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Boron monofluoride
Names
	Other names Boron fluoride; Boron(I) fluoride; Fluoroboronene; Fluoroborylene
Identifiers
CAS Number	13768-60-0 ;
3D model (JSmol)	Interactive image; Interactive image; Interactive image;
ChemSpider	4891729 ;
ECHA InfoCard	100.033.970
EC Number	237-383-0;
PubChem CID	6336604;
UNII	N01K2O1ZJH ;
CompTox Dashboard (EPA)	DTXSID501317099 ;
	InChI InChI=1S/BF/c1-2 Key: YFSQMOVEGCCDJL-UHFFFAOYSA-N ;
	SMILES [BH0]F; [B-]=[F+]; [B-2]#[F+2];
Properties
Chemical formula	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
Related isoelectronic 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 their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Boron monofluoride or fluoroborylene is a chemical compound with the formula BF, one atom of boron and one of fluorine. It is an unstable gas, but it is a stable ligand on 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.^[1]

Structure

The experimental B–F bond length is 1.26267 Å.^[2]^[3]^[4] 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.^[5]

Lewis dot diagram structures show three formal alternatives for describing bonding in boron monofluoride.

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.^[6]

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 (−196 °C).^[7]

Properties

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] The spectroscopic constants vibrational frequency ω_e of BF⁺ (X ²Σ⁺) is 1765 cm⁻¹ and for neutral BF (X ¹Σ⁺) it is 1402.1 cm⁻¹.^[2]^[9] The anharmonicity of BF is 11.84 cm⁻¹.^[9]

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.^[7]

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.^[7] Propene reacts to make a mix of cyclic and non-cyclic molecules which may contain BF or BF₂.^[2]

BF hardly reacts with C₂F₄ or SiF₄.^[2] 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.^[2]

BF reacts with oxygen: BF + O₂ → OBF + O; with chlorine: BF + Cl₂ → ClBF + Cl; and with nitrogen dioxide BF + NO₂ → OBF + NO.^[10]

Ligand

A naïve analysis would suggest that BF is isoelectronic with carbon monoxide (CO) and so could form similar compounds to metal carbonyls. As discussed above (see § Structure), BF has a much lower bond order, so that the valence shell around boron is unfilled. Consequently, BF as a ligand is much more Lewis 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 of BF₃ that decompose once coordinated.

In a 1968 conference report, Kämpfer et al claimed to produce Fe(BF)(CO)₄ via reaction of B₂F₄ with Fe(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 BF coordinated to a transition element is due to Vidovic and Aldrige, who produced [(C₅H₅)Ru(CO)₂]₂(μ₂-BF) (with BF bridging both ruthenium 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, by sterically hindering the formation of a dimer. In the molecule, boron is double-bonded to iron.^[16]

FBScF2, FBYF₂, FBLaF₂, and FBCeF₂ have been prepared in a solid neon matrix by reacting atomic metals with boron trifluoride.^[17]

