PLATINUM METALS REVIEW - Johnson Matthey Technology Review

68 Pages·2005·4.8 MB·English

PLATINUM METALS REVIEW
A quarterly survey of research on the platinum metals and of
developments in their application in industry
VOL. 32 OCTOBER 1988 NO. 4
Contents
Reactions of Complexes of Platinum Metals with Bio-Molecules
226
Water Soluble Rhodium Catalysts
226
The Destruction of Polychlorinated Biphenyls
186
Chemical Conversion of Carbon Dioxide
187
The Electrodeposition of Platinum and Platinum Alloys 188
Ruthenium in Cancer Chemotherapy
198
Lean-Burn Oxygen Sensor Material 199
Control of Corrosion in Molten Carbonate Fuel Cells
200
Flammable Gas Detection
203
International Congress on Catalysis
204
Geology and Geochemistry of the Platinum Metals
208
Abstracts
209
New Patents
217
Index to Volume 32 226
Communications should be addressed to
The Editor, Platinum Metals Review
Johnson Matthey Public Limited Company, Hatton Garden, London ECl N 8EE
Reactions of Complexes of Platinum
Metals with Bio-Molecules
By Professor A. G. Sykes
Department of Chemistry, The University, Newcastle upon Tyne, England
There is a growing recognition of the opportunities that may exist for in-
organic biochemistry to contribute to the solution of practical problems
that occur in many quite different areas, ranging from mineral extrac-
tion to medicine. While most traditional applications of the platinum
metals depend upon their ability to remain largely unchanged in highly
reactive environments, compounds of these metals can be quite reactive
and do react with biological materials. This paper reviews the present
position and indicates potential applications.
The reactions of co-ordination complexes of finds applications in the treatment of testicular,
the platinum metals and gold with bio- ovarian and lung cancers. Cis-platin was the
molecules are relevant to a number of areas. first successful anti-cancer drug, but it does
The most important of these currently relates to display unpleasant side effects, notably kidney
the medicinal applications of platinum and gold toxicity, nausea and vomiting, as well as
(I), with much potential still to be explored. neurotoxicity. Second generation drugs, carbo-
There are other applications, however, in- platin [a], which is now approved for use in
cluding the laboratory use of platinum metals as the U. K., iproplatin [bl and spiroplatin [cl ,a re
heavy-atom markers in the determination of less toxic and have better anti-tumour activity.
biological macromolecule crystal structures, the (3). There are additional advantages, thus
use of osmium tetroxide as a specific oxidant spiroplatin has higher water solubility, of
and biological stain, and the use of attached o.5M, which allows easier administration of the
ruthenium complexes as spectroscopic labels drug. It has been demonstrated that this form
and probes for studying the electron-transfer of drug is active against three animal tumours
reactions of bio-molecules. At the other ex- with a 30-fold greater activity than cis-platin.
treme there is the potential use of micro- The use of carboplatin and iproplatin in
organisms in mining processes, and for the synergistic combination with radiotherapy is
selective extraction/recovery of metals from being investigated. Third generation drugs
aqueous solutions (pH>2) with cells or cell ex- which contain combinations of malonate and
tracts. The formation of ruthenium as a fusion cyclobutanedicarboxylate with amine and
product in nuclear reactors-the isotopes lo3 Ru diamine ligands, so giving a broad spectrum of
and '06 Ru have half lives of 40 days and I year, activity, are currently under investigation (3,
respectively-and its release into the at- 4). Features of all these drugs are the use of
mosphere at the time of the Chernobyl mishap, neutral Pt(I1) or Pt(1V) complexes which con-
is also relevant. tain two cis amine groups (primary or secon-
The use of the Pt(I1) complex cis-platin, dary, but not tertiary) and two other good cis
cis-[Pt(NH,),CI,l, in chemotherapy was ap- leaving groups. Thus trans complexes, or cis
proved as recently as 1979, and this is now the complexes with poor leaving groups, that is the
leading anti-cancer drug in the U.S.A. (2). It is inert CN-, ONO-, NCS- or I-, are inactive.
