Electronic structure, molecular geometry, and bonding energetics in zerovalent yttrium and gadolinium bis(arene) sandwich complexes. A theoretical ab initio study

The first ab initio study of bonding and bonding energetics in zerovalent rare earth bis-(arene) sandwich complexes is presented. The d³ ground state and d²s¹ first excited state [(e_2g)³ and (e_2g)²(a_1g¹) configurations, respectively] electronic structures and geometries of the M(η⁶-C₆H₆)₂ complexes, where M = Y and Gd, have been investigated by using relativistic effective core potentials with MP2, MP4, and coupled-cluster ab initio wave functions. The two electronic states lie close in energy but exhibit significant differences in bonding interactions and geometries. The greater stabilities found in the ground states of both complexes are associated with greater metal → ligand back-bonding interactions and, hence, shorter M-C bond lengths. Computed ground state electronic configurations agree well with experimental EPR and magnetic susceptibility data, while metrical parameters are in very good agreement with diffraction data. The metal-arene interactions, both ligand-to-metal donation and metal-to-ligand back-donation, are similar in both complexes. The role of electron correlation appears crucial to correctly predicting the metal-arene dissociation energies. The Hartree-Fock energy is actually repulsive compared to that of the ground state fragments, while the inclusion of correlation effects through perturbative and coupled-cluster procedures predicts bound molecules, even though the binding energies remain underestimated.