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]inf inity determines the mechanism and the kinetics of both processes. It is shown that the degree of reversibility, in particular the exothermi c re-ammination, is governed by the conservation of the two-dimensiona l structural element [NiPt(CN)4]infinity. Indeed, only one type of bon d has to be broken, and reformed, i.e. the two Ni-NH3 bonds per Ni. Mi croscopic studies reveal that by starting with single crystals with av erage dimensions of few tenths of a mm, each cycle of de- and re-ammin ation leads to a continuous decrease of the size of crystalline domain s until an optimum geometry is reached for the given experimental cond itions. By semi-quantitative measurements it can be shown that this di rection-dependent kinetic course of the overall reaction is controlled by the diffusion of ammonia along the [NiPt(CN)4]infinity layers. If the macroscopic size of these layer fragments is very small, i.e. afte r several cycles of the reversible reaction, this diffusion control be comes negligible. The reaction is controlled by the availability of re active Ni sites and ammonia, i.e. its partial pressure.