Comparative study of the dipole polarizability of the metallocenes Fe(C₅H₅)₂,Ru(C₅H₅)₂and Os(C₅H₅)₂

Abstract

We report measurements of the density and frequency dependence of the gas-phase refractivity (n − 1) of ferrocene, Fe(C₅H₅)₂, ruthenocene, Ru(C₅H₅)₂, and osmocene, Os(C₅H₅)₂. From these measurements we directly obtain information on the frequency dependence of the mean dipole polarizability α(ω) in the frequency range between ω = 0.05 and 0.14 au. Combination of these results with available polarizability anisotropies Δα(ω) yields a two-term Kramers–Heisenberg dispersion relation, from which the frequency dependence of the electronic parts of both α(ω) and Δα(ω) can be obtained. From this analysis we yield the static values α(0) = 126.13(66) au, 133.1(1.2) au and 138.5(4.8) au and Δα(0) = 33.9(1.1), 35.1(2.0) and 35.6(7.7) au, for ferrocene, ruthenocene and osmocene, respectively. In all cases a dramatic increase in Δα(ω) with increasing frequency is observed, and the UV values of Δα at ω = 0.15 au are approximately twice the static values. All of these results are used to estimate dispersion interaction energy constants C₆ as well as the diamagnetic susceptibility χ of these compounds. Simple group contribution models are shown to fail in predicting the polarizability of the metallocenes.