Salts containing phosphaalkene radical anions have been isolated and characterized by
electron paramagnetic resonance (EPR) spectroscopy, UV–vis absorption spectroscopy,
and single-crystal X-ray diffraction. The radical anions feature elongated P–C bonds and an
aromatization of fulvene compared to the neutral phosphaalkene. Their EPR spectra and
theoretical calculations indicate the spin density of the radicals mainly resides on
phosphorus atoms. This work provides the first example of a crystalline phosphaalkene …