B4CO2: a new, observable sigma-pi diradical.

A new sigma-pi diradical, B4(CO)2, prepared in matrix isolation, was characterized unambiguously by isotopic-substitution infrared spectroscopy and by theoretical computations. Both open-shell singlet and triplet states have three pi electrons but are aromatic with moderately large NICS values. Quantum chemical calculations at various levels indicate that the open-shell singlet is slightly more stable than the triplet state. However, the singlet and triplet are computed to have very similar IR features which do not allow experimental differentiation.