Physical and Chemical Properties of Platinum Group Metals 2 Chapter 2 | Physical and Chemical Properties of Platinum Group Metals Contents

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PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS 2 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS CONTENTS

2.1 OVERVIEW OF PGMS 04

2.2 METALLIC PGMS 05

2.3 COMPOUNDS OF PLATINUM GROUP METALS 06 SIMPLE COMPOUNDS 06 COMPLEX COMPOUNDS 07

REFERENCES 13

2 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS SUMMARY

• Six elements of Groups 8, 9, and 10 in the periodic table constitute the platinum group metals (PGMs): platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os).

• The physical and mechanical properties of the PGMs and their compounds indicate a wide range of properties with widely varying densities and solubilities (see Table 2-1).

• Metallic forms of PGMs are generally considered to be ‘inert’, i.e., not chemically reactive. However, this is dependent in part on dimensional characteristics. Thus, while massive metal forms have low chemical reactivity, ﬁ nely-divided metal powders with high surface area show greater reactivity.

• Simple binary compounds exist for each of the PGMs. They also form a vast array of complex coordination compounds in which the central metal atom is bound to a variety of ligands by coordinate bonding, including halides, sulphur, amines, and other atoms and groups.

• This unique coordination chemistry has made PGM compounds of great industrial value, but also can have implications for the health of workers exposed to certain of these compounds due to the linkages with biological behaviour and toxicity (see Chapter 6).

• The complex halogenated platinum compounds (CHPS) are among those which are industrially and toxicologically important.

• Other commercially signiﬁ cant forms of PGMs include, but are not limited to: metallic PGMs; ammine complexes of platinum and palladium; chloro-compounds other than those in the CHPS series; rhodium salts; and the PGM nitrates.

3 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS 2.1 OVERVIEW OF PGMS

OVERVIEW OF PGMS

The six platinum group metals ‘noble metals’. In common with ruthenium), reds (rhodium) and, (PGMs) are transition metals the other transition elements, less commonly, purples and blues occurring in the d-block (groups their chemistry is characterised (ruthenium and iridium). They 8, 9, 10, periods 5 and 6) of the by multiple oxidation states form a vast range of coordination periodic table (Figure 2-1). and a wide range of coloured compounds with some of the compounds covering the entire most complicated and diverse These metals, along with silver spectrum from, commonly, almost chemistry in the periodic table. and gold, share a generally high colourless (osmium), through resistance to chemical attack and, yellows (platinum) to rich red- as a result, are also called the browns (palladium, iridium,

Manganese Iron Cobalt Nickel Copper 25 26 27 28 29 Mn Fe Co Ni Cu

Technetium Ruthenium Rhodium Palladium Silver 43 44 45 46 47 Tc Ru Rh Pd Ag

Rhenium Osmium Iridium Platinum Gold 75 76 77 78 79 Re Os Ir Pt Au

Bohrium Hassium Meitnerium Darmstadtium Roentgenium 107 108 109 110 111 Bh Hs Mt Ds Rg

Figure 2-1: Platinum Group Metals in the Periodic Table

4 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS 2.2 METALLIC PGMS

METALLIC PGMS

Metallic forms of PGMs are degrees by mineral acids such as generally considered to be ‘inert’, nitric, sulphuric or aqua regia but this is dependent in part on (a mixture of hydrochloric and dimensional characteristics. nitric acids), particularly when fi nely divided. On a laboratory Reactions of PGMs with most scale iridium, ruthenium, and substances, for example osmium (sometimes called oxygen, halogens, and acids, the ‘insoluble PGMs’), usually are infl uenced—both in rate and require fusion with an oxidising extent—by the particle size of alkali, such as sodium peroxide, the metal. Thus some reactions followed by acidifi cation with that apparently do not take place hydrochloric acid to reliably with massive metal can occur solubilise them, particularly after with fi nely-divided, high surface high-temperature treatment. area metal powders. The metals in colloidal form (as ‘blacks’) Metallic PGMs provide perhaps can exhibit striking catalytic the most commercially important capabilities in some reactions. class of catalysts known and include automotive exhaust PGMs are all susceptible to attack catalysts, petroleum platforming by elemental halogens, especially catalysts, ammonia oxidation chlorine and fl uorine. Palladium, catalysts, and fuel cells. See platinum, and to a lesser extent, Chapter 3 for further information rhodium are attacked to varying regarding applications of PGMs.

