Magnetic Anisotropy of Ferrocene and Chemical Bonding

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Abstract
Knowledge of the magnetic anisotropy of ferrocene was expected to throw some light on the controversial question concerning the bonding of the iron atom to the rings in ferrocene. With this in view, measurements of the anisotropy were made on small single crystals of ferrocene by Krishnan's flip‐angle method. The quartz torsion fibers were calibrated using the known crystal structure and magnetic susceptibilities of naphthalene and acenaphthene. Magnetic field strengths ranged from about 4 to 9 kG. Using the data of a complete three‐dimensional x‐ray study of ferrocene, the principal molecular susceptibilities were determined by the method worked out by Lonsdale and Krishnan. The susceptibility along the direction of the K 3 molecular axis, normal to the plane of the rings through the center of mass of the rings, was much greater than those along the K 1 and K 2 directions, in the plane of the rings. The anisotropy between K 3 and the average of K 1 and K 2 was found to be 49.5×10—6 cgs units, and is evidence in favor of a single d π—p πbond between each of the rings and the iron atom. Pauling's ``resonance structures'' also seem plausible; the studies do not support Fischer's ``complete π‐electron donation'' structure. Theoretical calculations based on Langevin's formula for diamagnetism show a close agreement with the experimental value.

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