Magnetic susceptibility and electron paramagnetic resonance of chromocene and 1,1′-dimethyl-chromocene: Evidence for the dynamic Jahn–Teller effect

The magnetic susceptibility of 1,1′-dimethyl-chromocene, Cr(C5H4CH3)2, has been studied between 1.00 and 321.6 K. The data are well reproduced by a calculation of the dynamic Jahn–Teller effect for the 3E2g(a1ge32g) ground state of D5d symmetry. Two sets of ‘‘best fit’’ parameter values are obtained: ζ=160 cm−1, δ=100 cm−1, kJT=1.10, κ=0.70 (set I); ζ=200 cm−1, δ=200 cm−1, kJT=1.15, κ=0.70 (set II). The magnetism of chromocene Cr(C5H5)2 is well approximated by the same set of parameters above ∼10 K. The calculated values of the lowest energies are in good agreement with recent measurements of the electronic Raman effect in chromocene. The results have been utilized to interpret the EPR spectra of the compounds in methyltetrahydrofuran solution.