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the sulfates are gained as solvent-free compounds. The typical

reaction route for the preparation of sulfates uses concentrated

or even diluted sulfuric acid at ambient conditions so that

hydrates are frequently obtained. A second advantage of our

route is that, at elevated temperatures, H

2

SO

4

and SO

3

are very

strong oxidisers and even noble metals like platinum and

palladium are attacked. Finally, the reaction conditions are very

suitable to obtain crystalline or even single crystalline material,

which is mandatory for structure elucidations.

Architectures with sulfate ions

The sulfate ion SO

4

2-

may act either as a chelating or a bridging

ligand. The chelating co-ordination mode is especially useful for

the stabilisation of metal-metal bonded dimers, as might be seen

from the structure of the platinum(III)sulfate Pt

2

(SO

4

)

2

(HSO

4

)

2

(Fig. 2). The compound which has been obtained from the

unexpected oxidation of elemental platinum with sulfuric acid

shows a [Pt

2

] dumbbell co-ordinated by four chelating sulfate

(and hydrogen sulfate) ions to give the so-called ‘paddlewheel

motif’. The complex [W

2

O

3

(SO

4

)

6

]

6-

ion illustrates other typical co-

ordination options of the SO

4

2-

group (Fig. 3). Here, the sulfate ion

acts, on one hand, as a monodentate terminal ligand; on the

other hand it is a bidentate-bridging ligand and stabilises the

dimeric structure of the complex. An even more complex structure

is the niobium sulfate K[Nb

8

O

12

](SO

4

)

8

, showing cubic [Nb

8

O

12

]

moieties which are linked by tridentate bridging sulfate groups.

This compound can be obtained from elemental niobium,

concentred sulfuric acid, and K

2

SO

4

at 250°C.

Formation and functionality of disulfate ions

The reaction of various metals, M, with SO

3

-enriched sulfuric acid

(‘oleum’) or even neat SO

3

leads to compounds containing

disulfate groups, S

2

O

7

2-

. Thus, the high SO

3

content forces the

condensation of [SO

4

] tetrahedra under formations of these

anions. The disulfate ion might exhibit different co-ordination

modes. For example, in the tris(disulfato)-metallates [M(S

2

O

7

)

3

]

2-

,

they act as chelating ligands towards the metal centre (Fig. 4).

Interestingly, this structural motif can be gained for a large number

of M atoms, like germanium, tin, titanium, palladium, and

platinum. The most unusual complex of this type, however,

contains an octahedrally co-ordinated silicon atom in its centre.

These complexes are of special interest because they can occur in

two different enantiomorphs, i.e. they are chiral compounds.While

the metallates [M(S

2

O

7

)

3

]

2-

are monomeric complexes, the

disulfate group might also act as a linker for the formation of

dimeric complexes. This has been observed in the structure of

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Fig. 5 Structure of the molecular disulfate Re

2

O

4

Cl

2

(S

2

O

7

)

2

Fig. 3 Monodentate and bidentate-bridging sulfate groups in the

complex [W

2

O

3

(SO

4

)

6

]

6-

ion

Fig. 4 The tris(disulfato)-metallates [M(S

2

O

7

)

3

]

2-

with chelating S

2

O

7

2-

ions