Magnetic interactions and spin densities in molecular compounds: an example

Abstract

The discovery of ferromagnetism in the charge transfer salt [Fe(Cp 2 *)] + [TCNE] − has raised a lot of interest. The nature of this magnetic coupling was controversial. The group of Miller invoked a McConnell II mechanism, with configuration interactions. Another view was given by Kahn and coworkers who proposed a coupling between spin carriers (McConnell I mechanism), implying a positive exchange due to an overlap between spin densities of opposed signs. In the first interpretation, there would be a positive spin density located on the carbon rings of the ferrocene, but in the second interpretation, this spin density should be negative. To clarify this mechanism, it was decided to investigate separately, by polarized neutron diffraction, the spin density of [TCNE] − , associated with a nonmagnetic donor and the magnetization density of [Fe(Cp 2 *)] + , associated with a nonmagnetic acceptor. For [TCNE] − , the measurement was straightforward: it was found that most of the spin density was located on the central carbon while a noticeable amount was delocalized on the terminal nitrogens. Several attempts to measure the magnetization density of [Fe(Cp 2 *)] + were unsuccessful due to a loss of symmetry on cooling. To overcome this difficulty, we have finally measured the magnetization density of [Fe(Cp 2 *)] + in a crystal of space group P 1 ̄ , where four of these ions are associated with the polyoxotungstate [SiW 12O 40] 4−. We have found that the Fe atoms carry a moment of 2.0 μ B and that the carbons of the rings carry −0.005±0.001 μ B. The signs of these carbon spin populations are consistent with the McConnell I mechanism, but their magnitudes are too small to account for the experimental interactions.