B[double bond, length as m-dash]P double bonds relieved from steric encumbrance: matrix-isolation infrared spectroscopy of the phosphaborene F 2 B-P[double bond, length as m-dash]BF and the triradical B[double bond, length as m-dash]PF 3 .

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 F 2 B-P[double bond, length as m-dash]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[double bond, length as m-dash]P double bond is found in the triradical B[double bond, length as m-dash]PF 3 , the direct adduct of laser-ablated atomic B and PF 3 . The single-bonded isomer F 2 B-PF and the dimer F 3 P-B[triple bond, length as m-dash]B-PF 3 are also tentatively assigned.