Chemical Thermodynamics of Tin - Volume 12 - OECD Nuclear ...

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VIII Group 17 (halogen) compounds and complexes
VIII.3 Aqueous halide complexes
Although a number of investigations have been published dealing with the halide
complexes of tin(II) and tin(IV), relatively few spectroscopic data are available for these
species in aqueous solutions. 19 F and 119 Sn NMR studies indicated the formation of
2−q
three SnIq
species (q = 1 to 3) [1959CON/POU], [1990KOK/RAK]. In the aqueous
solutions of SnX 2 + HX (X = Cl – , Br – , I – 2 q
) several species (probably SnI
− q
, q = 1 to 4)
are in rapid equilibrium. When a mixture of hydrogen-halogenides were used, all the ten
trihalogenostannate(II) anions ([Sn(Cl x Br y I z )] – , x + y + z = 3) have been identified by
119 Sn NMR in the diethylether extract [1989COD/TAY]. A recent EXAFS study of
tin(II) chloride solutions revealed the presence of the complexes SnCl −
2
3
and SnCl −
4
at
chloride concentrations between 0.5 and 2.5 M [2000SHE/RAG]. The relative stability
of the tetrachlorido species increased with the temperature. Mössbauer spectroscopic
studies conducted on a series of frozen aqueous solutions of tin(II) are also in favour of
the presence of SnX −
3 (X = F – , Cl – , Br – ) anions [1981CHE/HSU], [1984CHE/HSU],
[1984HSU/CHE]. The SnX −
3 anions are pyramidal due to the presence of a sterically
active nonbonding pair of electrons. This electron pair makes the SnX −
3 anions
(X = Cl – , Br – ) excellent ligands, resulting in a wide range of metal-metal bounded
transition metal complexes, such as [Pt(SnCl 3 ) 5 ] 3– [1964YOU/GIL], [1984PRE/RUE].
Tin(IV) has strong hydrolyzing tendency, even in 1 M HX solution partially
2
hydrolyzed halogeno complexes ( SnX y(OH) − 6−
y) may be present, which is often
disregarded by the researcher. This is especially true in case of the NMR measurements,
+
since the aqua and hydroxido complexes (e.g. SnX
3(H2O) 3
, SnX 3 (H 2 O) 2 OH(aq),
SnX 3(H2O)(OH) −
2
2 and SnX 3(OH) − 3 ) are indistinguishable due to the fast proton
exchange. Using varied concentrations of the halide ions the presence of all
2
[SnX 6 (H2O) ] n −
− n n complexes (n = 0 to 6, X = Cl – and Br – ) have been identified by
119 Sn NMR spectroscopy, including the cis/trans and fac/mer isomers
[1989MAO/YOU], [1989MAO/YOU2], [1992TAY/COD]. Mixtures of SnCl 4 and
SnBr 4 in aqueous solution give rise to practically all possible aquohalostannate(IV)
species with the composition of [SnCl x Br y (H 2 O)z] z–2 (where x + y + z = 6)
[1992TAY/COD]. An EXAFS study indicated, that in solution containing 0.11 M HCl
2
and 2.0 M NaCl, aqueous tin(IV) occurs exclusively as the SnCl − 6 complex between 25
and 250 °C [2000SHE/RAG]. Mössbauer spectroscopic investigations reflected the
stepwise formation of the [SnX 6 ] 2– (X = F – , Cl – ) complexes [1981CHE], [1981CHE2],
[1991KHR/ZAP].
VIII.3.1 Aqueous fluoride complexes
VIII.3.1.1 Aqueous Sn(II) fluorides
Polarographic [1954SCH/DAV], [1970BON/TAY], [1990BEA/MEN], potentiometric
measurements, using tin amalgam [1961CON/PAU], [1968HAL/SLA] and/or fluoride
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012