Rydberg transitions in the near-ultraviolet vapor-phase absorption spectra of 3d metallocenes

The electronic absorption spectra of Cp2M (M = V, Cr, Mn, Fe, Co, Ni; Cp = η5-C5H5), Cp′2Fe (Cp′ = η5-C5H4Me) and Cp*2Mn (Cp* = η5-C5Me5) in the vapor phase have been investigated. Vapor-phase spectra differ from the solution ones. In the most cases, this is caused by the presence of Rydberg bands in the vapor-phase absorption spectra. These bands disappear on going to the spectra of solutions. The 3d(a1g) → Rnp Rydberg transitions are revealed most clearly. The bands at 40 700, 41 000, 41 240, 41 570 cm−1 in the spectrum of Cp2Fe and the band at 39 650 cm−1 in the spectrum of Cp′2Fe are unambiguously assigned to the 3d(a1g) → R4p transition which displays a vibrational structure for ferrocene (0,0 transition at 41 000 cm−1). In addition, the vapor-phase spectrum of ferrocene shows bands at 42 490 and 42 800 cm−1 which can be assigned to the second component of the 3d(a1g) → R4p transition. The peaks at 39 400 and 46 470 cm−1 in the spectrum of vaporous vanadocene correspond to the two first members of the Rydberg series: 3d(a1g) → R4p and 3d(a1g) → R5p. Analogous assignment for the bands at 35 000 and 45 000 cm−1 in the vapor-phase spectrum of nickelocene seems to be reasonable. For Cp*2Mn, the 3d(a1g) → Rnp (n = 4, 5, 6) Rydberg series is observed. The energy of ionization from the 3d(a1g) orbital of Cp*2Mn (5.4 eV) has been determined as a convergence limit of the series. The shoulder at 48 450 cm−1 in the spectrum of vaporous chromocene may be interpreted as one of the first members of the Rydberg np series. The intensity of the band at 47 000 cm−1 in the spectrum of cobaltocene changes when going from the vapor phase to solution. This can be explained by the contribution of the 3d(a1g) → R4p Rydberg transition.