Borane carbonyl is the inorganic compound with the formula H₃BCO. This colorless gas is the adduct of borane and carbon monoxide. It is usually prepared by combining borane-ether complexes and CO. The compound is mainly of theoretical and pedagogical interest.^[2]

The structure of the molecule of borane carbonyl is H₃B⁻−C≡O⁺. The B−C≡O linkage is linear. The coordination geometry around the boron atom is tetrahedral. The bond distances are 114.0 pm for the C≡O bond, 152.9 pm for the C−B bond, and 119.4 pm for the B−H bonds. The H−B−H bond angle is 113.7°. The C≡O vibrational band is at 2164.7 cm⁻¹, around 22 cm⁻¹ higher than that of free CO.^[3]

Borane carbonyl has an enthalpy of vaporization of 19.7 kJ/mol (4750 cal/mol).^[4] It has electronic state ¹A₁ and point group symmetry C_3v.^[5]

Borane carbonyl was discovered in 1937 by reacting diborane with excess carbon monoxide, with the equation:

B₂H₆ + 2 CO ⇌ 2 BH₃CO.^[4]

The reaction quickly reaches equilibrium at 100°C, but at room temperature, the reverse reaction is slow enough to isolate borane carbonyl. This reaction is performed at high pressures, typically with a maximum pressure observed of 1000 to 1600 psi (68.95 to 110.32 bar).^[6] It can also be performed at atmospheric pressure, with ethers as a catalyst.^[7][8]

A more recent synthesis of borane carbonyl involves slowly bubbling carbon monoxide through a 1 M H₃B−THF solution. The resulting gas stream can be condensed and subsequently bubbled through ethanolic potassium hydroxide to produce the boranocarbonate anion ([H₃BCO₂]²⁻ or H₃B⁻−CO−2).^[8]