Platinum Metals Review

PLATINUM METALS REVIEW A quurterly survey of reseurch on the platinum metuls urrd of dwelopments in their applications in industry VOL. 16 OCTOBER 1972 NO. 4 Contents The Fluorine Compounds of the Platinum Metals Precision Fabrication in Platinum The Production of Ultra-pure Hydrogen Cobalt-Platinum Permanent Magnets Fifth International Congress on Catalysis The Work of James Lewis Howe Abstracts New Patents Index to Volume 16 Communications should be addressed to The Editor, Platinum Metals Review Johnson, Matthey & Co Limited, Hatton Garden, London EClP 1AE The Fluorine Compounds of the Platinum Metals THE NATURE AND IMPORTANCE OF RECENT WORK By John H. Holloway Chemistry Department, The University of Leicester This review summarises some of the chemistry of the platinum metal fluorides and outlines the part they have played in developments in inorganic chemistry. A number of new compounds in thisjeld have been identijed only recently for the jrst time. The last fifteen years has seen a revision historically significant in inorganic fluorine and considerable extension of the fluorine chemistry in the last sixty years. In order to chemistry of the platinum metals. In the form this type of compound an electron must earlier part of the period the new hexa- be removed from oxygen and transferred to fluorides PtF, (I), RuF, (2) and RhF, (3) RF,. The surprising thing about the were reported and the fluoride previously reaction is that the first ionisation potential thought to be OsF, was shown to be OsF, for oxygen is quite high. Until 02+PtF6-was (4). More recently OsF, itself has been made no compound containing 02+had ever prepared and characterised and the possible been reported. The discoverer of this unique existence of OsF, has been suggested (5). compound, Neil Bartlett, realised that the Unusual polymeric smctures have been fist ionisation potential of xenon is almost found in the crystalline pentduorides of identical with that of molecular oxygen: ruthenium (6), osmium (7), rhodium (8), iridium and platinum all of which I 177kJmol-l (g), (10), Op -Oa+ :-e have been either discovered or properly I 16glcJmol-l characterised for the first time since 1960. Xe *Xe++- e The first oxide fluorides have been discovered, but so far only RuOF, (11), Os09F, This led him to wonder if the reaction of (12), OsOF, (13), OsOF, (14), and PtOF, platinum hexafluoride with xenon would (IS) have been reported and some of these produce a xenon compound analogous to the are still not well characterised. oxygen compound. A simple experiment, In 1962 a compound of formula ROBF, the mixing of xenon with PtF, vapour, (16) was found among the products of the dmedthat it did (18). This first com- reaction of platinum and platinum salts with pound of xenon, Xe+PtF,-, was the pre- fluorine in glass and silica apparatus. Sub- cursor to the discovery of the whole new sequently it was shown that this same sub- chemistry of the noble gases (19). Now stance could be synthesised by oxidising compounds of krypton, xenon and radon are molecular oxygen with an equimolar quantity known (19) and there are hints (20) that of PtF, vapour (16, 17) and that the correct argon compounds may be discovered soon. formulation is Oa+PtF,- (17). This dis- Perhaps the most interesting recent aspect covery is without doubt one of the most of platinum metal fluoride chemistry has Platinum Metals Rev., 1972, 16, (4), 118-122 118 Fig. 1 The hexajuorides of the platinurn mads are highly volatile and er- tremely corrosive. Con- sequently their discovery awuited the deuelopment of special Mend and nickel uacuum and high-pressure systems for handling them under anhydrous condi- tions and such apparatus is shown here been the discovery of a new class of com- The variety of types of oxide fluoride and pounds, the transition metal carbonyl the fact that oxide fluorides are known only fluorides. Carbonyl fluorides of molyb- for three metals are indications that this is denum (21) and ruthenium (22) have been not the case. In fact, the chemistry of the claimed but only the ruthenium compound, platinum metals is quite diverse. This is difluorotricarbonylruthenium(I1) (22) has well illustrated by the fluorides, and trends been fully characterised and this has been are best determined by considering the shown to have a closely related structure to elements as members of the second and third those of the platinum metal pentafluorides transition series (See Table I). As is usual (see later). with transition elements each exhibits a variety of oxidation states. Through the Binary Fluorides series up to the first members of Group VIII Because the six platinum metals are usually the highest attainable oxidation state is referred to as a single “group”, it is often equal to the group number. Ruthenium and assumed that they are chemically similar. osmium