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Home , Allotropy, Iron group, Platinum group, Ruthenium

The in the Periodic System A COMPARATIVE STUDY OF THE TRANSITION METALS

By A. R. Powell, F.R.S.

The characteristics of the six platinum metals provide an interesting stndy in their variations with and position in the . This article discusses the inter-relationships of the physical and chemical properties of these elements and of those of their neighbouring transition metals.

It was in 1860 that the platinum metals intcr-relationships, finally announcing (I) his were first recognised as a distinct of view that the platinum metals formed “an elements. Sixteen years earlier Carl Ernst isolated metallic group, inseparable, and Claus, while Professor of at the solidly constituted”, having a number of University of in , had dis- properties in common, that could be arranged covered the sixth member of the group and in two supcrimposed groups of thrce, the named it in honour of his adopted “Nebenreihe” or second.ary series-ruthen- country. During the intervening ium, and -and the Claus published fourteen papers describing “Hauptreihe” or principal series-, the compounds of the six metals and their and platinum.

Mendeliev’s revised Periodic Table, published in 1871

Platinum Metals Rev., 1960, 4, (4), 144-149 144 The PIatinum Metals and their Neighbours in the Periodic Table

Sroup VIA 3oup VIIA Group VIII Group IB

First long period Cr Mn Fe Co Ni cu 24 25 26 27 28 29

Second long period Mo Tc Ru Rh Pd Ag 42 43 44 45 46 47

Third long period W Re 0s Ir Pt Au 74 75 76 77 78 79

Although Dobereiner had described his opposite page that the two platinum triads some thirty years before this, Claus was triads described by Claus, together with the some years ahead of the famous Newland triad , and , were placed in octaves, while the broader generalisations of Group VIII by Mendeliev, but that the Lothar Meyer and Mendeliev were still ten elements , and also appeared years away. in the same group although alternative posi- Curiously, it was also in 1860 that tions were left for them in Group I. Un- Cannizzaro addressed the gathering of over a fortunately, and probably owing to a mis- hundred chemists convened at Karlsruhe by understanding of the true nature of the rare Kekule, Welzien and Wurtz in an attempt to earth elements, MendelCev had now inserted clear up some of the muddle into which an extra series in the table, so leaving a gap chemical formulation had fallen. Cannizzaro, between the two platinum metal triads. This by reaffirming the disregarded views of had the inevitable result of prompting the re- Avogadro, succeeded in bringing order into examination of platinum for these the many conflicting opinions on the relation- apparently missing elements, and a number ship of the atomic and molecular weights, of “discoveries” were claimed, including and in so doing laid the foundations of a Guyard’s Uralium and Kern’s Davyum. common system on which Lothar Meyer and With the development of the electronic Mendeliev were to build. theory and the recognition that the periodic Meyer’s well-known curve of atomic classification of the elements was dependent volumes, published in 1870, clearly showed on atomic number and not on atomic weight the existence of a periodicity in the charac- this anomaly disappeared, and with the allo- teristics of the elements. A year earlier cation of the rare earth elements to a single Mendeliev had published his first periodic space in the table the apparent gap in the table in the course of a paper to the Russian platinum metal sequence vanished. Chemical Society, “On the Correlation of the The portion of the modern periodic table Properties and Atomic Weights of the Ele- in which the platinum metals occur is shown ments”, but further study of the problem above. This of course includes , dis- caused him to revise the table considerably covered by Noddack and his co-workers in and to publish it in its new form in 1871. 1925, and -formerly known as It will be seen in the reproduction on the masurium-predicted by MendelCev as eka-

Platinum Metals Rev., 1960, 4, (4), 145 , and discovered only in 1937 similarity in properties between each of these among the products of nuclear fission. pairs of metals and the metals above them in the first long period, but none the less certain Physical and Mechanical Properties similarities do exist between iron, cobalt and In general the platinum metals are char- nickel and the platinum metals below them. acterised by high melting points and small The structure of the metals in the atomic diameters. As Claus pointed out, the second and third long periods changes, as two triads show certain marked differences; shown in the table below, from b.c.c. in the upper triad comprising ruthenium, in Group VIA to c.p.h. in Groups VIIA rhodium and palladium the atomic weights and VIII(a) and then to f.c.c. in Groups are of the order of IOO and the VIII(b) and (c) and Group I. The metals around 12 g/cc, while in the heavy or lower of the first long period in Groups VIA, VIIA triad, osmium, iridium and platinum, the and VIII differ from those below them in atomic weights are around 190 and the den- the same group in exhibiting allotropy. sities around 22 g/cc. The specific heats of In the graph on page 147 the melting points the elements in the upper triad are about of the metallic elements are plotted against twice those of the elements in the lower triad. the atomic number; it will be seen that they Nevertheless, as is general in the periodic form three fairly smooth curves, the maximum system, the greatest similarity in properties is point of each being occupied by a Group VIA found in the vertical groups; this is well element and the elements of the following illustrated by comparing the physical and groups all being placed on the descending chemical properties of the metals in the side of the peak. second long period with those of the metal When the atomic diameter is plotted immediately below in the third long period; against the atomic number the transition there is a strong resemblance between elements are grouped closely together along ruthenium and osmium, between rhodium the troughs of the curves, indicating strong and iridium, and between palladium and inter-atomic bonding in these metals. In platinum. There is a much less marked both cases the position of manganese is

