Diborane (B2H6)

In diborane, B2H6, each boron atom is sp3 hybridised, even though boron itself supplies only three valence electrons. Of the four sp3 hybrid orbitals on each boron, three are half-filled (one electron each) while the fourth remains completely empty. Two of the half-filled hybrid orbitals on each boron overlap directly with the 1s orbitals of two terminal hydrogen atoms, giving four ordinary, two-centre-two-electron B-H bonds; these four terminal hydrogens lie, together with the two boron atoms, in a single plane. The remaining two hydrogen atoms bridge the two borons in a very different way: the 1s orbital of each bridging hydrogen overlaps simultaneously with the half-filled hybrid orbital of one boron and the empty hybrid orbital of the other boron, producing an unusual three-centre-two-electron bond (sometimes called a 'banana bond') for each bridge (see the Diborane-structure figure note). These two bridging hydrogens lie symmetrically above and below the B2H4 plane, and it is this pair of three-centr …