Contribution of ¹⁵⁵Gd Mössbauer data to the study of the magnetic interaction in heterodinuclear 3d-Gd (3d = Cu, Ni)...

The valence shell of the gadolinium element corresponds to 4f⁷ 5d¹ 6s², so that the trivalent Gd^III ion possesses free 5d and 6s orbitals. It has been previously shown by CASSCF methods that the 5d orbitals, along with the 6s Gd orbitals, which are expected to be unoccupied, present a slight spin density and that the magnetic behaviour of Cu^II-Gd^III complexes can only be reproduced if the 5d Gd orbitals are taken into account in the active space. ¹⁵⁵Gd Mössbauer isomer shifts of 3d-Gd complexes, L¹CuGd(NO₃)₃, L¹NiGd(NO₃)₃·acetone, L²Cu(acetone)Gd(NO₃)₃, L²Ni(H₂O)₂Gd(NO₃)₃ where L¹ and L² are hexadentate Schiff base ligand, are almost unchanged (0.62-0.64 mm s^-1 relative to ¹⁵⁵Eu/SmPd₃ source) though the values are slightly smaller than a typical ionic compound GdF₃ (0.67 mm s^-1). The very similar isomer shift values of the 3d-Gd complexes indicate that there is no change in the small electron density of the 6s orbital and that the spin delocalization or spin polarization concerns the 5d Gd orbitals, in agreement with the crystal structure and Mössbauer spectrum of the Gd complex of nitrogen-coordinating tridentate ligand PrⁿTBP, [Gd(PrⁿTBP)₃](OTf)₃. Thus the observed ¹⁵⁵Gd Mössbauer isomer shifts of 3d-Gd complexes give an experimental proof for the participation of 5d Gd orbitals to the magnetic interaction in these 3d-Gd complexes.