used in combination therapy with either Early experiments demonstrated that cis- and
adriamycin or bleomycin and binblastine, and trans-platin bind more strongly to RNA than to
Platinum Melals Rev., 1988, 32, (4), 170-178 170
DNA, and least strongly to proteins. However, Au(1) bis-diphosphine complexes such as
when their ability to suppress synthesis was [Au(dppe),lU may have a future as anti-
measured at low concentrations only the syn- cancer agents (11). Titanocene dichloride,
thesis of DNA was suppressed. It is believed that [Ti(Cp), U , 1, where Cp = cyclopentadienyl,
cis-platin type complexes are effective because represents an interesting example of an
they bind to the N-7p ositions of two adjacent organometallic anti-tumour reagent (12).
guanines of the DNA, and that this hinders Another type of interaction of simple
replication (5-7). A normal cell is able to op- transition-metalc omplexes with DNA is the in-
pose this attachment by means of its repair tercalation which m r s with planar
mechanisms, while a tumour cell has a deficien- heterocyclic chromophores. The heterocyclic
cy in repair proteins which could otherwise ligands can insert and stack between base pairs
recognise the damaged segment and cause a of the DNA helix. Such effects have recently
repair. Analogous trans complexes attach more been reviewed by Barton (8). The stereoselec-
readily to guanine in the initial stages but tive interaction of tris(phenanthr0line) com-
,+
decrease in concentration over 24 hour periods plexes such as [Rubhen) 1 has for example
,+
due to their removal by repair proteins. been noted, the A-[Ru(phen) 1 complex in-
A complex such as cis-platin is believed to re- tercalating more favourably than the A isomer,
main in the neutral dichloro form in the plasma which is inhibited by steric repulsions between
where the concentration of U- is high H-atoms of the phenanthroline and 0-atoms of
(-103mM). After W o n a cross the cell the DNA phosphate (13). There are also
membrane, in the presence of only ,~IM U- spectroscopic applications, and the tris
-
in the cytoplasm, the complex yields the (4,7 diphenylphenanthroline) ruthenium (11)
various aquated products (8). Since H,O is a complex can be used as a sensitive stain for
good leaving group the platinum can then at- helix conformation in chromosome studies
tach itself to the DNA. using fluorescence techniques. Electron-
Rhodium complexes, for example transfer reactions of intercalated metal com-
[Rh,(O,CR),l, have been reported to possess plexes, for example the Fe(I1)-EDTA
some activity as anticaocer agents (9). More derivative complex which has the aromatic
generally, when added to cell growth media, methidium intercalator attached by a short
complexes mm-[RhX,L,l (where L is a N- hydrocarbon chain, can with oxygen give rise to
heterocyclic such as pyridine) are known to in- single strand DNA scissions (14). The Fe(I1)
terfere with cell division and cause f3atmnt.a- activates the oxygen to yield 0,- or
tion in growth (10). A series of Pd(II) analogue OH' radicals (or Fe-0, complexes), which at
complexes have been tested, but show little high local concentrations can bring about
promise. It is known that Pd(I1) is more labile cleavage of the sugar phosphate backbone.
than Pt(I1) and this can lead to unavoidable tox- The binding of platinum complexes to
icity. The Pd(II) may not even reach the DNA. cytochrome c has recently been studied. A
Recent reports indicate that the four-cmrdinate metalloprotein has many advantages in such
Platinum Me& Rm., 1988, 32, (4) 171
investigations because structural features have number of reagents, possibly with some pertur-
been extensively explored, and the presence of bation of conformation, is however of low reac-
the metal helps in demonstrating whether at- tivity with [Pt(terpy)ClI+. This is particularly
tachment of a second metal has any disruptive interesting since, in separate studies with amino
effects on the protein structure. Kostili has acids or small peptides as entering ligands, the
shown that [PtCI I - and [ Pt(z-Fpy) ,C 1 ,I , complex is completely selective towards cys-
where 2-Fpy is 2-fluoropyridine, cross-link teine. From a recent crystallographic study on
horse cytochrome c by co-ordinating to the iso-1-cytochrome c, it has been confirmed that
S-atom of the thioether side chain of a the Cysroz is buried and in a hydrophobic
methionine, [dl, in this case Met65 (15). region.
