Magnetization and spin density in [FeCp2*](center dot+) - A look-back at the magnetic interactions in the chains [FeCp2*](center dot+)[TCNE](center dot-)

The discovery of ferromagnetism in the charge transfer salt [FeCp2*](.+)[TC NE](.-) has raised a controversy concerning the nature of the magnetic coup ling. The group of J.S. Miller invoked a McConnell II mechanism, with confi guration interactions. O. Kahn and his coworkers proposed a McConnell I mec hanism, implying a positive exchange due to an overlap between spin densiti es of opposed signs. In the first interpretation there would be a positive spin density located on the carbon of the Cp* rings, but in the second inte rpretation, this spin density should be negative. To clarify this mechanism , it was decided to independently determine, by polarized neutron diffracti on, the spin density of [TCNE](.-), associated with a nonmagnetic donor and the magnetization density of [FeCp2*](.+) associated with a nonmagnetic ac ceptor. For [TCNE](.-), the measurement was straightforward, but for [FeCp2 *](.+), we had several failures due to a loss of symmetry on cooling the sa mples. Finally, we could carry on this measurement on a triclinic crystal w here [FeCp2*](.+) was associated with a polyoxotungstate anion. On [TCNE](. -), most of the spin density is located on the central carbons, with a noti ceable amount on the terminal nitrogens. On [FeCp2*](.+), the Fe atoms carr y a moment of 2.0 mu (B), a combination of both spin and orbital moments, t he carbon of the rings carry -0.005 +/- 0.001 mu (B), and, surprisingly, th e methyl carbons carry + 0.008 +/- 0.001 mu (B). The signs of the spin popu lations support the McConnell I, but their magnitude accounts for a part of the experimental intrachain exchange interaction only. Ab initio DFT calcu lations predict the very small densities on the carbon atoms of [FeCp2*](.), with the correct signs, but with magnitudes which are three times larger than the experimental ones. (C) 2001 Elsevier Science Ltd. All rights rese rved.