1 of the Transition Metals t 1 Group VIA Group VIIA Group VIII IB I i Cr Mn* Fe CO Ni cu u b.c.c. U u b.c.c. uc.p.h. ac.p.h. P c.p.h. P y f.c.c. p f.c.c. p f.c.c. f.c.c. Y 6 b.c.c. 6 Tc Ru Rh Pd b.c.c. c.p.h. c.p.h. f.c.c. f.c.c. I Mo 1f.c.c. W Re 0s Ir Pt Au a b.c.c. c.p.h. c.p.h. f.c.c. f.c.c. f.c.c. 1

* Mn and p-W have complicated structures

Platinum Metals Rev., 1960, 4, (4), 146 350C

3000

2500

U -5 2000 t 0z 5 1500 Y

1000

500

0 35 40 45, 50, CS55 60 65 nc 20 25 30 -70 75 1 ATOMIC NUMBERBl,

The melting points of the metallic elements plotted against atomic number anomalous, probably owing to its peculiar than is the case with the platinum metals. lattice structure. The workability of the metals in this por- The greaiest hardness and the highest tion of the periodic table depends on their mechanical strengths are generally exhibited crystal structure, those with a f.c.c. structure by the metals at the peak of the being much more easily worked than those curves, the values of both these properties with a b.c.c. structure while these, in turn, generally decreasing with increasing distance are more easily worked than those with a of the metal from the peak. Thus in the two c.p.h. structure. Ruthenium and osmium platinum metal triads the hardness and are brittle, but can be worked-although only strength decrease from left to right and are with difficulty-at high temperaturcs if pro- greater in the second triad than they are in tected from oxidation; manganese is brittle the first. and unworkable owing to its complex struc- This same periodicity was observed at high ture, and thc b.c.c. metals require in general in a study of the creep strengths to be hot-worked in the initial stages. On in compression of a number of metals at the other hand the metals in the third column IOO~OCcarried out by Allen and Carrington of Group VIII and those in Group IB are (2); these authors reported, among other relatively soft and can be easily worked in the examples of periodicity, a marked fall of cold. strength between rhodium and palladium in An interpretation of these facts based on the second long period, and between iridium the electronic structure of the metals has and platinum in the third, this fall being been attempted by Pauling (3) and by Hume- continued to silver and gold respectively. Rothery and his collaborators (4), who have The metals of the triad differ attributed the observed differences in be- from those in the two triads haviour to hybridisation of the spd orbitals in that their melting points, hardnesses and and to a stabilising effect due to resonance strengths are much closer to one another between the various possible structures.

Platinum Metals Rev., 1960, 4, (4), 147 Chemical Properties solutions crystalline precipitates of the type In Mendelkev's table the iron triad was [Fe(NH,),JX,. Analogous compounds of placed above the two platinum metal triads bivalent ruthenium have recently been pre- to form Group VIII but, in considering the pared by reducing ruthenium compounds chemical properties of the group, greater with zinc dust in ammoniacal similarities in behaviour are observed in the halide solutions (5). There is evidence (6) vertical columns than in the horizontal. All that reduction of rhodium trichloride in the the metals in the first vertical column exhibit same way yields [FUI"(NH~)~]C~,. a principal valency of 3; iron has a secondary Turning now to the metals in the second valency of 2 whereas the main secondary vertical column of Group VIII, the chief com- valency of ruthenium and osmium is 8. Ferric pounds that are common to all three metals are chloride and bromide form anionic complexes the double nitrites R,M(NO,),, the ammines, of the type HFeX, analogous to the corres- e.g. [M(NH,),]X,, [M(NH,),X]X,, the ponding gold compounds and like them RM(SO,),.I~H,O, and the double cyanides extractable into ether. Ruthenium and R,M(CN),. The only analogous purely in- osmium halides on the other hand form organic compounds of the bivalent metals are anionic complexes of the type H,MX, and the ammines [M"(NH,),JX, where M is Co H,MX,, the latter being analogous to the or Rh; although no ammines of bivalent Ir corresponding iridium, palladium and plat- have yet been isolated some evidence has inum compounds and yielding ammonium been obtained of their existence (6). Anal- and potassium salts of relatively low ; ogous compounds of all three metals in the quadrivalent rhenium and technetium also bivalent state with organic ligands have, form this type of compound. however, been prepared within recent years. Iron, ruthenium, and osmium resemble one The metals of the third vertical column another in forming compounds of the type of Group VIII show analogies only in the K,MO, and stable complex cyanides with composition of the and anhydrous the formula R,M(CN),, 3H,O. halides, in which they are bivalent. The An interesting point of resemblance be- chemical behaviour of these compounds tween the two higher members of Groups differs markedly, however; NiCl, is readily VIA, VIIA and VIII(a) is the comparatively soluble in water from which it crystallises as high volatility of the highest . Molyb- the hexahydrate, whereas PdC1, and PtCl, denum trioxide volatilises above about 600°C~ are insoluble in water and dissolve only in above 1200"C, rhenium solutions of other chlorides forming the heptoxide above zso"C, and ruthenium and anions PdCl," and PtC1," respectively. In osmium tetroxides volatilise at around 10o"C; forming anionic complexes palladium the first three oxides are anhydrides and platinum resemble gold. Nickel forms whereas the last two behave neither as ammines of the type [Ni(NH,),]X,, whereas nor as bases. Like manganese, technetium, the bivalent palladium and platinum ammines and rhenium, ruthenium forms salts of the have the formula [M(NH,),]X, and the type KMO, in which it is heptavalent. The chemical behaviour of these tetrammines is compounds K,FeO,, K,RuO,H,O, and quite different from that of the nickel K,OsO,, zH,O have a similar composition hexammines. The two platinum metals form to the chromates, molybdates, tungstates and double nitrites of the type K,M(NO,),, rhenates, but are much less stable, and differ whereas the nickel compound has the formula in their water of crystallisation and crystal K,M(NO,),, but the three metals resemble structure. one another in yielding double cyanides of In the absence of air ferrous salts give with the type R,M(CN),, 2H,O, the nickel com- ammonium halides in strongly ammoniacal pound being, however, immediately decom-