The binding is illustrated in [el. With Recently it has been demonstrated that in-
[Pt(terpy)ClP however, selective covalent cubation of horse cytochrome c with the Rh(I1)
labelling of histidine residues, tfl, His33 (major dimer [Rh,(O,CCH,),l for 2 days at pH 7 (but
binding) and His26 (minor binding) is observ- not pH 5) gives the diprotein complex
ed, with no labelling of Met65 (16). The dif- [Rh,(O,CCH,),l(cyt c)~[,g l (19). Attachment
ference in behaviour can be attributed to the A
steric demands of the terpyridine ligand, %
which prevent it binding to the thioether
group. Interestingly, in the case of the com- OI ..>.GJ - G f l
HNVN- Rh‘- R6’- N-NH
plex [Pt(NH,) H, 01 + preliminary results *o’ I
indicate that binding occurs 55 per cent to yo
the methionine and 45 per cent to histidine
[gl
His33 (17).
is at a histidine residue, and model complexes
H3Nt - CI H-COO- 0,s‘- PLI t -C\sH 3 I[mRh =,( Oim,CidCaHzo,l)e,,( Iwmit)h,I tchaen Ibme pcroe-poarrdeidn,a wtehde irne
the axial position. Since no reaction is observed
(CH2)2 ‘0
I H3C LI with tuna cytochrome c, which does not have
:.s\
the His33 residue, it has been concluded that
CH3 attachment is at this residue on horse
L=CI- or
cytochrome c. Enhanced stability of the dimer
Met QF
[ dl [el adduct to hydrolysis, as compared to
[Rh,(O,CCH,),(Im),l, is attributed to the
t
H3N -CH -COO-
I steric bulk of the protein with the possibility of
some H-bonding in addition.
CH2
I Gold(1) drugs are extensively used in the
HF=F treatment of rheumatoid arthritis although
:Nb ,NH
again the mechanism is not well understood
C
H (20). The most commonly used drugs are
His Ifl
Myochrisin (gold sodium thiomalate) and
The reaction of cytochrome c from Candtda Solganal (gold thioglucose), which are ad-
krusei and baker’s yeast with [Pt(terpy)Cl] ministered by weekly or monthly intramuscular
+
has also been studied (IS), when modification injections. The structures of these compounds
of surface His33 and His39 residues (which are are not known precisely. The newer drug
in hydrophilic regions) is observed, but His26 Auranofin, the gold triethyl phosphine
(in a hydrophobic pocket) is largely shielded. thioglucose complex, can be administered oral-
The cysteine residue, HS-CH,-R (pK,- 8.3), ly (11). It seems likely that the role of the drug
Cys102 in baker’s yeast, which is reactive to a is anti-inflamatory and/or anti-enzymatic, and
Platinum Metals Rev., 1988, 32, (4) 172
residues, none of which are present in large
numbers in metalloproteins, and in some cases
provide unique sites. Some care is required
because mother liquors high in [CI-I or
AcO [SO, 2-1 concentrations could impede efficient
binding of [PtCI,lz-, which is first converted
The Structure of Auranofin to aqua and hydroxo forms of neutral or I+
charge before attaching to the residues in-
dicated (25). Other complexes which will react
that the body's immune responses are effected in the same way are [Pt(N02),12-,
in some way. There is some evidence that gold [Pt(NH,),CI,l or IPt(en)CI,l, where
accumulates in the red blood cells. The uptake en = ethylenediamine, whereas the inertness
of Et,PAu+ into such cells has been in- of [Pt(CN),12- ensures its retention as a
vestigated (21). With concentrations of 2- anion which will interact, if at all, with
,
Et PAu (up to 9mM) in excess of that achiev- positively charged residues (lysines, arginines
+
ed in therapeutic applications (25-5opM), in- and at pH<7 histidines) by electrostatic associa-
teractions with intracellular glutathione, and a tion. A wide range of mercury compounds in-
second site identified as the cysteine p-93 of cluding for example ethylmercury phosphate,
hemoglobin, takes place. Excess Et PAu' also are known to bind to cysteine or histidine
