EPR Studies of the Electronic Structure and Dynamic Jahn-Teller Effect in Iron(I) Sandwich Compounds

Abstract

Variable-temperature (4–300 K) EPR results are reported for the 19-electron sandwich complexes [η5-C5H5]Fe[η6-C6H6] = Fe(Cp,Bz) and [η6-C6(CH3)6]2Fe+PF6− = Fe(hmBz)2PF6 and some ring-alkylated derivatives of the former in the pure form as well as in the dilute form in several diamagnetic host compounds and frozen solutions. The results are consistent with a d7 configuration for iron, with the unpaired electron in a mainly metal-centered (dxz, dyz) molecular orbital. The variation of the g values in the series and their host lattice dependence are analyzed and explained on the basis of dynamic Jahn-Teller coupling in the effective 2π ground state and rhombic splitting caused by the unsymmetrical ligands and/or host lattice potential. The Jahn-Teller coupling strength, kJT, is of the order of unity in these systems, which means that the Jahn-Teller stabilization energy and the effective frequency of the vibronic coupling mode are of comparable magnitude. The Jahn-Teller effect is therefore completely dynamic even at the lowest temperature as has been previously observed also in the case of d7 metallocenes. The variation of the vibronic overlap integral and kJT with the rhombic distortion is evaluated and discussed. In the case of Fe(Cp,Bz), the variation of polycrystalline line width with temperature is fitted to an Orbach-type formula in order to derive information on the energy of the first excited Kramers' doublet. © 1982, American Chemical Society. All rights reserved.