Boron monofluoride


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; each molecule has 14 electrons.

Structure

The experimental B-F bond length is 1.26267 Å. Despite being isoelectronic to the triple-bonded species CO and N2, computational studies generally agree that the true bond order is much lower than 3. One reported computed bond order for the molecule is 1.4, compared with 2.6 for CO and 3.0 for N2.
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.

Preparation

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.

Properties

Boron monofluoride molecules 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. ωe is 1765 cm−1.

Reactions

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.
BF reacts with acetylenes to make the 1,4-diboracyclohexadiene ring system. BF can condense with 2-butyne forming 1,4-difluoro-2,3,5,6-tetramethyl-1,4-diboracyclohexadiene. Also, it reacts with acetylene to make 1,4-difluoro-1,4-diboracyclohexadiene. Propene reacts to make a mix of cyclic and non-cyclic molecules which may contain BF or BF2.
BF hardly reacts with C2F4 or SiF4. BF does react with arsine, carbon monoxide, phosphorus trifluoride, phosphine, and phosphorus trichloride to make adducts like 3B•AsH3, 3B•CO, 3B•PF3, 3B•PH3, and 3B•PCl3.
BF reacts with oxygen: BF + O2 → OBF + O; with chlorine: BF + Cl2 → ClBF + Cl; and with nitrogen dioxide BF + NO2 → OBF + NO.

Ligand

The first case of BF being a ligand on a transition element was demonstrated in 2009 with the compound 2Ru24. The BF was bound to both ruthenium atoms as a bridge.
Vidovic and Aldridge reacted NaRu2 with ·BF3. Note that the BF was formed in place rather than added on.
Earlier in 1968, K. Kämpfer, H. Nöth, W. Petz, and G. Schmid claimed that Fe4 was formed in the reaction of B2F4 with Fe5, however this has not been reproduced.
By reacting iron vapour with B2F4 and PF3, a substance with the formula 4FeBF was produced. Hafnium, thorium, titanium, and zirconium can form a difluoride with a BF ligand at the low temperature of 6K. These come about by reacting the atomic metal with BF3. The first fully characterized molecule featuring BF as a terminal ligand was synthesized by Drance and Figueroa in 2019.
BF is isoelectronic with carbon monoxide and so could form similar compounds to metal carbonyls. It is predicted to also bridge between two or three metal atoms. Working with BF as a ligand is difficult due to its instability in the free state.