reacts at weaker binding sites (nitrogen or residues (26), and can, because of their struc-
thioether ligands). Comparisons have been ture, readily penetrate into proteins to react
made with the binding of gold at weak and with buried side chains. Lanthanides (often
strong binding sites identified on serum samarium because of its large anomalous scat-
albumin (22). A facile interprotein gold transfer tering signal) and uranium compounds will
from gold modified hemoglobin to the -SH con- bind at carboxylate residues, although buffers
taining component of serum albumin has been other than phosphate (which will bind to such
noted in this work. The application of I'P metals) should be used. The complex ion
NMR spectroscopy has been important in these K, [HgI, 1, which in aqueous solution generates
studies. trigonal [HgI,]- , can bind electrostatically to
The use of heavy-atom markers was a major cations or, because of its flat structure,
breakthrough in the X-ray structure determina- penetrate into proteins (23).
tion of large bio-molecules. In this process the The strong oxidant osmium tetroxide, which
crystalline material is derivatised for phase is more stable than RuO, ,i s used as a reagent
determination by a method of isomorphous to give syn dihydroxy addition from the less
replacement (23). Procedures involve soaking hindered side of a double bond (27). The
the crystal in mother liquor containing the reagent adds rather slowly but quantitatively to
heavy-atom marker, or reacting the two give the intermediate as illustrated [il.
together prior to crystallisation. In order to ob-
-
tain the necessary information, the native dif-
I I
fraction intensities and those obtained from - c = c - + OSOL c - c -
crystals derivatised in at least two unique sites I I I I Iil
-
have to be determined. HO OH
I I
The reactivities of individual amino acids has
-C-C--
been summarised by Petsko and different I I
classes of heavy-atoms defined according to The latter is subsequently decomposed with
their affinities (23). The most extensively used sodium sulphite in, for example, ethanol. The
reagent is [PtCl,12- which is capable of bin- same reaction can be carried out more
ding to methionine, histidine and cysteine economically with hydrogen peroxide using
Platinum Metals Rev., 1988, 32, (4) 173
osmium tetroxide as a catalyst. The procedure on a-lytic protease and lysozyme can be used to
finds commercial use in small scale prepara- determine inter-residue distances (I 5.5 and
tions of scarce materials. Osmium tetroxide is 11.8& respectively) (32). At about the same
highly toxic however, and particularly hazar- time the preparation and characterisation of a
,
dous to the eyes because of its ready reaction (NH,) Ru(II1) to histidine-33 derivative of
with organic matter to give a black oxide, a pro- horse cytochrome c was reported (33). This
perty which is utilised in its application as a f=- type of attachment to electron transport
ative and stain in electron microscopy. This is metalloproteins has now been carried out in a
probably due to its ability to react with un- number of cases in order to explore fued
saturated fatty acid side chains of lipids (24). distance (crystallographically defined for the
Binding of [OsO, 1 * - to cis diols has been used unmodified protein) electron transfer from the
in the X-ray analysis of t-RNA where binding attached ruthenium (as Ru(II)), to the active
L:
is most likely to the 3'-ribose, [ii] (28). ; site in its oxidised form, heme Fe(II1) in the
1:: - case of cytochrome c (34). A feature of all these
,
studies is the special affinity of (NH ) Ru for
+ K~OSOL >OSQ histidine in preference to other amino acid
Iiil residues.
