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The studied compound belongs to the family of [MPy₄X₂] ⋅ 2Py isomorphous clathrates. Its crystal structure exhibits a van der Waals architecture formed by neutral [HgPy₄(NO₃)₂] host molecules, with the guest pyridine molecules included in the cavities of the host lattice. The host complex is formed by coordination of four pyridines, located near the equatorial plane, and two nitrates, located axially, to the Hg(II) cation. One of nitrates ligates as a monodentate ligand and another as a bidentate. The coordination polyhedron is 'HgN₄O₃', with average Hg-NPy and Hg-Onitrate distances of 2.38(5) and 2.68(1) Å, respectively. The crystal structure is complicated with a superlattice and the crystal symmetry reduced to monoclinic, as compared to the structure usually occurring in the [MPy₄X₂] ⋅ 2Py clathrates. The pyridine vapor pressure over the clathrate was measured in the 293-369 K temperature range by the static tensimetric method. Thermodynamic parameters of the clathrate dissociation were calculated from these data. For the reaction 1/3[HgPy₄(NO₃)₂] ⋅ 2Pysolid=1/3[HgPy₃(NO₃)₂] solid + Pygas the parameters are as follows: ΔHºav = 49.4(2) kJ/mol, ΔSºav = 127(2) J/(mol K) and ΔGº₂₉₈=11.4(3) kJ/mol. The results are compared with previously reported data on compounds of the [MPy₄(NO₃)₂] ⋅ 2Py series.

The [CuPy₄(NO₃)₂]*2Py (Py=pyridine) clathrate showed a phase transformation at 46 °C with an enthalpy change of 3.27(5) kJ/mol. The unit cell parameters were measured in the -173 to +77 °C temperature range by single-crystal XRD, showing an expansion of two unit cell parameters and a contraction of the third, with discontinuous changes at the transition point and nonlinearity over a wide temperature range. Both polymorphs are orthorhombic; above the transition point the primitive unit cell (Pnna) transforms into a C-centered cell (Ccca). The precise transition point was determined from the temperature dependence of the intensity of the characteristic reflections which became systematic absences for the high-temperature polymorph. The crystal structure of the high-temperature polymorph was determined at +52 °C and compared with available data for the low-temperature polymorph. In both forms, the host molecule has a copper atom in a distorted octahedral environment, coordinated by four pyridine nitrogens in the equatorial plane and two nitrate oxygens above and below the equatorial plane. Guest pyridine molecules are located in channels. The changes associated with the polymorphous transformation are related to the dynamics of the nitrate group in the structure, and they take place over the entire temperature range. The nitrate ligand plane is inclined to the axis of the complex and shows significant motion. Above the transition point the nitrate ligand acquires the ability to flip from one side of the axis of the complex to the other. The results of this study make it possible for previous data to be rationalized, and provide evidence of a generic relation between two structural types of the [MPy₄X₂]*2(guest) family of clathrates.