5 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS 2.3 COMPOUNDS OF PLATINUM GROUP METALS

COMPOUNDS OF PLATINUM GROUP METALS

For more comprehensive ruthenium(VIII) and osmium(VIII) reference sources on the oxides are frequently used characteristics, chemistry, and to recover these metals by physic-chemical properties of SIMPLE distillation. Although PGMs form PGMs and their compounds the binary anhydrous chlorides, these reader is referred to various COMPOUNDS are not commercially signiﬁ cant. reference sources (O’Neil, They are often substantially 2001; Sehrt and Grehl, 2012). insoluble in most solvents, and Supplier Company catalogues polymeric in structure. and websites also contain useful In the environment, platinum information on these aspects group metals occur separately, including those of Johnson or alloyed together, or alloyed Matthey, Umicore N.V., Heraeus with other metals (e.g., as GmbH and BASF. The biological osmiridium and ferroplatinum). properties and toxicology of There are also some distinct PGMs are separately addressed in compound mineral species which Chapter 6 of this Guide. illustrate the binary compounds: these include sperrylite (PtAs); PGMs form a number of cooperite (Pt,Pd)S; a series apparently simple binary of platinum and palladium compounds which are invariably chalcocides, such as sulfarsenides, structurally complicated. michenerite (PdBiTe) and They also form a vast array of moncheite (Pt,Pd)(Te,Bi); and coordination compounds in laurite, (Ru,Os)S; all of which which the central metal atom is occur naturally in metal sulphide bound to a number of ligands by deposits, such as in Sudbury, coordinate bonding, including Ontario in Canada and in the halides, sulphur, amines, and Merensky Reef in South Africa. other atoms/groups. This unique coordination chemistry has Binary compounds exist for made PGM compounds of great each metal. The most important industrial value, but also can compounds are those with have implications for the health oxygen or chlorine. Platinum(IV) of workers exposed to certain of oxide is a commercially used these compounds due to linkages catalyst. Iridium, rhodium, and with biological behaviour and palladium(IV) hydrated oxides toxicity (see Chapter 6). can be used to quantitatively hydrolyse these metals during reﬁ ning, and the volatile

6 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS 2.3 COMPOUNDS OF PLATINUM GROUP METALS

As a generalisation, in the early Chloride solutions are the 1960s Pearson developed a predominant media for the set of ‘rules’ for ‘hard/soft’ commercial separation and COMPLEX Lewis acids (Pearson, 1963). reﬁ ning of PGMs. In this The platinum group metals are application, PGMs form sequences COMPOUNDS categorised as ‘soft’ metals that of aquated species with the prefer to coordinate to ‘soft’ relatively weakly bound chlorides

ligands (e.g., cyanide, ammonia, such as the hexagonal [M(Cl)n 4-n amines, sulphur ligands and (H2O)6-n)] , where M is Pt(IV) or In common with all transition halides, etc.). They do not easily Ir(IV), or the square planar [M1 (Cln metals, the vast majority of the 4-n (2-n) form complexes with ‘hard’ (H2O) ] , where M1 is Pt(II) or commercial chemistry of PGMs ligands, such as oxo acids (e.g., Pd(II). The degree of aquation is concerned with their extensive nitrate, sulphur, phosphate, increases with decreasing chloride range of coordination complexes. etc.). The practical use of concentration. The ammonium In these, the central metal atom is complex compounds of PGMs in and potassium salts of some of bound to a number of ligands, or homogeneous and heterogeneous the hexachloro complexes are complexing agents. These ligands catalysis has generated further of relatively low water solubility are atoms or molecules which rapid development of their and those of Pt(IV), Pd(IV), and frequently act as electron donors. chemistry. This is discussed Ir(IV) are sometimes used for The biological signifi cance of below, and also in Chapter 6 with precipitation of those metals ligand substitution is discussed in specifi c reference to how the during refi ning and purifi cation more detail in Chapter 6. chemistry of these compounds processes.

relates to their toxicity.

