Comparative study on effects of bridging and terminal ligands on magnetic exchange interaction in [(NH3)5Cr((μ-X)Cr(NH3)4L]n+ (X=O, OH; L=OH, OH2, NH3; n=4, 5): density functional theory study

The magneto-structural correlations for the model molecules of the rhodo and erythro series of binuclear ammine complexes of Chromium (III), [(NH3)5Cr(μ-X)Cr(NH3)4L]n+ (X=O, OH; L=OH, OH2, NH3; n=4, 5) are investigated by using density functional theory combined with the broken-symmetry approach. Comparative studies on effects of bridging and terminal ligands demonstrated that the bridging ligand plays a significant role in magnetic exchange interaction between Cr(III) ions. The substitution of the bridging ligand μ-hydroxo by μ-oxo atom leads a remarkable effect on magnetic exchange interaction, while the magnetic coupling constant J decreases exponentially with the increase of the bridging Cr-O distance and increases exponentially with the increase of the bridging Cr-O-Cr angle. In comparison with the bridging ligand, the terminal ligands behave a minor effect in the magnetic exchange interaction. The substitution of the single terminal ligand t-OH by t-OH2 and t-NH3, and the variation of the terminal coordinate bond distances, Cr-(t-OH), Cr-(t-OH2) and Cr-(t-NH3) all exhibit a limited effect on the magnetic exchange interaction between Cr(III) ions. It is unexpected that the variation of the terminal ligands conformation significantly effects on the magnetic exchange interaction. The stronger magnetic exchange interactions are involved in the staggered conformational isomer, where the direction of N11-Cr2-O13 is in either perpendicular or overlapped one of O3-Cr2-N9. On the other hand, the eclipsed conformational isomer of the terminal ligands results in the weaker magnetic exchange interactions. Finally, molecular orbital analyses reveal a significant magnetic molecular orbital effect, consisting with HTH model. Spin population analyses show that the polarization mechanism occurs in magnetic super-exchange interaction of model 1a, leading a weaker coupling between the two Cr(III) centers.