2
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
3
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.
Figure 2-1: Platinum Group Metals in the Periodic Table
Ru
44
R
uthenium
Rh
45
R
hodium
Pd
46
P
alladium
Pt
78
P
latinum
Ir
77
Iridium
Os
76
O
sm
ium
Ag
4
7
S
i
l
v
e
r
Au
7
9
G
o
l
d
Rg
1
1
1
R
o
e
n
t
g
e
n
i
u
m
Ds
11
0
D
a
r
m
s
t
a
d
t
i
u
m
Mt
1
0
9
M
e
i
t
n
e
r
i
u
m
Hs
1
0
8
H
a
s
s
i
u
m
Bh
1
0
7
B
o
h
r
i
u
m
Re
7
5
R
h
e
n
i
u
m
Tc
4
3
Te
c
Te
c
Te
h
n
e
t
i
u
m
Mn
2
5
M
a
n
g
a
n
e
s
e
Fe
2
6
I
r
o
n
Co
2
7
C
o
b
a
l
t
Ni
2
8
N
i
c
k
e
l
2
9
C
o
p
p
e
r
C
u
4
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
2.1
The six platinum group metals
(PGMs) are transition metals
occurring in the d-block (groups
8, 9, 10, periods 5 and 6) of the
periodic table (Figure 2-1).
These metals, along with silver
and gold, share a generally high
resistance to chemical attack and,
as a result, are also called the
OVERVIEW
OF PGM
S
‘noble metals’. In common with
the other transition elements,
their chemistry is characterised
by multiple oxidation states
and a wide range of coloured
compounds covering the entire
spectrum from, commonly, almost
colourless (osmium), through
yellows (platinum) to rich red-
browns (palladium, iridium,
ruthenium), reds (rhodium) and,
less commonly, purples and blues
(ruthenium and iridium). They
form a vast range of coordination
compounds with some of the
most complicated and diverse
chemistry in the periodic table.
OVERVIEW OF PGMS
5
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
2.2
Metallic forms of PGMs are
generally considered to be ‘inert’,
but this is dependent in part on
dimensional characteristics.
Reactions of PGMs with most
substances, for example
oxygen, halogens, and acids,
are infl uenced—both in rate and
extent—by the particle size of
the metal. Thus some reactions
that apparently do not take place
with massive metal can occur
with fi nely-divided, high surface
area metal powders. The metals
in colloidal form (as ‘blacks’)
can exhibit striking catalytic
capabilities in some reactions.
PGMs are all susceptible to attack
by elemental halogens, especially
chlorine and fl uorine. Palladium,
platinum, and to a lesser extent,
rhodium are attacked to varying
METALLIC
PGM
S
degrees by mineral acids such as
nitric, sulphuric or aqua regia
(a mixture of hydrochloric and
nitric acids), particularly when
fi nely divided. On a laboratory
scale iridium, ruthenium, and
osmium (sometimes called
the ‘insoluble PGMs’), usually
require fusion with an oxidising
alkali, such as sodium peroxide,
followed by acidifi cation with
hydrochloric acid to reliably
solubilise them, particularly after
high-temperature treatment.
Metallic PGMs provide perhaps
the most commercially important
class of catalysts known and
include automotive exhaust
catalysts, petroleum platforming
catalysts, ammonia oxidation
catalysts, and fuel cells. See
Chapter 3 for further information
regarding applications of PGMs.
METALLIC PGMS
6
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
2.3
For more comprehensive
reference sources on the
characteristics, chemistry, and
physic-chemical properties of
PGMs and their compounds the
reader is referred to various
reference sources (O’Neil,
2001; Sehrt and Grehl, 2012).
Supplier Company catalogues
and websites also contain useful
information on these aspects
including those of Johnson
Matthey, Umicore N.V., Heraeus
GmbH and BASF. The biological
properties and toxicology of
PGMs are separately addressed in
Chapter 6 of this Guide.
PGMs form a number of
apparently simple binary
compounds which are invariably
structurally complicated.
They also form a vast array of
coordination compounds in
which the central metal atom is
bound to a number of ligands by
coordinate bonding, including
halides, sulphur, amines, and
other atoms/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 linkages
with biological behaviour and
toxicity (see Chapter 6).
