Insights into Single-Molecule-Magnet Behavior from the Experimental Electron Density of Linear Two-Coordinate Iron Complexes.

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 z2 orbital due to 3d z2-4s 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 iron d z2 orbital is more populated in 1 than in 2. This result can be interpreted as arising from greater stabilization of the d z2 orbital in 1, thus offering an unprecedented experimental rationale for the origin of magnetic anisotropy in 1.

Predictive concept for lone-pair distortions - DFT and vibronic model studies of AXn-(n-3) molecules and complexes (A = NIII to BiIII; X = F-I to I-I; n = 3-6).

The stereochemical and energetic consequences of the lone-pair effect in the title molecules and complexes have been studied by DFT calculations based on a vibronic coupling concept. The anionic complexes were examined as bare entities and, more realistically, in a polarizable charge-compensating solvent continuum. The tendency for distortions of AX3 compounds away from the high-symmetry parent geometry becomes more pronounced the larger the chemical hardness of a molecule and its constituents is; on the other hand, anionic complexes AXn-(n-3) (n = 4-6) become softer and less susceptible to distortion as compared to the corresponding AX3 molecule, the larger the coordination number and the anionic charge are. Thus, while all AX(3) compounds adopt the distorted C3v structure, only very few AX6(3-) species are calculated to deviate from the parent Oh geometry. If a complex possesses a low stabilization energy due to an unfavorable central ion/ligand size ratio, vibronic coupling may even lead to complete dissociation of one (SbF6(3-) --> SbF5(2-) + F-) or more (PF6(3-) --> PF4- + 2F-) ligands. The derived hardness rule perfectly covers the reported structural findings. The calculations indicate that the lone-pair effect is an orbital overlap phenomenon. The interpair repulsion within the valence shell, keeping the average bond distances constant, does not stabilize the distorted with respect to the parent geometry, in disagreement with the VSEPR model.