Characterization and crystal structure determination of 3d–4f cyano-bridged bimetallic assemblies, Ln(DMF) 4 (H 2 O) 3 Co(CN) 6 ·H 2 O (DMF=N,N′-dimethylformamide, Ln=Tb, Dy, Ho and Er) have been newly carried out. We have observed their isostructural features and gradual decreasing cell constants and corresponding changes of local bond distances and angles around the cyano-bridges in accord with the ionic radii changes due to lanthanoid contraction. In order to elucidate the reasons that only Nd–Fe and Nd–Co complexes can exhibit photo-induced magnetic functions, we have also investigated the electronic properties for several analogous 3d–4f complexes, Ln(DMF) 4 (H 2 O) 3 M(CN) 6 ·H 2 O (Ln=Ce, Nd, Sm, Gd, Tb, Dy, Ho and Er; M=Co and Fe), by means of X-ray photoelectron spectroscopy (XPS). We found that the characteristic bent features of Nd–N–C bond angles and the slight shift of binding energies of Ln 4d 3/2 peaks in the 3d–4f systems being closed to mixed-valence states might be the reasons for the novel functions.