Bonding in Sandwich Compounds

A study is made of the bonding in sandwich compounds by means of ESR and susceptibility measurements. Data obtained for vanadocene and nickelocene are discussed in terms of various calculated MO schemes. The experimental results are compatible with the level order $e_{\text{2g}}{\mathrm{\hspace{0.17em}\lesssim \hspace{0.17em}a}}_{\text{1g}}{\mathrm{\hspace{0.17em}<\hspace{0.17em}e}}_{\text{1g}}*$ for the highest‐bonding and lowest‐antibonding orbitals, all orbitals having a strong metal $\text{3d}$ character. The assignments given for the ground‐state configurations are quantitatively checked by comparison of calculated and experimental $D$ and $g$ values. Both spin–orbit and spin–spin coupling are shown to give a substantial contribution to the value of $D$ . The excitation energies used in the perturbation calculation of the $D_{\mathrm{LS}}$ and $g$ values are obtained from a ligand‐field treatment of the low‐energy bands in the optical absorption spectra.