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Chemistry · Ch 11 — The p-Block Elements

Diborane, B2H6

11.3.3

Diborane, B2H6

Diborane, B2H6B_2H_6

Diborane is the simplest known boron hydride.

Preparation. In the laboratory, it can be made by treating boron trifluoride with lithium aluminium hydride in diethyl ether:

4BF3+3LiAlH4→2B2H6+3LiF+3AlF34BF_3 + 3LiAlH_4 \rightarrow 2B_2H_6 + 3LiF + 3AlF_3

A convenient alternative oxidises sodium borohydride with iodine:

2NaBH4+I2→B2H6+2NaI+H22NaBH_4 + I_2 \rightarrow B_2H_6 + 2NaI + H_2

Industrially, diborane is manufactured by reacting boron trifluoride with sodium hydride at 450 K:

2BF3+6NaH→B2H6+6NaF2BF_3 + 6NaH \rightarrow B_2H_6 + 6NaF

Physical/chemical behaviour. Diborane is a colourless, highly toxic gas (b.p. 180 K) that catches fire spontaneously in air, burning with a large release of energy:

B2H6+3O2→B2O3+3H2OΔcH∘=−1976 kJ/molB_2H_6 + 3O_2 \rightarrow B_2O_3 + 3H_2O \qquad \Delta_cH^\circ = -1976\ kJ/mol

Most higher boranes are similarly spontaneously flammable. Diborane hydrolyses readily to give boric acid:

B2H6(g)+6H2O(l)→2B(OH)3(aq)+6H2(g)B_2H_6(g) + 6H_2O(l) \rightarrow 2B(OH)_3(aq) + 6H_2(g)

Adducts with Lewis bases. Diborane undergoes cleavage reactions with Lewis bases (L) to give adducts of the type BH3⋅LBH_3 \cdot L:

B2H6+2NMe3→2BH3⋅NMe3B2H6+2CO→2BH3⋅COB_2H_6 + 2NMe_3 \rightarrow 2BH_3 \cdot NMe_3 \qquad B_2H_6 + 2CO \rightarrow 2BH_3 \cdot CO

Its reaction with ammonia is different — it first forms B2H6⋅2NH3B_2H_6 \cdot 2NH_3, better written as the ionic species [BH2(NH3)2]+[BH4]−[BH_2(NH_3)_2]^+[BH_4]^-; further heating converts this into borazine, B3N3H6B_3N_3H_6:

3B2H6+6NH3→3[BH2(NH3)2]+[BH4]−→Δ2B3N3H6+12H23B_2H_6 + 6NH_3 \rightarrow 3[BH_2(NH_3)_2]^+[BH_4]^- \xrightarrow{\Delta} 2B_3N_3H_6 + 12H_2

Borazine is nicknamed "inorganic benzene" because it has a six-membered ring of alternating B–H and N–H units, geometrically analogous to benzene.

Structure. In diborane (Fig. 11.2a), the two boron atoms and the four terminal hydrogens all lie in one plane, with two bridging hydrogens above and below this plane. The four terminal B–H bonds are ordinary two-centre-two-electron bonds; the two bridging B–H–B bonds are unusual three-centre-two-electron bonds, often called "banana bonds" (Fig. 11.2b). Each boron is sp3sp^3 hybridised: of its four sp3sp^3 hybrids, one is left without an electron and takes part in the electron-deficient bridge. …

Figure 11.2(a)The structure of diborane, B2H6

What this figure shows. A ball-and-stick style geometric diagram of the diborane molecule showing two boron atoms (circled B) connected in the centre, with four terminal hydrogen atoms (circled H, two on each boron) drawn as a wide splayed 'bowtie'/hourglass shape lying in the plane of the page, each terminal B-H bond making an angle of 120 degrees with a labelled bond length of 119 pm. Two additional bridging hydrogen atoms (circled H) are shown above and below the B-B axis, connected to both boron atoms by dotted lines forming a diamond/rhombus bridge, with an angle of 97 degrees marked at boron and a bridge bond length of 134 pm labelled, illustrating the no …

Figure 11.2(b)Bonding in diborane. Each B atom uses sp3 hybrids for bonding. Out of the four sp3 hybrids on each B atom, one is without an electron shown in broken lines. The terminal B-H bonds are normal 2-centre-2-electron bonds but the two bridge bonds are 3-centre-2-electron bonds. The 3-centre-2-electron bridge bonds are also referred to as banana bonds.

What this figure shows. Two side-by-side schematic bonding diagrams of diborane. The left diagram shows two boron atoms (labelled B) each surrounded by four lobed sp3 hybrid orbitals drawn as circles/loops (some shaded solid, some outlined with broken/dashed lines) with terminal H atoms at the outer tips of the shaded lobes and the two unshaded (broken-line) lobes from each boron pointing toward each other in the bridge region, illustrating the sp3 hybridisation with one empty hybrid per boron used for bridge bonding. The right diagram is a simplified sketch of the two three-centre-two-electron 'banana' bridge bonds: two B atoms connected by two curved banana-shaped bond lines that each loop through a bridging H atom above and below, with two additional terminal H atoms attached to each B by straight lines, visually depicting the curved/banana shape of the bridging B-H-B bonds. …

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