Boron monofluoride

Boron(I) fluoride
Fluoroboronene

Boron monofluoride or fluoroborylene is a chemical compound with formula BF, one atom of boron and one of fluorine. It was discovered as an unstable gas and only in 2009 found to be a stable ligand combining with transition metals, in the same way as carbon monoxide. It is a subhalide, containing fewer than the normal number of fluorine atoms, compared with boron trifluoride. It can also be called a borylene, as it contains boron with two unshared electrons. BF is isoelectronic with carbon monoxide and dinitrogen and each molecule has 14 electrons.

The experimental B–F bond length is 1.26267 Å. One reported computed bond order for the molecule is 1.4.

BF is unusual in that the dipole moment is inverted with fluorine having a positive charge even though it is the more electronegative element. This is explained by the 2sp orbitals of boron being reoriented and having a higher electron density. Backbonding, or the transfer of π orbital electrons for the fluorine atom is not required to explain the polarization.

Boron monofluoride can be prepared by passing boron trifluoride gas at 2000 °C over a boron rod. It can be condensed at liquid nitrogen temperatures (−196 °C).

Boron monofluoride atoms have a dissociation energy of 7.8 eV or heat of formation −27.5±3 kcal/mole 760 kJ/mol. The first ionization potential is 11.115 eV. ω_e is 1765 cm⁻¹.

BF can react with itself to form polymers of boron containing fluorine with between 10 and 14 boron atoms. BF reacts with BF₃ to form B₂F₄. BF and B₂F₄ further combine to form B₃F₅. B₃F₅ is unstable above −50 °C and forms B₈F₁₂. This substance is a yellow oil.