The bimetallic mixed-ligand complexes [MHg(C6H6ON2)2(SCN)4], where M2+ = Co (I), Ni (II), Cu (III), and C6H6ON2 is nicotinamide (NA), have been synthesized from aqueous solutions and studied by chemical analysis and IR spectroscopy. Molecule NA can be either a bridging or bidentate ligand, the thiocyanate ligand is ambidentate, so the crystal structure of the complexes may be either polymeric or molecular. The structure of complexes I and III has been characterized by X-ray diffraction. Crystals are monoclinic, space group C2/c, Z = 8, and a = 26.6385(5) Å, b = 12.8622(2) Å, c = 19.033(4) Å. β = 133.3779(4)°, V = 4739.9 Å3, ρcalcd = 2.063 g/cm3 for complex I, and a = 26.2828(7) Å, b = 12.7435(4) Å, c = 18.9408(9) Å, β = 132.217(1)º, V = 4698.35 Å3, ρcalcd = 2.094 g/cm3 for complex III. The synthesis of single crystals suitable for X-ray diffraction analysis has been failed for the nickel(II) complex, but this compound has been studied by X-ray powder diffraction. The comparison of experimental and calculated X-ray diffraction patterns enables the conclusion about similar structures of the [CoHg(C6H6N2O)2(SCN)4] and [NiHg(C6H6N2O)2(SCN)4] phases. Crystals of complexes I–III are isostructural. The sulfur atoms of four rhodanide groups form the tetrahedral coordination environment of the mercury atoms. The octahedral coordination environment typical for the cobalt(II) complex becomes tetragonal pyramidal in the case of copper(II). M(NA)2 units and mercury atoms are linked into a 3D-framework by bridging rhodanide groups.