2- CI Cl NH CI CI 3 Pt Pt CI NH CI CI 3 CI

Hexagonal Square planar cis diamminodichloroplatinum(II) hexachloroplatinate(IV)

Figure 2-2: Example structures of Pt coordination complexes

7 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS 2.3 COMPOUNDS OF PLATINUM GROUP METALS

Another commercially complexes with di erent including their toxicity (see important class of compounds ligands (e.g., cisplatin or cis- Chapter 6). are the amine and ammine diamminodichloroplatinum(II), complexes of platinum(II), carboplatin, and oxaliplatin) are Increasingly important classes of palladium(II) and platinum(IV). used in cancer chemotherapy compounds are the so-called PGM The ammine complexes of (so-called “platins”). As a ‘soft’ ‘nitrates’ which can be used for the latter fi nd application in metal, platinum also has a great catalyst impregnation. These are electroplating and automotive a nity for sulphur, such as with generally formed by dissolution of exhaust catalyst production. dimethyl sulfoxide (DMSO); the metal or an oxide/hydroxide Diamminedichloropalladium(II) numerous DMSO complexes compound in nitric acid. As with is substantially insoluble in acid have been reported (see Figure most aqueous PGM chemistry, chloride media and may be 2-3). This ability to form strong, these are not simple binary used to precipitate palladium stable complexes with amine compounds and involve a range and, because it can be re- -N and sulphur -S centres, of complexes, some of which may dissolved in excess ammonia which are present in biological include bridged polymeric species and re-precipitated, to purify the macromolecules, undoubtedly (see Figure 2-4). metal in some refi nery circuits. infl uences the biological activity Certain platinum(II) coordination of certain platinum compounds,

(H) o Me (H) o o (H)

Me2(o)S Pt S(O)Me2 Pt Pt

o o (H) CI (H)

(H)o n Pt

Sulphur-bonded Proposed oligomer of the Pt-bridged structure in ‘Pt(II) nitrate’ trans chloromethylbis(dimethylsulphoxide)Platinum (II) [Danan Dou et al., (2001) Structure and chemical properties of Pt nitrate and application in three-way automotive emission catalysts. Applied Catalysis B Environmental 30: 11-24.]

Figure 2-3: Example of a Pt-DMSO complex Figure 2-4: Postulated structure for a form of Pt nitrate

8 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS

TABLE 2-1: PHYSICAL PROPERTIES OF PGMs AND 2.3 SELECTED COMPOUNDS

The physical and mechanical properties of the platinum group metals indicate a wide range of properties with widely varying densities and solubilities. The following tables list some of the more important platinum group metal compounds found in the workplace, along with their melting points, solubility in water, and their chemical formulae.

Molecular Melting Point Density Industry Name Formula Solubility in Water Weight (°C) (kg/m3)

