We have investigated the electronic structure of a series of thiocarboxylate-bridged Fe-Pt heterobimetallic lantern complexes using a combined field-dependent Mössbauer spectroscopic and theoretical study. While these complexes exhibit similar isomer shifts typical of high-spin Fe(II) centers, their quadrupole splitting, ΔEQ, show a variation which is much larger than expected. Moreover, simulations of the temperature- and field-dependent Mössbauer spectra yielded positive values for the axial zero-field (ZFS) parameter D, indicating an easy-plane magnetic anisotropy. These results are contradictory to the anticipated negative D values spawned by the spin-orbit coupling acting on a ground orbital quasi-doublet. We hypothesize that the changes in quadrupole splitting are rooted in the spin-orbit coupling of complexes’ orbital ground state with the first low-lying orbital excited state. Interestingly, the energy of the excited state and therefore the strength of the spin orbit coupling, is modulated by the combination of pseudo Jahn-Teller and steric interactions between the apical pyridine ligand and bridging thiocarboxylates. Our theoretical analysis suggests that the positive D value may be traced to the vibronic coupling which raises the energy of the lowest excited orbital state to value comparable to that of the second excited state. The spin-orbit interaction of the ground state with the two lowest excited states leads to two contributions to the ZFS tensor along two spatial directions of comparable magnitude and consequently, to a positive D value. The combined spin-orbit-vibronic model used here may be used to investigate the electronic structures of other complexes with an approximate fourfold symmetry including those of Fe(pinF)2-, where pinF = perfluoropinacolate, which have an unusual square-planar geometry.

Spectroscopic and theoretical investigation of Fe-Pt lantern complexes and other related four-coordinate iron(II) systems

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