Insights into the origin of magnetic anisotropy in linear iron complexes from the experimental electron density

In 2013, Zadrozny et al. [1,2] discovered zero-field slow magnetic relaxation and hysteresis for the linear iron(I) complex [Fe(C(SiMe 3 ) 3 ) 2 ] , which has one of the largest spin-reversal barriers reported for mononuclear transitionmetal single-molecule magnets.Theoretical calculations suggested that the magnetic anisotropy is due to pronounced stabilization of the iron 3d z 2 orbital in this complex compared to the neutral iron(II) complex Fe(C(SiMe 3 ) 3 ) 2 [3].Experimental support for this interpretation has however remained lacking.In the present study[4], we have determined the experimental electron density from high-resolution single-crystal X-ray diffraction data in Fe(C(SiMe 3 ) 3 ) 2 ] and Fe(C(SiMe 3 ) 3 ) 2 , which shows that the d z 2 orbital is indeed more populated in Fe(C(SiMe 3 ) 3 ) 2 ] than in Fe(C(SiMe 3 ) 3 ) 2 .This can be interpreted as arising from a greater stabilization of the d z 2 orbital in Fe(C(SiMe 3 ) 3 ) 2 ] than in Fe(C(SiMe 3 ) 3 ) 2 , thus providing unprecedented experimental evidence for the origin of magnetic anisotropy in [Fe(C(SiMe 3 ) 3 ) 2 ] and the corresponding slow magnetic relaxation.