A procedure for detecting genetic changes in Relevant to these studies is the earlier Taube
new strains of viruses has recently been work, and the observation that both Ru(I1) and
reported using hydroxylamine and osmium Ru(II1) are inert to substitution. Modification
tetroxide (29). is carried out using a -50-fold excess of
Attachment of pentaammineruthenium(II1) [Ru(NH, ) H, 01 2+ [iiil , -
to the N-3 position of the imidazole side chain
+
of a histidine residue, [hl in proteins was IRu(NH,),H,OI2+ His-Protein
reported (NH,),RuHis-Protein Iiiil
,CH2CHCO$ after which the reaction is terminated by
I
HN-C NH; chromatographic filtration to remove excess of
/1 \\
HC4,CH the ruthenium reagent. The fully oxidised form
N
Ru(III)Fe(III) is obtained by oxidation with
[Fe(CN), 1 after which further purification
3-
(by FPLC) and characterisation is carried out.
NH3 [hl In order to study the intramolecular electron-
by Matthews and coworkers in 1978 (30). transfer process, rapid pulse-radiolysis or flash-
Earlier in the 1970s it had been shown that im- photolysis in situ reduction has to be achieved to
idazole and histidine complexes readily form in generate the Ru(II)Fe(III) combination; about
aqueous solution, and are extremely stable at 10 per cent reduction is appropriate. The in-
neutral and acidic pHs. The first studies were tramolecular electron-transfer process [ivl,
-
with the ribonuclease A protein which has four
histidine residues. Three derivatives each con- Ru(ll)Fe(I ll) Ru( III)Fe( II) [ivl
taining a single (NH,), Ru-histidine complex can then be monitored spectrophotometrically.
were synthesised and purified. It was found In the cytochrome c case protein concentrations
that sensitivity of the charge-transfer spectrum are sufficiently dilute so that there is no con-
of one of these derivatives to temperature and tribution from the intermolecular (bimolecular)
urea-induced unfolding could be used to ex- path [vl.
-
plore conformational changes in the vicinity of
Ru(II)Fe(lll) + Ru(lll)Fe(lll)
the complex (31). It has also been shown that
Ru(lll)Fe(lll) + Ru(lll)Fe(ll) Ivl
fluorescent energy transfer between trypto-
phans and a (NH,),Ru attached to a histidine The reduction potentials for the couples
Platinum Metals Rev., 1988, 32, (4) 174
[Ru(NH,),(His)12+/’+( 0.08V)a nd cytochrome
c (II)/(III) (0.26V) ensure that there is a N lHis87)
positive driving force for [ivl. An interesting
variation on this experiment is to attach other
ligands to the ruthenium which have much
more strongly oxidising couples, for example,
[R~(NH,),(isn)(H,0)1~+(’o~.+em V) or
[Ru(NH,),(py)(H,O)I (o.33mV), so that
,+’I+
a variation on intramolecular electron transfer
[vil can be monitored. This type of study is at Fig. 1 Structural model showing selected
parts and the relative orientations (with
present under investigation.