both exhibit octavalence in their Platinum Metals Rev., 1972, 16, (4) 119 Table I The Platinum Metals as Members of the Second and Third Transition Series Group IIIA IVA VA VIA VIIA VIII IB Element Y Zr Nb Mo Tc Ru Rh Pd Ag Electronic Configuration Element La Hf Ta W Re 0s Ir Pt Au Electronic 5d16s2 5d26s2 5d36s3 5d46s2 5d56sz gd66s2 5d76sZ 5d96s1 5d106s1 Configuration tetroxides and some hint of the existence of In 1966, Glemser and his co-workers (5) OsF, has also been obtained (5). Beyond these succeeded in preparing OsF, by heating first members of Group VIII, however, the osmium plus fluorine mixtures at high oxidation state maximum diminishes dramati- temperatures (500 to 600°C) and pressures cally from Ru to Ag and from 0s to Au in (350 to 400 atm.). As expected the pale spite of the availability of the necessary yellow compound loses fluorine at room numbers of valence electrons. There is no temperature to give OsF,. Osmium hepta- firm evidence for Rh(1X) and Ir(1X) compounds or for decavalent palladium or Table II platinum. Indeed, even hepta- and octa- The Known Fluorides and Oxide Fluorides valent states for the elements beyond of the Platinum Metals and the Highest ruthenium and osmium have not been Oxidation-state Oxides attained. I In Table I1 the known fluorides and Ru RuF, Rh RhF, oxide fluorides of the platinum metals are RuF, RhF5 listed along with the highest oxidation-state RuF, RhF, PdF, oxides. A number of interesting features RuF, RhF, PdF, are at once apparent. For example, although PdF, RhF, is known Rho, is not; the highest RuOF, known fluoride of ruthenium is RuF, in RuO, Rho, Pd,O, spite of the fact that RuO, exists. Some explanations for observations such as these 0s OsF, Ir Pt have been derived from thermodynamic OsF, IrF, PtF, considerations (23). OsF, IrF, PtF, Octafluorides and Heptafluorides OsF, IrF, PtF, In spite of the demise of so-called “OsFs” IrF, in 1958 (4) there has always been hope that OsO,F, PtOF, a higher osmium fluoride than OsF, might OsOF, be formed since, unlike RuF,, OsF, is OsOF, stable and the preceding element in the oso, IrO, PtO, third transition series forms a heptafluoride, Platinum Metals Rev., 1972, 16, (4) J 20 fluoride was shown to have a pentagonal bipyramidal (D5h symmetry) structure. n During these experiments mass spectro- metric, magnetic susceptibility, e.s.r. and i.r. evidence was accumulated which suggested the possible existence of some OsF, in the reaction product. However no con- clusive evidence for OsF, has yet been obtained. Hexafluorides The discovery of the hexafluorides RuF, 0-F~ (2)~OsF6 (4)~~FG (3h and PtF6 (1) was I the work of one research group under the Fig. 2a (above) leadership of Bernard Weinstock at Argonne Fig. 26 (below) National Laboratory in the U.S.A. Their success came largely through their develop- ment of special Monel and nickel vacuum and high-pressure systems for the handling of these highly volatile and corrosive materials. (Similar apparatus is shown in Fig. I.) With the exception of palladium then, a hexafluoride is known for each of the platinum metals. The physical properties of some of these molecules have been carefully studied but very little is known of their chemistry (24, 25). Although there are no good heat of formation data available, fundamental vibra- tion frequencies for the algmode in the infra- red and Raman spectra (26, 23) indicate a decrease in bond strength from left to right in Fig. 2 (a) Tetrameric crystalline pentajuorides of each series and this is confirmed by the the platinum metals with metal atoms M at the corners of a rhombus and with non-linear bridging increasing tendency for the hexafluoride to jhorine atoms between the metals. F=$uorine atom dissociate to lower fluoride and fluorine with (b) The tetramer from the side showing clearly increasing atomic number. This decrease in the close packed layers of fluorine atoms stability has been explained in terms of there being an increase in the electron withdrawing The structures are based on a hexagonal power of the central atom across each series close-packed arrangement of fluorine atoms, (23). and the tetramer seen from the side clearly shows the close packed layers of fluorine Pentafluorides and Lower Fluorides atoms (see Fig. zb). Unusual tetrameric structures were dis- Other recent interesting aspects of platinum covered in the crystalline pentafluorides of metal pentafluoride chemistry include adduct Ru (6), 0s (71, Rh (81, Ir (9), and Pt (10) formation of RuF, and OsF, with XeF,. The (see Fig. za) in the early 1960s. In these MFj.XeF,, MF,.(XeF,), (M=Ru or 0s) tetramers the metal atoms lie at the corners and (RuF,),.XeF, adducts are known and of a rhombus and there are non-linear may be prepared in BrF, solution (28) or, bridging fluorine atoms between the metals.

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