A breakthrough in the study of single-molecule magnets occurred with the discovery of zero-field slow magnetic relaxation and hysteresis for the linear iron(I) complex [Fe(C(SiMe3)(3))(2)](-) (1), which has one of the largest spin-reversal barriers among mononuclear transition-metal single-molecule magnets. Theoretical studies have suggested that the magnetic anisotropy in 1 is made possible by pronounced stabilization of the iron d(z)(2) orbital due to 3d(z)(2)-4s mixing, an effect which is predicted to be less pronounced in the neutral iron(II) complex Fe(C(SiMe3)(3))(2) (2). However, experimental support for this interpretation has remained lacking. Here, we use high-resolution single-crystal X-ray diffraction data to generate multipole models of the electron density in these two complexes, which clearly show that the 5 iron d(z)(2) orbital is more populated in 1 than in 2. This result can be interpreted as arising from greater stabilization of the d(z)(2) orbital in 1, thus offering an unprecedented experimental rationale for the origin of magnetic anisotropy in 1.