3D model (JSmol)
|Molar mass||29.81 g·mol−1|
|200.48 J K−1 mol−1|
Std enthalpy of
|115.90 kJ mol−1|
|aluminium monofluoride |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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. 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.
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 (BF2)3B•AsH3, (BF2)3B•CO, (BF2)3B•PF3, (BF2)3B•PH3 and (BF2)3B•PCl3.
Vidovic and Aldridge reacted NaRu(CO)2(C5H5) with (Et2O)·BF3. Note that the BF was formed in place rather than added on.
By reacting iron vapour with B2F4 and PF3 a substance with the formula (PF3)FeBF 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.
BF is isoelectronic with carbon monoxide (CO) and so could form similar compounds to metal carbonyls. It is predicted to also bridge between two or three metal atoms (μ2 and μ3). Working with BF as a ligand is difficult due to its instability in the free state.
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