Structural Principles in Octahedral Fluorocomplexes

Inquiring the causes of the appearance of certain crystal structures, the structimal elements which invariably show up in different structure types 

The following discussion is restricted to such structural elements already observed in transition metal fluorides, whereas others, examples of which are not known yet, were omitted. Later on influences will be described that may affect crystal structures by means of size, charge, electronic configuration and bonding of the constituent ions of a compound. 

The structural elements in a crystal may be isolated or expanded into 1, 2 or 3 dimensions. Consequently the MeFe-octahedra occur in finite or infinite groups of number z and linked into d =0, 1, 2 or 3 dimensions. These letters z andrf, similar to the writing of MacAa scMt (276), may serve to characterize the structural elements according to % [MeFg]. To allow for stoichiometry the shared fluoride ions are to be covmted half only, resulting in f [MeFa /2Fy] -, with x -i- y = 6 and x/2 +y) being the number of fluoride ions per cation Me in the compound n is the formal charge of the structural unit described. 

These s3Tnbols, that will be used in the following, already imply an answer as to the question of complex or coordination compound. The transition from an isolated group to a 3-dimensional framework is dictated above aU by the stoichiometry of the compounds and is identical with the transition from a typical complex structme to a coordination lattice, in which isolated MeFe-octahedra are no longer detectable. Of comse there are further modifications by influences of ionic sizes and charges to be discussed later. In this context the reader is referred to a paper on this topic by Hoppe (756). 

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