Electron localization in the bis-arene complexes [(η-C6H6)2Cr] and [(η-C6H5Me)2Mo]: an investigation by photoelectron spectroscopy with variable photon energy

Relative partial photoionization cross sections have been measured for the valence bands of [(η-C 6 H 6 ) 2 Cr] and [(η-C 6 H 5 Me) 2 Mo] over the photon energy ranges 24–80 eV ([(η-C 6 H 6 ) 2 Cr]) and 24–100 eV ([(η-C 6 H 5 Me) 2 Mo]). Both compounds show pronounced intensity increases of the metal-based ionizations (a −1 1g and e −1 2g ) in the region of the n p subshell ionization potentials, attribute to n d resonant photoemission following n p→ n d giant resonant absorption. Shape resonances are also observed in the cross sections of these bands. For [(η-C 6 H 5 Me) 2 Mo], the two resonance processes are almost coincident in photon energy, but can be distinguished more clearly in the [(η-C 6 H 6 ) 2 Cr] cross sections. Evidence for metal d—carbon 2pπ covalency in the e 1g set of molecular orbitals is found, with features in the cross sections of the metal-based bands correlating with those in the e −1 1g ionizations. The e −1 1u bands show an essentially monotonic cross section decrease with photon energy.