Metal node design is crucial for obtaining structurally diverse coordination polymers (CPs) and metal–organic frameworks with desirable properties; however, FeII ions are exclusively six-coordinated. Herein, we present a cyanide-bridged three-dimensional (3D) CP, FePd(CN)4, bearing four-coordinate FeII ions, which is synthesized by thermal treatment of a two-dimensional (2D) six-coordinate FeII CP, Fe(H2O)2Pd(CN)4⋅4 H2O, to remove water molecules. Atomic-resolution transmission electron microscopy and powder X-ray and neutron diffraction measurements revealed that the FePd(CN)4 structure is composed of a two-fold interpenetrated PtS topology network, where the FeII center demonstrates an intermediate geometry between tetrahedral and square-planar coordination. This four-coordinate FeII center with the distorted geometry can act as a thermo-responsive flexible node in the PtS network.
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