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Re

2

O

4

Cl

2

(S

2

O

7

)

2

, which is the first known molecular disulfate (Fig.

5). The compound results from the reaction of ReCl

5

and oleum.

In most cases we observed higher co-ordination numbers and

stronger linkage for the S

2

O

7

2-

ions. A very exciting example is the

palladium compound Pd(S

2

O

7

), which forms in the reaction of

elemental palladium and SO

3

. The compound exhibits a Pd

2+

ion

in octahedral co-ordination of oxygen atoms, leading to the

paramagnetic behaviour of the compound (Fig. 6). Moreover,

below 11.6K a ferromagnetic ordering is observed, the first

example of such behaviour for a Pd

2+

compound.

Polysulfates; rare species

As mentioned above, the tendency to form polyanions is quite

limited for sulfates, at least compared to silicates and

phosphates. Thus, higher polysulfates than the discussed

polysulfates have rarely been seen. Nevertheless, our special

reaction conditions using very high concentrations of SO

3

indeed

yielded some of these species. They have been obtained either in

isolated form or as co-ordinating ligands in complexes; for

example, in the noble metal complexes [Au(S

3

O

10

)

2

]

-

and

[Pd(S

4

O

13

)

2

]

2-

(Fig. 7), the noble metal atoms are attached by two

chelating trisulfate and tetrasulfate ions, respectively. Compared

to the isolated species observed, for instance, in Pb(S

3

O

10

) and

(NO

2

)

2

[S

4

O

13

], the co-ordination of the polysulfate anions leads

to a significant change of the bond lengths within the anions.

Nevertheless, theoretical calculations reveal that the stability of

polysulfate ions decreases with increasing chain length, so the

preparation of higher polysulfates is really challenging. Up to now

we have been able to extend the series up to a hexasulfate,

prepared as the potassium salt K

2

(S

6

O

19

).

Sulfate derivatives

The most simple derivatisation of sulfate ions is the formal

substitution of one oxygen atom for either an OH, NH

2

, CH

3

or CF

3

group. This leads to the formation of a hydrogen sulfate,

amidosulfate, methanesulfonate, and trifluoromethanesulfonate

anion, respectively (Fig. 8). These well known anions are very

similar to each other as they all have a tetrahedral shape and the

same charge of −1. In this way they are also very similar to the

perchlorate anion; however, due to a different charge distribution

they are the better ligands compared to the latter. Because the

hydrogen sulfate ion is always present in sulfuric acid, it is clear

that hydrogen sulfates may readily occur in reaction with sulfuric

acid. The above-mentioned example of Pt

2

(SO

4

)

2

(HSO

4

)

2

shows

that this is indeed the case. The preparation of amidosulfates is

somewhat special because the related amidosulfuric acid is a

solid due its zwitterionic character. Contrastingly, methanesulfonic

and trifluoromethanesulfonic acid (‘triflic acid’) are liquids with

properties very similar to sulfuric acid. That means that our typical

preparation methods can be easily applied, and a plethora of new

anhydrous compounds of these acids could be gained.

Because one vertex of the tetrahedral anion cannot be used for

co-ordination, i.e. the CH

3

and the CF

3

group, the co-ordination

behaviour is very different from that of the sulfate (and also

hydrogen sulfate) ion. In general this leads to a weaker linkage of

the anions, and very often chain or layer-type structures are seen.

For example, the palladium methanesulfonate Pd(CH

3

SO

3

)

2

has a

chain structure with the anions acting as bidentate-bridging

ligands. These chains are connected in the crystal structure only

by weak interactions, often leading to the mechanical lability of

the compounds.This is even more true for trifluoromethanesulfonates

(‘triflates’) because the CF

3

group is even more weakly interacting.

A nice example is the structure of Zr(CF

3

SO

3

)

4

, which shows

chains of Zr

4+

ions linked by eight bidentate-bridging triflate

groups (Fig. 9). The rods formed in that way are ‘decorated’ by CF

3

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Fig. 8 Derivatives of the sulfate anion

Fig. 6 Single crystals of Pd(S

2

O

7

)

Fig. 7 The [Pd(S

4

O

13

)

2

]

2-

ion