COMPOUNDS OF PLATINUM
GROUP METALS
ruthenium(VIII) and osmium(VIII)
oxides are frequently used
to recover these metals by
distillation. Although PGMs form
binary anhydrous chlorides, these
are not commercially signifi cant.
They are often substantially
insoluble in most solvents, and
polymeric in structure.
COMPOUNDS OF PLATINUM GROUP METALS
SIMPLE
COMPOUNDS
In the environment, platinum
group metals occur separately,
or alloyed together, or alloyed
with other metals (e.g., as
osmiridium and ferroplatinum).
There are also some distinct
compound mineral species which
illustrate the binary compounds:
these include sperrylite (PtAs);
cooperite (Pt,Pd)S; a series
of platinum and palladium
chalcocides, such as sulfarsenides,
michenerite (PdBiTe) and
moncheite (Pt,Pd)(Te,Bi); and
laurite, (Ru,Os)S; all of which
occur naturally in metal sulphide
deposits, such as in Sudbury,
Ontario in Canada and in the
Merensky Reef in South Africa.
Binary compounds exist for
each metal. The most important
compounds are those with
oxygen or chlorine. Platinum(IV)
oxide is a commercially used
catalyst. Iridium, rhodium, and
palladium(IV) hydrated oxides
can be used to quantitatively
hydrolyse these metals during
refi ning, and the volatile
CI
2-
Pt
CI
CICI
CICI
Pt
NH
3
Cl
NH
3
CI
7
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
Chloride solutions are the
predominant media for the
commercial separation and
refi ning of PGMs. In this
application, PGMs form sequences
of aquated species with the
relatively weakly bound chlorides
such as the hexagonal [M(Cl)
n
(H
2
O)
6-n
)]
4-n
, where M is Pt(IV) or
Ir(IV), or the square planar [M
1
(Cl
n
(H
2
O)
4-n
]
(2-n)
, where M
1
is Pt(II) or
Pd(II). The degree of aquation
increases with decreasing chloride
concentration. The ammonium
and potassium salts of some of
the hexachloro complexes are
of relatively low water solubility
and those of Pt(IV), Pd(IV), and
Ir(IV) are sometimes used for
precipitation of those metals
during refi ning and purifi cation
processes.
2.3
COMPOUNDS OF PLATINUM GROUP METALS
COMPLEX
COMPOUNDS
In common with all transition
metals, the vast majority of the
commercial chemistry of PGMs
is concerned with their extensive
range of coordination complexes.
In these, the central metal atom is
bound to a number of ligands, or
complexing agents. These ligands
are atoms or molecules which
frequently act as electron donors.
The biological signifi cance of
ligand substitution is discussed in
more detail in Chapter 6.
As a generalisation, in the early
1960s Pearson developed a
set of ‘rules’ for ‘hard/soft’
Lewis acids (Pearson, 1963).
The platinum group metals are
categorised as ‘soft’ metals that
prefer to coordinate to ‘soft’
ligands (e.g., cyanide, ammonia,
amines, sulphur ligands and
halides, etc.). They do not easily
form complexes with ‘hard’
ligands, such as oxo acids (e.g.,
nitrate, sulphur, phosphate,
etc.). The practical use of
complex compounds of PGMs in
homogeneous and heterogeneous
catalysis has generated further
rapid development of their
chemistry. This is discussed
below, and also in Chapter 6 with
specifi c reference to how the
chemistry of these compounds
relates to their toxicity.
Figure 2-2: Example structures of Pt coordination complexes
Hexagonal
hexachloroplatinate(IV)
Square planar cis diamminodichloroplatinum(II)
Figure 2-3: Example of a Pt-DMSO complex
Sulphur-bonded
trans chloromethylbis(dimethylsulphoxide)Platinum (II)
Figure 2-4: Postulated structure for a form of Pt nitrate
Proposed oligomer of the Pt-bridged structure in ‘Pt(II) nitrate’
[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.]
o
o
o
o
n
(H)
(H)
(H)
(H)
(H)
(H)
o
o
Pt Pt
Pt
Pt
Me
Pt
CI
Me
2
(o)S S(O)Me
2
8
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
2.3
COMPOUNDS OF PLATINUM GROUP METALS
Another commercially
important class of compounds
are the amine and ammine
complexes of platinum(II),
palladium(II) and platinum(IV).