- edge to edge separation) of the donor and
acceptor sites in ruthenium-modified
Ru(lll)Fe(ll) Ru(ll)Fe(lll) [vil A. variabilis plastocyanin (Ref. 35)
With hind-sight, cytochrome c(1I) with its
high overall positive charge (estimated as + 8 at
-
pH 7) was not in fact a good choice in the first to. Plastocyanin (35) and azurin (36) each have
instance for modification with a positively a single copper active site, and utilise
charged complex. The time for modification Cu(II)/Cu(I) redox states. High-potential iron-
s,
(24-72h) is substantially longer than that re- sulphur protein (HIPIP) has a cuboidal Fe,
quired for acidic negatively charged proteins. cluster which can be either Fe,S,’+”+ (37),
Thus with the single copper protein plasto- while both the cytochromes c and cs5,c ontain
cyanin the procedure for [iii] requires only 20 a heme Fe and have Fe(III)/Fe(II) stable states
minutes in the case of the acidic plastocyanin (38, 39). It should be noted that not all proteins
(charge 9-) from the green algal source have histidine residues, and that not all
Scenedesmus obliquus, but requires approx- histidines are modified by ruthenium. For ex-
imately 4 hours for the basic plastocyanin ample in the case of cytochrome c, His33 is
(charge I+) from the blue-green algal source readily modified but His26 is not.
Anabaena variabilis (35). Some care is required in defining the distance
Modification of proteins by the attachment of (d) for intramolecular electron transfer from the
(NH,),Ru is not trivial, and extensive Ru(I1). For the copper proteins the Cu-S (cys-
characterisation of the products is required for teine) is likely to be the lead-in group, and since
each new protein studied to ensure that attach- there is delocalisation between copper and
ment is indeed at a histidine residue, and (if sulphur the relevant distance is to the S-atom,
more than one histidine) which histidine is in- Figure I (35). Similarly there is delocalisation
volved (3s ). Techniques used include Induc- at the imidazole ring of the histidine to which
tively Coupled Plasma (ICP) atomic emission the ruthenium is attached, and the nearest
spectroscopy to determine the metal content, point of the imidazole to the active site is con-
NMR to demonstrate that the histidine C,H sidered relevant. A computer graphics
proton resonance is no longer present due to the representation for P. srutzeri cytochrome c5,, is
line broadening effect of paramagnetic Ru(III), illustrated in Figure 2 (39). The direct
and testing with diethyl pyrocarbonate (DEPC) polypeptide link between His47 and the heme
for modification of a histidine (with attendant is circuitous in the extreme, and cannot be rele-
U.V. spectrophotometricc hanges) which can no vant. Electron transfer to the axially co-
longer occur when ruthenium is present. There ordinated Met61 has been assumed, but
is no evidence to suggest that ruthenium attach- transfer to the heme ring, which is strongly
ment is at all disruptive. electron delocalised, is also possible. For
Three different types of electron-transport HIPIP, Figure 3, the distance to the nearest
protein have been modified and will be referred point of the Fe,S, cube, one of the ~.c,-sulphido
Platinum Metals Rev., 1988, 32, (4) 175
\ His16
Fig. 2 Structural model showing
selected parts and relative orienta-
tions of the donor and acceptor
heme and His47 (with Ru(NH~)~
attached) sites in P. stutreri
cytochrome csJl. The buried pro-
pionate attached to the heme at
position 7 is also indicated (Ref. 39)
ligands, is considered relevant (37). Of all the the outset the distance separating the Ru(I1)
studies to date HIPIP is the only protein in from the metal active site, and the driving force
which the modified histidine is linked directly were expected to be the prime rate determining
by a short polypeptide chain to the active site factors. The results obtained clearly indicate
(Cys43 is co-ordinated to the Fe,S,). The that this is not so and that biological electron
through bond distance (saturated bonds!) is transfer is far from simple. It is concluded that
13A, whereas the through space distance is protein structure and the nature of the in-
only 7.9A. No benefits seem to accrue from tervening polypeptide material must be impor-
this type of attachment. tant. Of current interest is just how influential
Distances (d) for electron transfer coinciden- any intervening aromatic residues might be.
tally fall into two groups which are close to 12A Further results from this work are awaited.
