Magnetic interactions and spin densities in molecular compounds: an example

The discovery of ferromagnetism in the charge transfer salt [Fe(Cp-2(*))](. +)[TCNE](.-) has raised a lot of interest. The nature of this magnetic coup ling Was controversial. The group of Miller invoked a McConnell II mechanis m, with configuration interactions. Another view was given by Kahn and cowo rkers 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 int erpretation, this spill density should be negative. To clarify this mechani sm, it was decided to investigate separately, by polarized neutron diffract ion, the spin density of [TCNE](.-), associated with a nonmagnetic donor an d the magnetization density of [Fe(Cp-2(*))](.+), associated with a nonmagn etic 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 att empts to measure the magnetization density of [Fe(Cp-2(*))](.+) were unsucc essful due to a loss of symmetry on cooling. To overcome this difficulty, w e have finally measured the magnetization density of [Fc(Cp-2(*))](.+) in a crystal of space group PT, where four of these ions are associated with th e polyoxotungstate [SiW12O40](4-). We have found that the Fe atoms carry a moment of 2.0 mu, and that the carbons of the rings early - 0.005 +/- 0.001 mu (B). The signs of these carbon spin populations are consistent with the McConnell T mechanism, but their magnitudes are too small to account for t he experimental interactions. (C) 2001 Elsevier Science B.V. All rights res erved.