron-diffraction results for deuterodiborane also appear) Even in the absence of microwave data, we can say that the parameters of diborane are now as accurately known as those for almost any other molecule of con arable size. 

B10H14 and some other borane molecules have also been found to have structures based on the icosahedron, with some of the corners not occupied by boron atoms, and with some bridging hydrogen atoms, as in diborane. The B12 icosahedron is also present in elementary boron and in the hard substance B4C. 

Figure 2 Diagrams corresponding to those in Figure 1 for the plane of the bridging hydrogen atoms in diborane. This plane contains the unique surface of the central ring cp. (Reproduced with permission of the American Chemical Society from J. Aw. Chew.. SV r., 1979, 101, 1389)...

Figure 6.14 The orbitals of two BH2 groups and of two hydrogen atoms used in the formation of the MOs of the bridge bonding in diborane...

Further confirmation of the bridge type of structure comes from the methylation of diborane. It is found possible to replace only four of the hydrogen atoms with methyl groups and on further methylation instead of B (CH3) two molecules of B(GH3)3 are produced. The molecule BH (0113)2 does not exist as the monomer, but like BH3, dimerizes imme-... 

---