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.[1]

Boron monofluoride
Names
Other names
Boron fluoride

Boron(I) fluoride
Fluoroboronene

Fluoroborylene
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.033.970
EC Number
  • 237-383-0
UNII
Properties
BF
Molar mass 29.81 g·mol−1
Thermochemistry
200.48 J K−1 mol−1
115.90 kJ mol−1
Related 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

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] 760 kJ/mol.[2] The first ionization potential is 11.115 eV.[2] ωe is 1765 cm−1.[2]

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.[9]

Ligand

The first case of BF being a ligand on a transition element was demonstrated in 2009 with the compound (C5H5)2Ru2(CO)4(μ-BF).[10] The BF was bound to both ruthenium atoms as a bridge.[11]

Vidovic and Aldridge reacted NaRu(CO)2(C5H5) with (Et2O)·BF3.[12] 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 Fe(BF)(CO)4 was formed in the reaction of B2F4 with Fe(CO)5, however this has not been reproduced.[12]

By reacting iron vapour with B2F4 and PF3, a substance with the formula (PF3)4FeBF was produced.[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.[13]

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).[14] Working with BF as a ligand is difficult due to its instability in the free state.[15]

References
  1. 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.
  2. Vidovic, Dragoslav; Aldridge, Simon (2011). "Coordination chemistry of group 13 monohalides". Chemical Science. 2 (4): 601. doi:10.1039/C0SC00508H.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. Timms, P. L. (1972). "Low Temperature Condensation". Advances in Inorganic Chemistry and Radiochemistry. p. 143. ISBN 0-12-023614-1.
  8. Eyring, Leroy (1967). Advances in High Temperature Chemistry volume 1. Academic Press. p. 70. ISBN 9781483224343.
  9. 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.
  10. Vidovic, Dragoslav; Aldridge, Simon (4 May 2009). "Coordination and Activation of the BF Molecule". Angewandte Chemie. 121 (20): 3723–3726. doi:10.1002/ange.200901022. PMID 19373822.
  11. Xu, L.; Li, Q.; Xie, Y.; King, R. B.; Schaefer, H. F. (2010). "Prospects for Making Organometallic Compounds with BF Ligands: Fluoroborylene Iron Carbonyls". Inorganic Chemistry. 49 (3): 1046–1055. doi:10.1021/ic901964f. PMID 20041690.
  12. 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.
  13. 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. doi:10.1126/science.aaw6102. PMID 30872521. S2CID 78094683.
  14. 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.
  15. 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.
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