Pan European Networks - Horizon 2020 - page 34

the most dominant for these sulfates. For
palladium, no reaction is observed under the
same conditions, and even at a higher
temperature, the metal is not attacked at all.
The oxidation of elemental palladium is only
possible if a sulphur trioxide-rich sulphuric acid,
so-called ‘oleum’, is used, and in this case,
violet crystals of Pd(HS
2
O
7
)
2
are obtained.
This hydrogen disulfate has a unique structure
because it shows the palladium atom in an
untypical octahedral co-ordination of oxygen
atoms. This leads to the paramagnetic
behaviour of the compound and even low-
temperature ferromagnetic ordering, which has
not been observed before for divalent palladium
compounds. If the oxidation reaction is carried
out with neat SO
3
, there is also no formation of
tri or tetravalent palladium compounds
observed. Instead, Pd(S
2
O
7
) forms, which is
also a paramagnetic palladium(II) disulfate.
Fig. 2 shows the octahedral co-ordination of
Pd
2+
ions by monodentate HS
2
O
7
ions on the
left hand side, whilst the graph illustrates the
move towards paramagnetic behaviour of
Pd(HS
2
O
7
)
2
on the right hand side.
Thermally labile nitrates
If the pure metals are not used in the reactions
with concentrated mineralic acids and their
anhydrides, suitable compounds of palladium
T
he elements palladium (Pd) and platinum (Pt) are neighbours in
group ten of the periodic table and play an important role in
various fields of chemistry. In addition, group ten comprises
nickel and darmstadtium. Nickel, palladium and platinum are also
classified as transition metals.
In their element form, both palladium and platinum are used as inert
materials, for example for building laboratory apparatus, designing
jewellery and as catalysts for various purposes; they are inactive towards
oxygen and are noble metals. The chemistry of palladium and platinum
is strongly stamped by the oxidation states +II and +IV. The latter is easier
to realise with the heavier group member, i.e. platinum, but the structures
of the compounds are very similar for the same oxidation states.
By contrast, in the course of our research, which has focused on the
preparation of oxoanionic compounds and the elucidation of their
structures and properties, we have discovered that palladium and
platinum often behave very differently with respect to both reactivity
and structures.
Metal-metal bonding
Undertaking investigations in this field, a very striking example is the
reaction of the metals with concentrated sulphuric acid under harsh
conditions. Typically, both metals are regarded as resistant materials
against mineralic acids. However, at a temperature above 300°C, metallic
platinum reacts with sulphuric acid under formation of Pt
2
(SO
4
)
2
(HSO
4
)
2
.
Interestingly, this reaction leads to a complex sulfate of trivalent platinum
bearing a dumbbell-shaped [Pt
2
]
6+
cation with a short platinum-platinum
bond. This cation is stabilised by the co-ordination of four chelating
tetrahedral anions forming the typical ‘paddlewheel’ motif. Fig. 1 shows
the dumbbell-shaped [Pt
2
]
6+
cation surrounded by four chelating and two
monodentate tetrahedral anions (SO
4
2−
and HSO
4
) as a typical structural
motif for platinum(III) sulfates.
As a consequence, this building unit has been found in a large number
of platinum sulfates, proving that the uncommon oxidation state +III is
I S S U E S I X
H O R I Z O N 2 0 2 0 P R O J E C T S : P O R TA L
34
E X C E L L E N T S C I E N C E
The unlikely neighbours
The metals platinum and palladium ‘do not behave like neighbours in the periodic
table’ when assessing their co-ordination compounds with oxoanions, as
Oldenburg University’s
Professor Dr Mathias S Wickleder
explains
Fig. 1
Fig. 2
1...,24,25,26,27,28,29,30,31,32,33 35,36,37,38,39,40,41,42,43,44,...244
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