A study, by inelastic neutron scattering, of adsorbed benzene, of metal π-Arene complexes and of hydrogen attached to metal clusters

Abstract

Studies of transition metal hydridocarbonyls show that the hydrogen vibrations are decoupled from the metal-carbonyl skeleton. The hydrogen modes of (μ(_2)-H) species (HW(_2)(CO)(_9)NO, HRe(_3)(CO)(_14), H(_3)Mn(CO)(_12), H(_3)Re(_3)(CO)(_12), H(_2)FeRu(_3)(CO)(_13) and H(_4)Ru(_4)(CO)(_12)), (μ (_3)-H) species (H(_4)Re(_4)(CO)(_12), HFeCo(_3)(CO)(_12) and H(_2)Ru(_6)(CO)(_18), (μ(_6) species (CsHCo(_6)(CO)(_15)) and (μ(_2)-H)(_2) species (H(_2)Os(_3)(CO)(_10) can be described in terms of C(_2v), C(_3v), O(_h) and D(_2h) ‘local’ symmetry respectively. The (μ(_2)-H) spectra are complex and a description that involves framework metal-carbonyl deformations with associated hydrogen motion is postulated. The vibrational data has assisted in the analysis of previous spectra of hydrogen adsorbed on transition metal surfaces. Evidence is overwhelmingly in favour of (μ(_3)-H) species on Ni while (μ(_2)-H) and (μ(_3)-H) species are found on Pt. It is suggested the hydrogen site(s) cannot be described confidently for the H/W system with present data. A study of C(_6)H(_6) and, to a lesserextent, C(_7)H(_7), π -complexes was carried out to assist in adsorbed benzene studies. Adsorbed benzene on both Na13X and Ag13X zeolites resembled that found in weak C(_6)H(_6) complexes. The only major difference between the two M(^+)-C(_6)H(_6) interactions was that a higher torsional frequency existed in Ag13X. A stronger complexing of benzene took place on Pt black. This was deduced from the high frequency skeletal vibrations, the large increase in ͌ _11) and the large torsional effective force constant. The benzene lies in a position, above single Pt atoms, of at least C(_2v) symmetry. Only one surface structure was indicated at ≤1 monolayer coverage whereas at ~2 monolayers the IINS spectra resembled that of solid and clathrated benzene.