THE C-60(2-) FULLERIDE ION

Synthetic, structural, and magnetic aspects of discrete C-60(2-) salts have been investigated to bring coherence to conflicting descriptions of the electronic structure of the Buckminsterfulleride(2-) ion. The C-13 NMR chemical shift in DMSO solution appears as a broad signal at 184 ppm indicative of a similar to 40 ppm downfield paramagnetic shift relative to C-60 Variable-temperature magnetic susceptibility studies are consistent with essentially equi-energy singlet (S = 0) and triplet (S = 1) spill states. The EPR spectrum of C-60(2-) at 4 K consists of two signals. One signal is a typical axial triplet with 2D similar to 25 G. The second is a narrower, doublet-like signal proposed to arise from a triplet with 20 too small to be resolved. A further axial triplet signal is observed at higher temperatures and is ascribed to thermal occupation of a low-lying excited state. The presence of three triplet states is a natural consequence of the reduction of the symmetry of C-60(2-) from I-h to C-i as required by the Jahn-Teller theorem and observed in the X-ray crystal structure of [PPN+](2)[C-60(2-)] (PPN+ = bis(triphenylphosphine)iminium). Collectively, the NMR, magnetic susceptibility, and EPR data show that, contrary to indications from recent EPR studies, C-60(2-) is paramagnetic.