Platinum Substances

a b f Platinum Pt 195.09 1768.2 21.45 0.08 µ/l

Platinum(II) oxide PtO 211.08 325 c 14.1 Insoluble

f Platinum(IV) oxide PtO2 227.08 >450 11.8 0.11mg/l

c Platinum(IV) sulphide PtS2 259.21 225-250 7.85 Insoluble

c Platinum(II) chloride PtCl2 265.99 581 6.0 Insoluble

c Platinum(IV) chloride PtCl4 336.89 327 4.30 Slightly soluble

Mixed Pt(IV) 'Platinum(II) nitrate' - - - Soluble complexes

Mixed complexes in Platinum(IV) sulphate tetrahydrate - - - Soluble solution

c Ammonium tetrachloroplatinate(II) (NH4)2 [PtCl4] 372.97 - 2.94 Soluble

c Potassium tetrachloroplatinate(II) K2 [PtCl4] 415.09 500 3.38 191g/l at 20°C

d f d d Hexachloroplatinic(IV) acid H2[PtCl6] 409.81 60 2.43 1400g/l at 18°C

c Ammonium hexachloroplatinate(IV) (NH4)2 [PtCl6] 443.87 380 3.07 5g/l at 20°C

c Potassium hexachloroplatinate(IV) K2 [PtCl6] 485.99 250 3.5 7.7g/l at 20°C

c d Sodium hexachloroplatinate(IV) Na2 [PtCl6] 453.77 250 3.5 Very soluble

c Cis diamminedichloro-platinum(II) [Pt(NH3)2Cl2] 300.05 270 - 2.5g/l at RT

c Tetraammineplatinum(II) dichloride [Pt(NH3)4]Cl2 333.98 80 2.7 240g/kg

a The boiling point of platinum metal is 3825°C. e May dissolve in own water of crystallization.. b At 20°C. f By transformation/dissolution kinetics tests at 100mg/l after 7 days. c Decomposes. d Hexahydrate. RT = Room Temperature. - = No data.

9 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS

TABLE 2-1: PHYSICAL PROPERTIES OF PGMS AND 2.3 SELECTED COMPOUNDS

Molecular Melting Point Density Industry Name Formula Solubility in Water Weight (°C) (kg/m3)

Palladium Substances

f Palladium Pd 106.4 1554.8 12.0 <0.01 µ/l

Palladium(II) oxide PdO 122.40 750 c 8.7 <0.01mg/l

c Palladium(II) dichloride PdCl2 177.33 679 4.0 4g/l at RT

Palladium(II) dibromide PdBr2 266.22 500 - Insoluble

c Palladium(II) diiodide Pdl2 360.21 350 6 Insoluble

c Palladium diacetate Pd(C2H3O2)2 225 200 - Insoluble

Palladium(II) acetate Trimer {Pd(C2H3O2)2}3 673.53 - - 0.35g/l at 20°C

Mixed complexes in Palladium(II) sulphate - 125 c - 280g/l solution

Mixed Pd(II) 'Palladium(II) nitrate' Decomposes - >10g/l complexes

c Diammonium hexachloropalladate(IV) (NH4)2[PdCl6] 355.22 240 - >10g/l at RT

Potassium hexachloropalladate(IV) K2[PdCl6] 397.3 >450 2.7 3.41g/l at RT

Dipotassium tetrachloropalladate K2[PdCl4] 326.4 524 2.7 Soluble

c Disodium tetrachloropalladate(II) Na2[PdCl4] 294.21 395 - 622g/l

Diamminedinitro-palladium(II) Pd(NO2)2(NH3)2 232.5 230 - Slightly soluble

c Trans diamminedichloropalladium(II) PdCl2(NH3)2 211.39 323 - 0.63g/l at RT

c Tetraamminepalladium(II) dichloride [Pd(NH3)4]Cl2 263.5 300 1.91 327g/l

Tetraamminepalladium(II) hydrogen [Pd(NH ) ](HCO ) 296.6 - - 56.2g/l at 20°C carbonate 3 4 3 2

Dihydrogen tetrachloropalladate(II) H2[PdCl4] 250.2 - - -

c Bis(acetylacetonato) palladium(II) Pd(C5H7O2)2 304.64 180 - 0.011mg/l

Bis(dibenzylidene-acetone) Pd(C H O) 575.02 150 - - palladium(0) 17 14

Dichlorobistripheny-phosphine PdCl [(C H ) P] 701.90 254 c - <7x10-5 g/l at 20°C palladium(II) 2 6 5 3 2

Dichloro(1,5-cyclooctadiene) PdCl (C H ) 285.51 210 c - - palladium(II) 2 8 12

10 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS

TABLE 2-1: PHYSICAL PROPERTIES OF PGMS AND 2.3 SELECTED COMPOUNDS

Molecular Melting Point Density Industry Name Formula Solubility in Water Weight (°C) (kg/m3)

Rhodium Substances

f Rhodium Rh 102.91 1960 12.41 <0.1 µ/l

c f Rhodium(III) oxide Rh2O3 253.81 1100 8.2 0.5 µ/l

Rhodium(III) trichloride RhCl3 209.26 - - Insoluble

Rhodium(III) trichloride, hydrated Mixed complexes - >450°C - 696g/l

'Rhodium(III) nitrate' Mixed complexes - <160 c - 1170g/l at RT

Rh2(SO4)3•12H2O Rhodium(III) sulphate Mixed complexes in - >500°C - >100g/l solution.

