We report an entirely new application of the experimental electron density (EED) in the study of magnetic anisotropy of single-molecule magnets (SMMs). Among those SMMs based on one single transition metal, tetrahedral Co(II)-complexes are prominent, and their large zero-field splitting arises exclusively from coupling between the dx2-y2 and dxy orbitals. Using very low temperature single-crystal synchrotron X-ray diffraction data, we obtained an accurate electron density (ED) for a prototypical SMM, and we have used the experimental d-orbital populations to quantify the dxy-dx2-y2 coupling, which simultaneously provides us with the composition of the ground state Kramers doublet wave function. Based on this experimentally determined wave function, we calculate an energy barrier for magnetic relaxation in the range 193-268 cm-1 , which is in full accordance with the previously published value of 230 cm-1 obtained from near-infrared spectroscopy. These results provide the first clear and direct link between ED and molecular magnetic properties.