The ammine complexes of
the latter fi nd application in
electroplating and automotive
exhaust catalyst production.
Diamminedichloropalladium(II)
is substantially insoluble in acid
chloride media and may be
used to precipitate palladium
and, because it can be re-
dissolved in excess ammonia
and re-precipitated, to purify the
metal in some refi nery circuits.
Certain platinum(II) coordination
complexes with di erent
ligands (e.g., cisplatin or cis-
diamminodichloroplatinum(II),
carboplatin, and oxaliplatin) are
used in cancer chemotherapy
(so-called “platins”). As a ‘soft’
metal, platinum also has a great
a nity for sulphur, such as with
dimethyl sulfoxide (DMSO);
numerous DMSO complexes
have been reported (see Figure
2-3). This ability to form strong,
stable complexes with amine
-N and sulphur -S centres,
which are present in biological
macromolecules, undoubtedly
infl uences the biological activity
of certain platinum compounds,
including their toxicity (see
Chapter 6).
Increasingly important classes of
compounds are the so-called PGM
‘nitrates’ which can be used for
catalyst impregnation. These are
generally formed by dissolution of
the metal or an oxide/hydroxide
compound in nitric acid. As with
most aqueous PGM chemistry,
these are not simple binary
compounds and involve a range
of complexes, some of which may
include bridged polymeric species
(see Figure 2-4).
9
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
a
The boiling point of platinum metal is 3825°C.
b
At 20°C.
c
Decomposes.
d
Hexahydrate.
e
May dissolve in own water of crystallization..
f
By transformation/dissolution kinetics tests at 100mg/l after 7 days.
RT = Room Temperature.
- = No data.
Industry Name Formula
Molecular
Weight
Melting Point
(°C)
Density
(kg/m
3
)
Solubility in Water
Platinum Substances
Platinum
a
Pt 195.09 1768.2
21.45
b
0.08
µ
/l
f
Platinum(II) oxide PtO 211.08
325
c
14.1 Insoluble
Platinum(IV) oxide PtO
2
227.08 >450 11.8
0.11mg/l
f
Platinum(IV) sulphide PtS
2
259.21
225-250
c
7.85 Insoluble
Platinum(II) chloride PtCl
2
265.99
581
c
6.0 Insoluble
Platinum(IV) chloride PtCl
4
336.89
327
c
4.30 Slightly soluble
'Platinum(II) nitrate'
Mixed Pt(IV)
complexes
- - - Soluble
Platinum(IV) sulphate tetrahydrate
Mixed complexes in
solution
- - - Soluble
Ammonium tetrachloroplatinate(II) (NH
4
)
2
[PtCl
4
] 372.97
-
c
2.94 Soluble
Potassium tetrachloroplatinate(II) K
2
[PtCl
4
] 415.09
500
c
3.38 191g/l at 20°C
Hexachloroplatinic(IV) acid H
2
[PtCl
6
] 409.81
60
d f
2.43
d
1400g/l at 18°C
d
Ammonium hexachloroplatinate(IV) (NH
4
)
2
[PtCl
6
] 443.87
380
c
3.07 5g/l at 20°C
Potassium hexachloroplatinate(IV) K
2
[PtCl
6
] 485.99
250
c
3.5 7.7g/l at 20°C
Sodium hexachloroplatinate(IV) Na
2
[PtCl
6
] 453.77
250
c
3.5
Very soluble
d
Cis diamminedichloro-platinum(II) [Pt(NH
3
)
2
Cl
2
] 300.05
270
c
- 2.5g/l at RT
Tetraammineplatinum(II) dichloride [Pt(NH
3
)
4
]Cl
2
333.98
80
c
2.7 240g/kg
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.