Rhodium(III) chloropentaammine Rh[Cl(NH ) ]Cl 294.42 - Low solubility dichloride 3 5 2

c Triammonium hexachlororhodate(III) (NH4)3[RhCl6] 369.74 260 - 99.8g/l at RT

Tris(triphenylphosphine) rhodium(I) [RhCl{P(C H ) } ] 925.23 103 c - <1x10-4 g/l chloride 6 6 3 3

Carbonyltrishydrido [RhH(CO) 918.19 97 c - 6x10-6 g/l at 20°C (triphenylphosphine) rhodium(I) {P(C6H6)3}3]

Ruthenium Substances

f Ruthenium Ru 101.7 2334 12.45 0.02 µ/l

Ruthenium(II) dichloride RuCl2 171.98 - - Insoluble

c Ruthenium(III) trichloride RuCl3 207.43 ~500 - Insoluble

Ruthenium(III) trichloride hydrated Mixed complexes - 90 c - 1140g/l at RT

Ruthenium tetrachloride RuCl4 242.88 - - Slightly soluble

c f Ruthenium(IV) dioxide, hydrated RuO2.xH20 - 1300 7.05 0.08 µ/l

Ruthenium(VIII) tetroxide RuO4 164.7 25 - Decomposes

>22.6g/l Diammonium hexachlororuthenate(IV) (NH ) [RuCl ] 349.87 320 c 2.21 4 2 6 (may hydrolyse)

c Tris(nitrate-O) nitrosylruthenium(III) [Ru(NO3)3(NO)] 317.09 140 - 690g/l

Dichlorotris(triphenyl-phosphine) [RuCl {P(C H ) } ] 958.83 116 c - <4x10-5 g/l at 20°C ruthenium(II) 2 6 5 3 2

11 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS

TABLE 2-1: PHYSICAL PROPERTIES OF PGMS AND 2.3 SELECTED COMPOUNDS

Molecular Melting Point Density Industry Name Formula Solubility in Water Weight (°C) (kg/m3)

Osmium Substances

Osmium Os 190.2 3050 22.61 Insoluble

Osmium dioxide OsO2 222.2 - 7.91 Insoluble

Osmium(VIII) tetroxide OsO4 254.20 40 4.91 72g/l

Soluble, Osmium dichloride OsCl 261.11 - - 2 decomposes

Iridium Substances

f Iridium Ir 192.22 2446 22.56 <0.01 µ/l

c Iridium dioxide IrO2 224.20 1100 11.7 0.0002g/l at RT

c Iridium(III) trichloride IrCl3 298.58 763 - Insoluble

IrCl .xH 0, mixed Iridium(III) trichloride, hydrated 3 2 - - 5.3 220g/l complexes

c Dihydrogen-hexachloroiridate(IV) H2[IrCl6].6H2O 515.04 80 - 456g/l

c Diammonium hexachloroiridate(IV) (NH4)2[IrCl6] 441.01 60 2.86 -

Di-µ-chlorobis((1,2,5,6-eta)cycloocta-1,5- c [IrCl(C8H12)]2 671.71 202 - - diene))diiridium(I)

12 CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS REFERENCES

O’Neil M., ed. (2001) Platinum and platinum compounds. In: The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals. 13th Edition. Whitehouse Station, NJ. Merck & Company, Inc., pp 306-307, 403-404, 1350, 1369, 1373. ISBN: 0911910131.

Pearson R.G. (1963) Hard and soft acids and bases. JACS, 85: 3533-3539.

Sehrt U., Grehl M., eds. (2012) Precious Materials Handbook. 1st Edition. Hanau-Wolfgang, Germany, Umicore AG & Co KG, pp. 576. ISBN: 9783834332592.