Selecting metal ammine storage materials

1 Coordination complexes a well-known class of materials 

The stable vapour pressure of ammonia over different metal salts at a 

Most metal ammines desorb ammonia in a number of steps with different thermodynamic properties, where equatorial ligands tend to desorb before the apical (see Fig. 19.5). This means that the temperature necessary to achieve the desired ammonia pressure over the initial metal ammine is not sufficient to maintain this pressure when an ammine with a lower coordination number remains after partial desorption of the coordinated ammonia. In the ideal case, all ammonia is released in a one-step process at a single temperature as is the case for Ba(NH3)gCl2 (Biltz, 1923) alternatively, the smallest possible 

6 Van t Hoff plots of the equilibrium ammonia pressure versus temperature for the 90 metal ammines listed in Table 19.1. The bold lines represent the different stages of Mg(NH3)eCl2 decomposition. 

The final and, in many cases probably, decisive parameters in determining which metal ammine is optimal for an application will be the price and availability of the metal salt, i.e. NaCl, MgCl2 or CaCl2 are cheap and abundant, while Bel2 or CoBr2 would be much harder to come by (http // www.sigmaaldrich.com). 

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