Q6(d). Diborane: industrial synthesis, structure and B–H bonds

Synthesis

1. Industrial: boron trifluoride with sodium hydride at about 450 K: 2BF₃ + 6NaH → B₂H₆ + 6NaF. Also by reducing boron trichloride with hydrogen in an electric discharge: 2BCl₃ + 6H₂ → B₂H₆ + 6HCl
2. Laboratory: metathesis in ether: 3LiBH₄ + 4BF₃ → 3LiBF₄ + 2B₂H₆ (or with LiAlH₄); and 2NaBH₄ + I₂ → B₂H₆ + 2NaI + H₂ in diglyme. The work is done in a closed apparatus because diborane ignites in air.

Structure

1. Each B is surrounded by four H in a distorted tetrahedron. The two B atoms and the four terminal H lie in one plane; the two bridging H lie above and below it, between the B atoms (D₂ₕ).
2. B–Hₜ 119 pm, B–H_b 133 pm, B…B 177 pm; Hₜ–B–Hₜ 122°, H_b–B–H_b 97°. The bridge bonds are longer and weaker than the terminal bonds.

Nature of the B–H bonds

1. B₂H₆ has only 12 valence electrons (2×3+6×1). A Lewis structure with eight two-centre bonds would need 16: the molecule is electron deficient.
2. Each B is roughly sp³. Two hybrids per B form ordinary two-centre two-electron bonds with the terminal H (8 electrons).
3. The other two hybrids of each B overlap the 1s orbital of a bridging H. Three orbitals combine into a bonding, a non-bonding and an antibonding MO; the bonding MO holds two electrons, spread over B–H–B: a three-centre two-electron (3c–2e) bond (“banana bond”). Two such bridges use the remaining 4 electrons.
4. There is no direct B–B bond; the bridges hold the molecule together. Evidence: two kinds of H in the IR and ¹H NMR, and B₂H₆ is cleaved by Lewis bases (B₂H₆ + 2NMe₃ → 2Me₃NBH₃).

Application. diborane is the reagent for hydroboration of alkenes (anti-Markovnikov alcohols after oxidation) and the starting material for higher boranes; the 3c–2e idea also explains Al₂(CH₃)₆ and the boron clusters.