2.3
TABLE 2-1: PHYSICAL PROPERTIES OF PGMs AND
SELECTED COMPOUNDS
10
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
Industry Name Formula
Molecular
Weight
Melting Point
(°C)
Density
(kg/m
3
)
Solubility in Water
Palladium Substances
Palladium Pd 106.4 1554.8 12.0
<0.01
µ
/l
f
Palladium(II) oxide PdO 122.40
750
c
8.7 <0.01mg/l
Palladium(II) dichloride PdCl
2
177.33
679
c
4.0 4g/l at RT
Palladium(II) dibromide PdBr
2
266.22 500 - Insoluble
Palladium(II) diiodide Pdl
2
360.21
350
c
6 Insoluble
Palladium diacetate Pd(C
2
H
3
O
2
)
2
225
200
c
- Insoluble
Palladium(II) acetate Trimer {Pd(C
2
H
3
O
2
)2}
3
673.53 - - 0.35g/l at 20°C
Palladium(II) sulphate
Mixed complexes in
solution
-
125
c
- 280g/l
'Palladium(II) nitrate'
Mixed Pd(II)
complexes
Decomposes - >10g/l
Diammonium hexachloropalladate(IV) (NH
4
)
2
[PdCl
6
] 355.22
240
c
- >10g/l at RT
Potassium hexachloropalladate(IV) K
2
[PdCl
6
] 397.3 >450 2.7 3.41g/l at RT
Dipotassium tetrachloropalladate K
2
[PdCl
4
] 326.4 524 2.7 Soluble
Disodium tetrachloropalladate(II) Na
2
[PdCl
4
] 294.21
395
c
- 622g/l
Diamminedinitro-palladium(II) Pd(NO
2
)
2
(NH
3
)
2
232.5 230 - Slightly soluble
Trans diamminedichloro- palladium(II) PdCl
2
(NH
3
)
2
211.39
323
c
- 0.63g/l at RT
Tetraamminepalladium(II) dichloride [Pd(NH
3
)
4
]Cl
2
263.5
300
c
1.91 327g/l
Tetraamminepalladium(II) hydrogen
carbonate
[Pd(NH
3
)
4
](HCO
3
)
2
296.6 - - 56.2g/l at 20°C
Dihydrogen tetrachloropalladate(II) H
2
[PdCl
4
] 250.2 - - -
Bis(acetylacetonato) palladium(II) Pd(C
5
H
7
O
2
)
2
304.64
180
c
- 0.011mg/l
Bis(dibenzylidene-acetone)
palladium(0)
Pd(C
17
H
14
O) 575.02 150 - -
Dichlorobistripheny-phosphine
palladium(II)
PdCl
2
[(C
6
H
5
)
3
P]
2
701.90
254
c
- <7x10
-5
g/l at 20°C
Dichloro(1,5-cyclooctadiene)
palladium(II)
PdCl
2
(C
8
H
12
) 285.51
210
c
- -
2.3
TABLE 2-1: PHYSICAL PROPERTIES OF PGMS AND
SELECTED COMPOUNDS
a
The boiling point of platinum metal is 3825°C.
b
At 20°C.
c
Decomposes.
d
Hexahydrate.
e
May dissolve in own water of crystallization..
f
By transformation/dissolution kinetics tests at 100mg/l after 7 days.
RT = Room Temperature.
- = No data.
11
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
Industry Name Formula
Molecular
Weight
Melting Point
(°C)
Density
(kg/m
3
)
Solubility in Water
Rhodium Substances
Rhodium Rh 102.91 1960 12.41
<0.1
µ
/l
f
Rhodium(III) oxide Rh
2
O
3
253.81
1100
c
8.2
0.5
µ
/l
f
Rhodium(III) trichloride RhCl
3
209.26 - - Insoluble
Rhodium(III) trichloride, hydrated Mixed complexes - >450°C - 696g/l
'Rhodium(III) nitrate' Mixed complexes -
<160
c
- 1170g/l at RT
Rhodium(III) sulphate
Rh
2
(SO
4
)
3
•12H
2
O
Mixed complexes in
solution.
