COMPARATIVE-STUDY OF THE DIPOLE POLARIZABILITY OF THE METALLOCENES FE(C5H5)(2), RU(C5H5)(2) AND OS(C5H5)(2)

We report measurements of the density and frequency dependence of the gas-phase refractivity (n-1) of ferrocene, Fe(C5H5)(2), ruthenocene, R u(C5H5)(2), and osmocene, Os(C5H5)(2). From these measurements we dire ctly obtain information on the frequency dependence of the mean dipole polarizability alpha(omega) in the frequency range between omega=0.05 and 0.14 au. Combination of these results with available polarizabili ty anisotropies Delta alpha(omega) yields a two-term Kramers-Heisenber g dispersion relation, from which the frequency dependence of the elec tronic parts of both alpha(omega) and Delta alpha(omega) can be obtain ed. From this analysis we yield the static values alpha(0)=126.13(66) au, 133.1(1.2) au and 138.5(4.8) au and Delta alpha(0)=33.9(1.1), 35.1 (2.0) and 35.6(7.7) au, for ferrocene, ruthenocene and osmocene, respe ctively. In all cases a dramatic increase in Delta alpha(omega) with i ncreasing frequency is observed, and the UV values of Delta alpha at o mega=0.15 au are approximately twice the static values. All of these r esults are used to estimate dispersion interaction energy constants C- 6 as well as the diamagnetic susceptibility chi of these compounds. Si mple group contribution models are shown to fail in predicting the pol arizability of the metallocenes.