Is a 1.90 Å C–C bond length in polymeric fullerides possible?

The very long C–C intrafullerene bond distance of rlong=1.90(15) Å in RbC60 polymeric fulleride observed by Stephens et al. is unprecedented. Large scale full geometry optimizations using non-local density functional theory on C120, C1202−, C1206−, as well as on the model dinaphthalene dimer were performed in exploring the potential energy surface along the long C–C bond in question. For neutral C120, rlong should be close to 1.61 Å, a value that is in agreement with the experimental value for the C120 buckyball dimer and is close to the lower limit of the Stephens C–C value.