Ph₃PC - A Monosubstituted C(0) Atom in Its Triplet State

This study introduces a novel class of carbon-centered diradicals: a monosubstituted C atom stabilized by a phosphine. The diradical Ph₃P→C was photochemically generated from a diazophosphorus ylide precursor (Ph₃PCN₂) and characterized by EPR and isotope-sensitive ENDOR spectroscopy at low temperatures. Ph₃P→C features an axial zero-field splitting parameter D=0.543 cm^-1 with a vanishingly small rhombicity |E|/D=0.002. Time- and temperature-dependent measurements confirm a triplet ground state with a lifetime of approximately 10 min at 127 K in toluene-d₈. Multireference electronic structure calculations predict a clear triplet ground state with a singlet-triplet gap greater than 20 kcal/mol. In contrast to divalent C(0) compounds, such as Ph₃P→C←PPh₃, in which carbon needs excitation into a highly-excited closed-shell 2s⁰2p⁴ configuration, Ph₃P→C can be explained by direct involvement of carbon in its natural ³P state arising from the 2s²2p² configuration.