- >500°C - >100g/l
Rhodium(III) chloropentaammine
dichloride
Rh[Cl(NH
3
)
5
]Cl
2
294.42 - Low solubility
Triammonium hexachlororhodate(III) (NH
4
)
3
[RhCl
6
] 369.74
260
c
- 99.8g/l at RT
Tris(triphenylphosphine) rhodium(I)
chloride
[RhCl{P(C
6
H
6
)
3
}
3
] 925.23
103
c
- <1x10
-4
g/l
Carbonyltrishydrido
(triphenylphosphine) rhodium(I)
[RhH(CO)
{P(C
6
H
6
)
3
}
3
]
918.19
97
c
- 6x10
-6
g/l at 20°C
Ruthenium Substances
Ruthenium Ru 101.7 2334 12.45
0.02
µ
/l
f
Ruthenium(II) dichloride RuCl
2
171.98 - - Insoluble
Ruthenium(III) trichloride RuCl
3
207.43
~500
c
- Insoluble
Ruthenium(III) trichloride hydrated Mixed complexes -
90
c
- 1140g/l at RT
Ruthenium tetrachloride RuCl
4
242.88 - - Slightly soluble
Ruthenium(IV) dioxide, hydrated RuO
2
.xH
2
0 -
1300
c
7.05
0.08
µ
/l
f
Ruthenium(VIII) tetroxide RuO
4
164.7 25 - Decomposes
Diammonium hexachlororuthenate(IV) (NH
4
)
2
[RuCl
6
] 349.87
320
c
2.21
>22.6g/l
(may hydrolyse)
Tris(nitrate-O) nitrosylruthenium(III) [Ru(NO
3
)
3
(NO)] 317.09
140
c
- 690g/l
Dichlorotris(triphenyl-phosphine)
ruthenium(II)
[RuCl
2
{P(C
6
H
5
)
3
}
2
] 958.83
116
c
- <4x10
-5
g/l at 20°C
2.3
TABLE 2-1: PHYSICAL PROPERTIES OF PGMS AND
SELECTED COMPOUNDS
a
The boiling point of platinum metal is 3825°C.
b
At 20°C.
c
Decomposes.
d
Hexahydrate.
e
May dissolve in own water of crystallization..
f
By transformation/dissolution kinetics tests at 100mg/l after 7 days.
RT = Room Temperature.
- = No data.
12
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
Industry Name Formula
Molecular
Weight
Melting Point
(°C)
Density
(kg/m
3
)
Solubility in Water
Osmium Substances
Osmium Os 190.2 3050 22.61 Insoluble
Osmium dioxide OsO
2
222.2 - 7.91 Insoluble
Osmium(VIII) tetroxide OsO
4
254.20 40 4.91 72g/l
Osmium dichloride OsCl
2
261.11 - -
Soluble,
decomposes
Iridium Substances
Iridium Ir 192.22 2446 22.56
<0.01
µ
/l
f
Iridium dioxide IrO
2
224.20
1100
c
11.7 0.0002g/l at RT
Iridium(III) trichloride IrCl
3
298.58
763
c
- Insoluble
Iridium(III) trichloride, hydrated
IrCl
3
.xH
2
0, mixed
complexes
- - 5.3 220g/l
Dihydrogen-hexachloroiridate(IV) H
2
[IrCl
6
].6H
2
O 515.04
80
c
- 456g/l
Diammonium hexachloroiridate(IV) (NH
4
)
2
[IrCl
6
] 441.01
60
c
2.86 -
Di-
µ
-chlorobis((1,2,5,6-eta)cycloocta-1,5-
diene))diiridium(I)
[IrCl(C
8
H
12
)]
2
671.71
202
c
- -
2.3
TABLE 2-1: PHYSICAL PROPERTIES OF PGMS AND
SELECTED COMPOUNDS
a
The boiling point of platinum metal is 3825°C.
b
At 20°C.
c
Decomposes.
d
Hexahydrate.
e
May dissolve in own water of crystallization..
f
By transformation/dissolution kinetics tests at 100mg/l after 7 days.
RT = Room Temperature.
- = No data.
13
CHAPTER 2 | PHYSICAL AND CHEMICAL PROPERTIES OF PLATINUM GROUP METALS
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,
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