The relationship between the structures of Cu(II) complexes and their chemical transformations

Abstract

The thermal dehydration of the compounds M~[MII(H20)6](SeO4)2, where M 1 = NH4, K, Rb, Cs and T1, and M II = Cu and Ni, was studied in order to correlate the course of the decomposition with the known crystal structures. It was found that the stoiehiometry of the reactions is the same as that established for the analogous sulphato compounds of Cu(II) and Ni(II), respectively. Because of the discrepancies between the room-temperature crystal structures and the observed decomposition stoiehiometries, high-temperature powder diffractograms were taken. These indicated structural changes of the copper(II) compounds during heating. The powder patterns for different structure changes were calculated and compared with the experimental ones. It was shown that during the heating two axial Cu-HzO bonds are shortened and two equatorial bonds are lengthened. The observed decomposition stoichiometry is compatible with the formation of four nearly equal Cu-H20 bonds. The activation energies (E*) and pre-exponential factors (log A) for the first dehydration reaction of the Cu(II) compounds display the following sequence of MI: TI > Rb > NH4 > K, and they are the higher, the shorter the split equatorial Cu(II) bonds. For the compounds of Ni(II) the sequence of E* and log A values is K > TI > NH4 > Rb > Cs. It was shown previously [1, 2] that the symmetry of the Cu(II) coordination polyhedron is one of the important factors influencing the stoichiometryand the kinetics of the thermal decomposition of coordination compounds with Cu(II) as central atom. The fact that compounds with rhombically deformed coordination polyhedra, such as M~[Cu(H20)6](SO4)2, exhibit decomposition stoichiometry which would correspond better to tetragonal John Wiley & Sons, Limited, Chichester Akaddmiai lO'ad6, Budapest

Temperature induced structural changes of Cu(II) and Ni(II) complexes and their thermal decomposition

“…We conclude therefore that during the decomposition nearly all chemical bonds are newly formed. This mean that the heating leads to the activation of all present chemical bonds and not entirely those, which are split-off in the decomposition reaction [10].…”