Synthesis and Crystal Structures of Framework Tetrarhodanomercurate Mn(II), Fe(II), and Cd(II) Complexes with Nicotinamide

New bimetallic complexes of the composition [MHg(C₆H₆N₂O)₂(SCN)₄] have been synthesized, where M = Mn²⁺ (I), Fe²⁺ (II), Cd²⁺ (III) and C₆H₆N₂O is nicotinamide (NA). The compounds were obtained from aqueous solutions and studied by CHNS/O analysis, IR spectroscopy, inductively coupled plasma optical emission spectrometry (ICP-OES), and single-crystal X-ray diffraction. Compounds I–III are isostructural and crystallize in the monoclinic syngony (space group C2/c). The coordination environment of the M²⁺ atom is formed by two donor nitrogen atoms of two monodentately coordinated NA and four nitrogen atoms of the SCN groups, which form bridges between the M²⁺ and Hg²⁺ ions, connecting them into a three-dimensional framework. Hg²⁺ ions have a tetrahedral coordination environment consisting of four S atoms of four SCN groups.