References

^ ^a ^b Hildenbrand, Donald L.; Murad, Edmond (1965). "Dissociation Energy of Boron Monofluoride from Mass-Spectrometric Studies". The Journal of Chemical Physics. 43 (4): 1400. Bibcode:1965JChPh..43.1400H. doi:10.1063/1.1696932.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Vidovic, Dragoslav; Aldridge, Simon (2011). "Coordination chemistry of group 13 monohalides". Chemical Science. 2 (4): 601. doi:10.1039/C0SC00508H.
^ Nesbet, R. K. (1964). "Electronic Structure of N2, CO, and BF". The Journal of Chemical Physics. 40 (12): 3619–3633. Bibcode:1964JChPh..40.3619N. doi:10.1063/1.1725063.
^ Cazzoli, G.; Cludi, L.; Degli Esposti, C.; Dore, L. (1989). "The millimeter and submillimeter-wave spectrum of boron monofluoride: Equilibrium structure". Journal of Molecular Spectroscopy. 134 (1): 159–167. Bibcode:1989JMoSp.134..159C. doi:10.1016/0022-2852(89)90138-0. ISSN 0022-2852.
^ Martinie, R. J.; Bultema, J. J.; van der Wal, M. N.; Burkhart, B. J.; van der Griend, D. A. & de Kock, R. L. (2011). "Bond Order and Chemical Properties of BF, CO, and N₂". Journal of Chemical Education. 88 (8): 1094–1097. Bibcode:2011JChEd..88.1094M. doi:10.1021/ed100758t.
^ Fantuzzi, Felipe; Cardozo, Thiago Messias; Nascimento, Marco Antonio Chaer (28 May 2015). "Nature of the Chemical Bond and Origin of the Inverted Dipole Moment in Boron Fluoride: A Generalized Valence Bond Approach". The Journal of Physical Chemistry A. 119 (21): 5335–5343. Bibcode:2015JPCA..119.5335F. doi:10.1021/jp510085r. PMID 25531385.
^ ^a ^b ^c Timms, P. L. (1972). "Low Temperature Condensation". Advances in Inorganic Chemistry and Radiochemistry. Academic Press. p. 143. ISBN 0-12-023614-1.
^ Eyring, Leroy (1967). Advances in High Temperature Chemistry volume 1. Academic Press. p. 70. ISBN 9781483224343.
^ ^a ^b Dyke, John M.; Kirby, Colin; Morris, Alan (1983). "Study of the ionization process BF⁺ (X ²Σ⁺ ) ← BF(X ¹Σ⁺ ) by high-temperature photoelectron spectroscopy". J. Chem. Soc., Faraday Trans. 2. 79 (3): 483–490. doi:10.1039/F29837900483.
^ Light, G. C.; Herm, R. R.; Matsumoto, J. H. (November 1985). "Kinetics of some gas-phase elementary reactions of boron monofluoride" (PDF). The Journal of Physical Chemistry. 89 (23): 5066–5074. doi:10.1021/j100269a036. Archived (PDF) from the original on June 1, 2022.
^ Xu, Liancai; Li, Qian-shu; Xie, Yaoming; King, R. Bruce; Schaefer, Henry F. (15 March 2010). "Major Difference between the Isoelectronic Fluoroborylene and Carbonyl Ligands: Triply Bridging Fluoroborylene Ligands in Fe3(BF)3(CO)9 Isoelectronic with Fe3(CO)12". Inorganic Chemistry. 49 (6): 2996–3001. doi:10.1021/ic902511m. PMID 20143841.
^ Xu, Liancai; Li, Qian-shu; King, R. Bruce (May 2012). "Fluoroborylene ligands in binuclear ruthenium carbonyls: Comparison with their iron analogues". Polyhedron. 38 (1): 44–49. doi:10.1016/j.poly.2012.02.003.
^ Drance et al. 2019: "Previously, Vidovic and Aldridge reported that two equivalents of the ruthenium-based nucleophile Na[CpRu(CO)₂] (Cp, cyclopentadienyl; [C₅H₅]⁻) reacts with boron trifluoride diethyl etherate (BF^·
₃Et
₂O) with the formal loss of two equivalents of sodium fluoride (NaF) to produce the bridging BF complex ((μ₂-BF)[CpRu(CO)₂]₂) (20). The latter is the only crystallographically characterized compound in which BF functions as a ligand to a metal center."
^ ^a ^b Xu, L.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. III (2010). "Binuclear fluoroborylene manganese carbonyls". Inorganica Chimica Acta. 363 (13): 3538–3549. doi:10.1016/j.ica.2010.07.013.
^ Vidovic, Dragoslav; Aldridge, Simon (4 May 2009). "Coordination and Activation of the BF Molecule". Angewandte Chemie. 121 (20): 3723–3726. Bibcode:2009AngCh.121.3723V. doi:10.1002/ange.200901022. PMID 19373822.
^ Drance, M. J.; Sears, J. D.; Mrse, A. M.; Moore, C. E.; Rheingold, A. L.; Neidig, M. L.; Figueroa, J. S. (2019). "Terminal Coordination of Diatomic Boron Monofluoride to Iron". Science. 363 (6432): 1203–1205. Bibcode:2019Sci...363.1203D. doi:10.1126/science.aaw6102. PMID 30872521. S2CID 78094683.
^ Xu, Bing; Li, Li; Pu, Zhen; Yu, Wenjie; Li, Wenjing; Wang, Xuefeng (18 February 2019). "Fluoroborylene Complexes FBMF 2 (M = Sc, Y, La, Ce): Matrix Infrared Spectra and Quantum Chemical Calculations". Inorganic Chemistry. 58 (4): 2363–2371. doi:10.1021/acs.inorgchem.8b02801.