Vibronic spectroscopy of unsaturated transition metal complexes: $CrC2H,$ $CrCH3,$ and $NiCH3$

Vibronically resolved resonant two-photon ionization and dispersed fluorescence spectra of the organometallic radicals $CrC2H,$ $CrCH3,$ and $NiCH3$ are reported in the visible and near-infrared wavelength regions. For $CrC2H,$ a complicated vibronic spectrum is found in the 11 100–13 300 cm⁻¹ region, with a prominent vibrational progression having $ωe′=426.52±0.84 cm−1,$ $ωe′xe′=0.74±0.13 cm−1.$ Dispersed fluorescence reveals a $v″=1$ level of the ground state with $ΔG1/2″=470±20 cm−1.$ These vibrational frequencies undoubtedly pertain to the $Cr–C2H$ stretching mode. It is suggested that the spectrum corresponds to the $Ã 6Σ+←X̃ 6Σ+$ band system, with the $CrC2H$ molecule being linear in both the ground and the excited state. The related $CrCH3$ molecule displays a vibronic spectrum in the 11 500–14 000 cm⁻¹ region. The upper state of this system displays six sub-bands that are too closely spaced to be vibrational structure, but too widely separated to be K structure. It is suggested that the observed spectrum is a $E6←X̃ A16$ band system, analogous to the well-known $B 6Π←X 6Σ+$ band systems of CrF and CrCl. The ground state $Cr–CH3$ vibration is characterized by $ωe″=525±17 cm−1$ and $ωe″xe″=7.9±6 cm−1.$ The spectrum of $NiCH3$ lies in the 16 100–17 400 cm⁻¹ range and has $ωe′=455.3±0.1 cm−1$ and $ωe′xe′=6.60±0.03 cm−1.$ Dispersed fluorescence studies provide ground state vibrational constants of $ωe″=565.8±1.6 cm−1$ and $ωe″xe″=1.7±3.0 cm−1.$ Again, these values correspond to the $Ni–CH3$ stretching motion.