Photomagnetic Switching of Heterometallic Complexes [M(dmf) 4 (H 2 O) 3 (μ‐CN)Fe(CN) 5 ]⋅H 2 O (M=Nd, La, Gd, Y) Analyzed by Single‐Crystal X‐ray Diffraction and Ab Initio Theory

Single‐crystal X‐ray diffraction measurements have been carried out on [Nd(dmf) 4 (H 2 O) 3 (μ‐CN)Fe(CN) 5 ]⋅H 2 O ( 1 ; dmf=dimethylformamide), [Nd(dmf) 4 (H 2 O) 3 (μ‐CN)Co(CN) 5 ]⋅H 2 O ( 2 ), [La(dmf) 4 (H 2 O) 3 (μ‐CN)Fe(CN) 5 ]⋅H 2 O ( 3 ), [Gd(dmf) 4 (H 2 O) 3 (μ‐CN)Fe(CN) 5 ]⋅H 2 O ( 4 ), and [Y(dmf) 4 (H 2 O) 3 (μ‐CN)Fe(CN) 5 ]⋅H 2 O ( 5 ), at 15(2) K with and without UV illumination of the crystals. Significant changes in unit‐cell parameters were observed for all the iron‐containing complexes, whereas 2 showed no response to UV illumination. Photoexcited crystal structures have been determined for 1 , 3 , and 4 based on refinements of two‐conformer models, and excited‐state occupancies of 78.6(1), 84(6), and 86.6(7) % were reached, respectively. Significant bond‐length changes were observed for the Fe–ligand bonds (up to 0.19 Å), the cyano bonds (up to 0.09 Å), and the lanthanide–ligand bonds (up to 0.10 Å). Ab initio theoretical calculations were carried out for the experimental ground‐state geometry of 1 to understand the electronic structure changes upon UV illumination. The calculations suggest that UV illumination gives a charge transfer from the cyano groups on the iron atom to the lanthanide ion moiety, {Nd(dmf) 4 (H 2 O) 3 }, with a distance of approximately 6 Å from the iron atom. The charge transfer is accompanied by a reorganization of the spin state on the {Fe(CN) 6 } complex, and a change in geometry that produces a metastable charge‐transfer state with an increased number of unpaired electrons, thus accounting for the observed photomagnetic effect.