Platinum Metals Rev., 1960, 4, (4), 148 posed by acid, whereas the platinum ruthenium, by all acids or mixtures of acids. metal compounds are stable. Osmium is slowly converted by boiling nitric In the metallic state there are obvious acid or into the volatile tetroxide analogies between palladium and silver, and except when in the massive form. between platinum and gold. The former pair To sum up, it will be seen that in their of metals dissolve readily in nitric acid and physical and chemical properties, ruthenium in boiling concentrated sulphuric acid, and and osmium more closely resemble technet- react superficially with at room tem- ium and rhenium, or in certain respects perature. Platinum and gold resist all acids and tungsten, than they do except aqua regia and are not oxidised on iron. Rhodium and iridium are more closely heating, or affected by iodine. Rhodium dis- allied to cobalt than to any other metal, solves slowly in boiling concentrated sulphuric while palladium and platinum have close acid, in which respect it resembles palladium, analogies both with nickel and with the but otherwise is unaffected, like iridium and precious metals of Group I.

References

I C. E. Claus ...... J. prakt. Chew., 1860, 80,282; Chemical News, 1861, 3, I94 2 N. P. Allen and W. E. Carrington . . J. Inst. Met., 1953-4, 82, 525 3 L. Pading .. .. *. =. Phys. Rev., 1938, (ii), 54, 889 4 W. Hume-Rothery, H. M. Irving and Prac. Roy. Soc., 1951, (A), 208, 431 R. P. J. Williams 5 F. M. Lever and A. R. Powell . , International Conference on Co-ordination Chemistry, London, April 1959; Platinum Metals Rev., 1959, 3, 90 6 A.R.Powel1 ...... Unpublished work

Improved Stainless Steel Reactor Material

RESISTANCE TO INCREASED BY PLATINUM ADDITION

The corrosion resistance in uranyl sulphate 35-36 millyear to 15 millyear by the addition and uranyl nitrate solutions of 304 stainless of 0.5 per cent platinum. No significant steel alloyed with small amounts of platinum improvement was observed at a higher solu- or copper has been investigated as part of tion velocity of 68 ftlsec. In a heavy water the reactor materials research programme of solution containing uranyl nitrate and copper the Oak Ridge National Laboratory. Pre- nitrate, however, the corrosion resistance at liminary results reported by J. C. Griess 250°C of the steel was increased by the et al in three recent Homogeneous Reactor addition of 0.5 per cent platinum at both low Project Progress Reports (ORNL-2743, 2879 and high solution velocities. At a velocity and 2920) show that the resistance to cor- of 17 ft:sec the corrosion rates for the un- rosion of the 18-8 steel is increased by the alloyed and alloyed material were 10 and addition of small amounts of either element. I .6 miljyear respectively. With the same Of the alloys investigated, 304 stainless steel uranyl nitrate solution and a velocity of containing 0.5 per cent platinum had the 70 ftisec an even more significant decrease lowest corrosion rate. from 300 to 14 mil/year is reported. Further Working at 250°C and at a solution velocity corrosion tests on the 0.5 per cent platinum- of 17 ftlsec with a heavy water solution alloyed 304 stainless steel are to be undertaken containing uranyl sulphate and copper sul- in view of its possible applications in homo- phate, it was found that the corrosion rate geneous reactors in which uranyl sulphate for 304 stainless steel was reduced from and nitrate solutions may be used as fuels.

Platinum Metals Rev., 1960, 4, (4), 149-149 149