Packing principles in the structures of metal ammine salts

From simple considerations of the different ways in which ammonia may be introduced into the structures of metal ammine salts it is evident that while the ammonia molecules tend to be coordinated to the metal ions they otherwise behave as rather inactive groups towards their remaining environment. Accordingly a factor of major importance for the formation of these structures is the form of the most favourable packing of the large groups, namely the ammonia molecules and negative ions. This will be particularly true for the halides since the interaction between ammonia and halide ions appears to be quite small in general. Some metal ammine salt structures studied earlier, mainly halides, are briefly reviewed with the object of investigating to what extent such simple packing is realized in practice. It is shown that in the majority of such compounds the arrangement of the ammonia molecules and negative ions may be described approximately as cubic or hexagonal close-packed or as body-centred cubic, with the metal ions in tetrahedral or octahedral interstices.