and 8A, respectively, and values for the ther- Sperm-whale myoglobin, an oxygen binding
modynamic driving force (AEO) lie in a fairly protein, has also been used by the Gray group
narrow range 180-3mmV, see the Table. At to explore factors affecting electron transfer
\I p 10 I
v Fig. 3 Structural model show-
ing selected parts and relative
orientations of the Fe,S, active
site of high-potential iron-
sulphur protein, which is co-
ordinated by Cys43, and His42
which is ruthenium modified
(Ref. 37)
Met49
Platinum Metals Rev., 1988, 32, (4) 176
I
A Comparison of Rate Constants for Intramolecular Electron Transfer from
Ru(I1) in Ruthenium-Modified Metalloproteins
Protein Site of d AE O k Reference
Modification (A) (mV) (S-
Cytochrome c (horse) His33 11.8 180 30, 53a 38
Azurin (P.a.) His83 11.8 240 1.9 36
Plastocyanin (So.) His59 10-12 300 <0.26 35
Plastocyanin (A.v . His59 11.9 260 <0.08 35
Cytochrome c, 5, (P.s. His47 7.9 200 13 39
HlPlP (C.V.) His42 7.9 270 1 8b 37
a Values obtained by flash photolysis and pulse radiolysis. respectively
Values 1 and 13s-’ have been obtained by the Gray group for two HlPlPs modified at His42
reactivity. This protein has four surface Methods for the selective extraction of gold
histidines His12, His48, His81 and His116 (all and platinum ions from an aqueous solution at
at different distances 14.6-22.d from the pH>2 with cells or cell extracts of a micro-
heme Fe) each of which can be ruthenium- organism (green or blue-green algae) (40) and
modified. Four different singly-modified fungi (41), and subsequent elution with S-
Ru(NH,), derivatives have been prepared, and donor ligands are being actively explored. This
in this case a rate constant (k) against distance is the subject of a recent U.S. patent (40).
(d) relationship In k versus d appears to hold. Fungi have also be used to remove precious
Unlike the studies with electron transport pro- metals from dilute aqueous solutions (41).
teins reorganisation energy requirements at the Finally, mention should perhaps be made
active site are more significant. This is because here of the application of bacterial leaching pro-
metMb has an axial H,O ligand whereas cesses in the extraction of certain metals from
deoxyMb is five co-ordinate with no H,O. their ores which in the case of uranium and cop-
The myoglobin intramolecular reactions are per is already being exploited commercially (@,
somewhat slower therefore on this account. 43). 7liobacillus fernoxidam is for example able
The successful attachment of ruthenium to to oxidise iron pyrites (FeS) to soluble Fe”
proteins and various applications that have and SO:- with very beneficial results (43).
resulted poses the question: if ruthenium, why The use of other closely related micro-
not other metals? As far as electron transfer organisms, which are active in hot spring
studies are concerned it is of crucial importance regions, in order to make available the metallic
that the ruthenium stays attached (it is gold present in pyrites deposits is currently
substitution-inert t,>>I min) in both the receiving attention (4).
Ru(I1) and Ru(II1) oxidation states, and that This short review illustrates the wide range of
the reduction potentials are of the required potential applications of platinum metal com-
magnitude. Another metal which may behave pounds centering around their reactivity with a
in this way is osmium, but here very little is variety of different bio-molecules. Relevant
known of the (11) state solution chemistry. As areas stretch from laboratory applications to
far as the other platinum metals are concerned medicine via the further development of pro-
Rh(II1) and Ir(II1) are too inert to be readily at- cedures for mining and recovery. Many of the
tached, and the (11) states are comparatively applications make use of specific chemical pro-
rare. In the case of palladium and platinum the perties identified in the already well studied
stable oxidation states (11) and (Iv) are and extensive inorganic solution chemistry of
separated by two electrons. these elements.
Platinum Metals Rev., 1988, 32, (4) 177
Additional abbreviations used in the text, without explanation:
dppe = bis(diphenylphosphho)ethylene, EDTA = ethylenediamine tetraacetic acid,
terpy = terpyridine, py = pyridine, isn = isonicotinamide, Mb = myoglobin.
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