Unusual polymerization in theLi4C60fulleride

${\mathrm{Li}}_{4}{\mathrm{C}}_{60}$, one of the best representatives of lithium intercalated fullerides, features an unusual type of two-dimensional polymerization. Extensive investigations, including laboratory x-ray and synchrotron radiation diffraction, $^{13}\mathrm{C}$ NMR, magic angle spinning, and Raman spectroscopy, show a monoclinic $I2∕m$ structure, characterized by chains of $[2+2]$-cycloaddicted fullerenes, sideways connected by single $\mathrm{C}--\mathrm{C}$ bonds. This leads to the formation of polymeric layers, whose insulating nature, deduced from the NMR and Raman spectra, denotes the complete localization of the electrons involved in the covalent bonds.