Iridium tetrafluoride

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. 

Sir Humphry Davy attempted to isolate this unidentified element through electrolysis—but failed. It was not until 1824 that Jons Jakob Berzehus (1779—1848), who had earlier discovered cerium, osmium, and iridium, became the first person to separate the element silicon from its compound molecule and then identify it as a new element. Berzehus did this by a two-step process that basically involved heating potassium metal chips with a form of silica (SiF = silicon tetrafluoride) and then separating the resulting mixture of potassium fluoride and silica (SiF + 4K —> 4KF + Si). Today, commercial production of sihcon features a chemical reaction (reduction) between sand (SiO ) and carbon at temperatures over 2,200°C (SiO + 2C + heat— 2CO + Si). 

The 1 1 chlorine trifluoride-platinum pentafluoride adduct, m. p. I7l , and the 1 1 iodine pentafluoride adduct, m. p. 140° resemble, in their physical properties, the selenium tetrafluoride adducts of osmium pentafluoride (SeF4,OsFg, m. p. 158°) and iridium pentafluoride (SeF. IrFg, m. p. 133°). They are probably bifluorine-bridged monomers ... 

We have now established the pentafluoride of iridium and believe that the previously reported tetrafluoride was the pentafluoride. 

The establishment of pentafluorides of rhodium and iridium [5] as well as the trifluorides raised the possibility of each element also making a tetrafluoride[17]. On the other hand the mixed oxidation state character [13] of PdFj (Pd(II)Pd(IV)F6) raised the possibility of the tetrafluorides RhF4 and IrF4 being M(III)M(V)F8 compounds. At the same time the existence of Pd(IV) in the compound Pd(II)Pd(IV)F6 indicated the probable existence of PdF4. 

The tetrafluorides of iridium, rhodium and palladium [13] have been established by the work reported in this paper and together with Ptp4 constitute a structurally related set. 

The dark-brown product disproportionates above 400° into Irp3 and the volatile IrPs. The structure features (IrPe) octahedra which share 4 P atoms, each with one other IrPg group, leaving a pair of cis vertices unshared this is essentially a rutile type structure (p. 961) from which alternate metal atoms have been removed from each edge-sharing chain. It was the first 3D structure to have been found for a tetrafluoride. Claims have been made for the isolation of all the other iridium tetrahalides, but there is some doubt as to whether these can be substantiated. This is an unexpected situation since -1-4 is one of iridium s common oxidation states and, indeed, the derived anions [IrXe] - (X = P, Cl, Br) are well known. 

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