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

The very long C-C intrafullerene bond distance of r_long=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, C₁₂₀^2-, C₁₂₀^6-, 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, r_long 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.