Spatial distribution of valence electrons in metallocenes studied by Penning ionization electron spectroscopy

Abstract

Penning ionization electron spectra resulting from thermal collisions of He*(1s2s, 2³S) metastable atoms with gaseous metallocenes, M(C₅H₅)₂ (M = V, Cr, Mn, Fe, Co, Ni, Ru, and Os), were measured to directly probe the spatial electron distribution of the metal d-derived orbitals. Owing to the strong antibonding d–π interactions the metallic e₁″ orbitals in Mn(C₅H₅)₂, Ni(C₅H₅)₂, and especially Co(C₅H₅)₂, show rather diffuse electron distributions. In contrast, the Cp π-derived e₁″ orbitals exhibit compact distributions due to the bonding d–π interactions, which is clearly seen in the cases of Ru(C₅H₅)₂ and Os(C₅H₅)₂. For the metallic e₂′ and a₁′ orbitals with essentially nonbonding character, the radial extent of the d orbitals increase on going from Fe(C₅H₅)₂ to Ru(C₅H₅)₂ and Os(C₅H₅)₂, as expected. Mn(C₅H₅)₂ is anomalous among the 3d metallocenes, because the nonbonding e₂′ and a₁′ orbitals are significantly contracted in space, reflecting their highly ionic character.