[en] The superconducting fulleride (NH₃)_xNaK₂C₆₀ has a cubic structure with lattice parameter (a) and transition temperature (T_c) depending on x. The relation between these two parameters was found, however, to be opposite to what is expected from the BCS theory (and observed in the other fullerides). To better understand the origin of this anomaly we have measured the electronic spin susceptibility with SQUID magnetometry and NMR in two differently doped samples. The relation between T_c and the density of states at the Fermi energy is found to be opposite to the Migdal-Eliashberg prediction. The ¹³C-MAS measurement of the isotropic part of the ¹³C Knight shift qualitatively confirms this result. ¹³C NMR relaxation measurements validate the interpretation of the spin susceptibility in terms of density of states and rule out the presence of strong antiferromagnetic correlations in the Fermi liquid. (orig.)