The electronic structures of phenylnitrenes with anionic it-donating substituents are investigated by using mass spectrometry and electronic structure calculations. Reactions of para-CH2--substituted phenylnitrene, formed by dissociative deprotonation of p-azidotoluene, with CS2 and NO indicate that it has a closed-shell singlet ground state, whereas reactions of p-oxidophenylnitrene formed by dissociative deprotonation of p-azidophenol indicate either a triplet ground state or a singlet with a small singlet triplet splitting. The ground electronic state assignments based on ion reactivity are consistent with electronic structure calculations. The stability of the closed shell singlet states in nitrenes is shown by Natural Resonance Theory to be very sensitive to the amount of deprotonated-imine character in the wave function, such that large changes in state energies can be achieved by small modifications of the electronic structure.