The magnetic properties of the two uranium coordination compounds, [K(18c6)][U(OSi-((OBu)-Bu-t)(3))(4)] and [K(18c6)][U(N(SiMe3)(2))(4)], both presenting the U-III ion in similar pseudotetrahedral coordination environments but with different O- or N-donor ligands, have been measured. The static magnetic susceptibility measurements and density functional theory studies suggest the presence of different ligand fields in the two compounds. Alternating-current susceptibility studies conducted at frequencies ranging from 95 to 9995 Hz and at temperatures in the 1.7-10 K range revealed for both compounds slow magnetic relaxation already at zero static magnetic field with similar energy barriers U similar to 24 K.