Comparison of vibrational dynamics, thermal behaviour, and phase transition in [Ni(NH3)4](ReO4)2 and [Ni(NH3)6](ReO4)2

Abstract

The differences between thermal and vibrational properties of [Ni(NH 3 ) 4 ](ReO 4 ) 2 and [Ni(NH 3 ) 6 ] (ReO 4 ) 2 are reported. The differential scanning calorimetry revealed that tetraamminenickel(II) perrhenate exhibits, in the temperature range of 300-140 K, one phase transition at ca. T c h = 188 K (on heating) and T c c = 185 K (on cooling). In the case of hexaamminenickel(II) perrhenate, no phase transition was observed in the same temperature region. Thermogravimetric measurements showed that the decomposition proceeds in the two main stages. In the first stage, a complete deammination takes place and next Re 2 O 7 is released. TG measurements showed that NH 3 molecules are not equivalently bonded to central atom. The final and intermediate products of decomposition were analysed by means of infrared spectroscopy. The final product of thermal decomposition of both compounds is nickel(II) oxide. The analysis of far infrared spectra revealed that anions in [Ni(NH 3 ) 6 ](ReO 4 ) 2 have disturbed tetrahedral symmetry, whereas in [Ni(NH 3 ) 4 ](ReO 4 ) 2 they seem to form polymeric chains. The variation of the activation energies of the deammination and decomposition steps of [Ni(NH 3 ) 6 ](ReO 4 ) 2 was calculated through the model-free isoconversional Kissinger-Akahira-Sunose method and model-free Kissinger method. Infrared spectra were calculated by the DFT method and quite a good agreement with the experimental data was obtained.

Crystal structure, phase transitions and vibrations of H2O molecules in [Ca(H2O)2](ReO4)2

“…The list of the band positions, their relative intensities and tentative assignments are listed in Table 4. The assignments of these vibrations were proposed by comparing their frequencies with the literature data collected for titled compounds and similar ionic aquametal(II) complexes [23,24] and perrhenate complexes [4,24,25]. All characteristic frequencies of H 2 O and ReO 4 -internal modes were identified.…”

“…complex cations. Similar compounds with the same ligand [1][2][3] and anion [4,5] exhibit also an interesting polymorphism.…”

“…However, in the literature one can find that this compound crystallises in noncentrosymmetric monoclinic space group Cc [7] or C2 [8]. The ReO 4 -anions have a mostly regular tetrahedral geometry with an average Re-O bond length of 1.72 Å [7]. The calcium atoms in this structure are surrounded by eight oxygen atoms with an average Ca-O distance of *2.46 Å .…”

“…The calcium atoms in this structure are surrounded by eight oxygen atoms with an average Ca-O distance of *2.46 Å . The Ca 2 (H 2 O) 4 O 10 polyhedral long-chain structure is formed [6][7][8].…”

“…It was thought interesting to see whether a compound of the same type (with H 2 O ligands or ReO 4 -anions) will possess a phase polymorphism similar to that of those compounds just mentioned. The general aim of the present study is to investigate the polymorphism of [Ca(H 2 O) 2 ] (ReO 4 ) 2 in the temperature range of 120-300 K and find the connections of recorded phase transition with eventual changes of the crystal structure and/or of the rate of stochastic reorientational motions of H 2 O ligands.…”