Hyperfine Coupling Constants and Covalency in Sandwich Complexes

An analysis of the isotropic proton hyperfine coupling constants of V(C₅H₅)₂, Ni(C₅H₅)₂, and Cr(C₆H₆)₂⁺ is given. The unpaired electrons in the $e_{\text{1g}}*$ molecular orbitals in Ni(C₅H₅)₂ are found to be rather strongly delocalized into the carbon ${\text{2p}}_z\mathrm{–\pi }$ framework of the ligands. In agreement with bonding theories this indicates that the interaction between the metal ${\text{3d}}_{\mathrm{xz}}$ and ${\text{3d}}_{\mathrm{yz}}$ orbitals and the ligand $e_{\text{1g}}\mathrm{–\pi }$ molecular orbitals is very important for the chemical bonding in sandwich complexes. In Cr(C₆H₆)₂⁺ and V(C₅H₅)₂ the proton coupling constants are determined mainly by $\sigma$ delocalization of the unpaired electron in the $a_{\text{1g}}{\text{(3d}}_{z^2})$ orbital into the hydrogen $\text{1s}$ orbitals. In V(C₅H₅)₂, also, delocalization into the ligand $\pi$ framework is present and this is thought to be due to the $e_{\text{2g}}$ orbitals. It is concluded that the interaction of the metal ${\text{3d}}_{\mathrm{xy}}$ and ${\text{3d}}_{x^2{\mathrm{-y}}^2}$ orbitals with the ligand $e_{\text{2g}}\mathrm{–\pi }$ molecular orbitals is appreciable.