Effect of a single methyl substituent on the electronic structure of cobaltocene studied by computationally assisted MATI spectroscopy

Metallocenes represent archetypical organometallic compounds playing key roles in various fields of fundamental and applied chemistry. Many of their unique properties arise from low ionization energies (IE) which can be tuned by introducing substituents into the rings. Here we report the first mass-analyzed threshold ionization (MATI) spectrum of a methylmetallocene, (Cp ')(Cp)Co (Cp ' = eta 5-C5H4Me, Cp = eta 5-C5H5). The presence of a single Me group allows us to study the pure effect of methylation without the mutual influence of substituents. The MATI technique provides an extremely high accuracy in determining the adiabatic IE of (Cp ')(Cp)Co which equals 5.2097(6) eV. The effect of a Me group on the IE of cobaltocene appears to be 36% stronger than that in bis(eta 6-benzene)chromium. The MATI spectrum of (Cp ')(Cp)Co shows a rich vibronic structure from which vibrational frequencies of the free ion are determined. This information provides a solid basis for testing the quality of quantum chemical calculations. Various levels of the DFT and coupled cluster computations are used to describe the structural and electronic transformations accompanying the detachment of an elctron from (Cp ')(Cp)Co. New aspects of the methyl substituent influence on the potential energy surfaces, as well as on the inhomogeneous changes in charge density and electrostatic potential caused by ionization, are discussed. Extremely accurate ionization energy and vibrational frequencies of methylcobaltocene are determined by threshold ionization spectroscopy. New aspects of substituent effects are revealed by DFT and coupled cluster calculations.

Automated summary

Metallocenes, including methylcobaltocene, play important roles in various fields of chemistry. The ionization energy and vibrational structure of (Cp ')(Cp)Co can be influenced by introducing methyl substituents. The mass-analyzed threshold ionization spectrum and DFT calculations provide accurate information about the properties and transformations of (Cp ')(Cp)Co.