Structural and morphological changes characterizing reversible de-ammination/re-ammination processes in NiPt(CN)4(NH3)2

The topotactic structural mechanism of de- and re-ammination of single-crystalline NiPt(CN)₄(NH₃)₂ is characterized by means of structural, morphological and thermoanalytical studies. Structural investigations give evidence that the two-dimensional structural motif NiPt(CN)₄]_∞ determines the mechanism and the kinetics of both processes. It is shown that the degree of reversibility, in particular the exothermic re-ammination, is governed by the conservation of the two-dimensional structural element NiPt(CN)₄]_∞. Indeed, only one type of bond has to be broken, and reformed, i.e. the two Ni?NH₃ bonds per Ni. Microscopic studies reveal that by starting with single crystals with average dimensions of few tenths of a mm, each cycle of de-and re-ammination leads to a continuous decrease of the size of crystalline domains until an optimum geometry is reached for the given experimental conditions. By semi-quantitative measurements it can be shown that this direction-dependent kinetic course of the overall reaction is controlled by the diffusion of ammonia along the NiPt(CN)₄]_∞ layers. If the macroscopic size of these layer fragments is very small, i.e. after several cycles of the reversible reaction, this diffusion control becomes negligible. The reaction is controlled by the availability of reactive Ni sites and ammonia, i.e. its partial pressure.