Free phosphaborenes have a labile boron–phosphorus double bond and therefore require extensive steric shielding by bulky substituents to prevent isomerisation and oligomerisation. In the present work, the small free phosphaborene F2B–P=BF was isolated by matrix-isolation techniques and was characterised by infrared spectroscopy in conjunction with quantum-chemical methods. In contrast to its sterically hindered relatives, this small phosphaborene exhibits an acute BPB angle of 83° at the CCSD(T) level. An alternative orbital structure for the B=P double bond is found in the triradical B=PF3, the direct adduct of laser-ablated atomic B and PF3. The single-bonded isomer F2B–PF and the dimer F3P–B[three-bound]B–PF3 are also tentatively assigned.
