Electron spin multiplicities of transition-metal aromatic radicals and ions: M[C₆(CH₃)₆] and M⁺[(C ₆(CH₃)₆] (M = Ti, V, and Co)

Determination of electron spin multiplicities of transition-metal radicals and ions challenges both experimentalists and theoreticians. In this work, we report preferred electron spin states of M[C₆(CH₃) ₆] and M⁺(C₆(CH₃)₆], where M = Ti, V, and Co. The neutral radicals were formed in a supersonic metal cluster beam source, and their masses were measured with time-of-flight mass spectrometry. Precise ionization energies of the radicals and metal-ligand stretching frequencies of the ions were measured by pulsed field ionization zero electron kinetic energy spectroscopy. C-H stretching frequencies of the methyl group in the radicals were obtained by infrared-ultraviolet two-photon ionization. Electron spin multiplicities of the radicals and ions were investigated by combining the spectroscopic measurements, density functional theory, and Franck-Condon factor calculations. The preferred spin states are quintet, sextet, and quartet for the neutral Ti, V, and Co radicals, respectively; for the corresponding singly charged cations, they are quartet, quintet, and triplet. In these high-spin states, the aromatic ring remains nearly planar. This finding contrasts to the previous study of Sc(hmbz), for which low-spin states are favored, and the aromatic ring is severely bent.