| Names | |
|---|---|
| Other names Boron fluoride Boron(I) fluoride | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.033.970 |
| EC Number |
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| UNII | |
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| Properties | |
| BF | |
| Molar mass | 29.81 g·mol−1 |
| Thermochemistry | |
Std molar entropy(S⦵298) | 200.48 J K−1 mol−1 |
Std enthalpy of formation(ΔfH⦵298) | 115.90 kJ mol−1 |
| Related compounds | |
Relatedisoelectronic compounds | Carbon monoxide,dinitrogen,nitrosonium,cyanide,acetylide |
Related compounds | aluminium monofluoride aluminium monochloride aluminium monoiodide gallium monofluoride |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Boron monofluoride orfluoroborylene is a chemical compound with the formula BF, one atom ofboron and one offluorine. It is an unstable gas, but it is a stableligand ontransition metals, in the same way ascarbon monoxide. It is asubhalide, containing fewer than the normal number of fluorine atoms, compared withboron trifluoride. It can also be called aborylene, as it contains boron with two unshared electrons. BF isisoelectronic with carbon monoxide anddinitrogen; each molecule has 14 electrons.[1]
The experimental B–Fbond length is 1.26267 Å.[2][3][4] Despite beingisoelectronic to CO and N2, each of which is typically described as having atriple bond, computational studies generally agree that the truebond 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.[5]
BF is unusual in that thedipole moment is inverted, with fluorine having a positive charge even though it is the moreelectronegative 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.[6]
Boron monofluoride can be prepared by passingboron trifluoride gas at 2000 °C, at reduced pressure (below 1 mm Hg) over a boron rod. It can be condensed at liquid nitrogen temperatures (−196 °C).[7]
Boron monofluoride molecules have a dissociation energy of 7.8 eV or heat of formation −27.5±3 kcal/mole[1][8] or 757±14 kJ/mol.[2] The first ionization potential is 11.115 eV.[2] Thespectroscopic constants vibrational frequency ωe of BF+ (X2Σ+) is 1765 cm−1 and for neutral BF (X1Σ+) it is 1402.1 cm−1.[2][9] The anharmonicity of BF is 11.84 cm−1.[9]
BF can react with itself to form polymers of boron containing fluorine with between 10 and 14 boron atoms. BF reacts withBF3 to formB2F4. BF and B2F4 further combine to form B3F5. B3F5 is unstable above −50 °C and forms B8F12. This substance is a yellow oil.[7]
BF reacts with acetylenes to make the 1,4-diboracyclohexadiene ring system. BF can condense with2-butyne forming 1,4-difluoro-2,3,5,6-tetramethyl-1,4-diboracyclohexadiene. Also, it reacts withacetylene to make 1,4-difluoro-1,4-diboracyclohexadiene.[7] Propene reacts to make a mix of cyclic and non-cyclic molecules which may contain BF or BF2.[2]
BF hardly reacts withC2F4 orSiF4.[2] BF does react witharsine,carbon monoxide,phosphorus trifluoride,phosphine, andphosphorus trichloride to make adducts like (BF2)3B•AsH3, (BF2)3B•CO, (BF2)3B•PF3, (BF2)3B•PH3, and (BF2)3B•PCl3.[2]
BF reacts with oxygen: BF + O2 →OBF + O; with chlorine: BF + Cl2 → ClBF + Cl; and withnitrogen dioxide BF + NO2 →OBF + NO.[10]
A naïve analysis would suggest that BF is isoelectronic with carbon monoxide (CO) and so could form similar compounds tometal carbonyls. As discussed above (see:§ Structure), BF has a much lower bond order, so that thevalence shell around boron is unfilled. Consequently, BF as a ligand is much moreLewis acidic; it tends to form higher-order bonds to metal centers, and can also bridge between two or three metal atoms (μ2 and μ3).[11]
Working with BF as a ligand is difficult due to its instability in the free state.[12] Instead, most routes tend to use derivatives ofBF3 that decompose oncecoordinated.
In a 1968 conference report, Kämpfer et al claimed to produce Fe(BF)(CO)4 via reaction ofB2F4 withFe(CO)5, but modern chemists have not reproduced the synthesis, and the original compound has no crystallographic characterization.[13][14] The first modern demonstration of BFcoordinated to atransition element is due to Vidovic and Aldrige, who produced[(C5H5)Ru(CO)2]2(μ2-BF) (with BF bridging bothruthenium atoms) in 2009.[15] To make the compound, Vidovic and Aldridge reacted NaRu(CO)2(C5H5) with (Et2O)·BF3; the boron monofluoride ligand then formed in-place.[14]
Vidovic and Aldridge also developed a substance with the formula (PF3)4FeBF by reacting iron vapour with B2F4 and PF3.[2] 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.[2]
The first fully characterized molecule featuring BF as a terminal ligand was synthesized by Drance and Figueroa in 2019, bysterically hindering the formation of a dimer. In the molecule, boron isdouble-bonded toiron.[16]
FBScF2, FBYF2, FBLaF2, and FBCeF2 have been prepared in a solid neon matrix by reacting atomic metals with boron trifluoride.[17]
