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