Rhenium pentafluoride, structure

It has been suggested (79) that polymorphism may occur for transition metal pentafluorides as it does for oxide tetrafluorides (91). X-Ray powder photographs of products from the reduction of hexafluorides in anhydrous hydrogen fluoride (79) showed that the samples of rhenium and osmium pentafluorides had different structures from those previously reported, but no unit-cell dimensions could be derived. 

Although a number of complex fluorides of quinquevalent iridium, (e.g. KIrF,) are known, previous attempts to establish the simple fluoride have failed. The absence of a pentafluoride of iridium has become increasingly anomalous as the pentafluorides of the neighbouring elements, rhenium, osmium, and platinum have been prepared. Previous work - indicated that reactions which might have yielded the pentafluoride gave the tetrafluoride instead. The physical properties of this tetrafluoride. (m.p. 106—107 b.p. > 300 ), however, resembled those of a pentafluoride or oxide tetrafluoride. This indicated that iridium tetrafluoride differed structurally from its neighbouring tetrafluorides. 

Technetium oxide tetrafluoride like the pentafluoride is formed as a by-product of TcFf, in the lluorination of technetium metal [59,78,79]. TCOF4 forms two phases that may be separated from TcFs and from one another by vacuum sublimation. One phase of TcOF forms blue needles, has a chain structure, and is isostructural with its rhenium analogue. The other modification is a more volatile green compound of tri-meric structure [78,79]. 

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