Moessbauer spectra of tin(IV) iodide complexes

Article abstract of DOI:10.1039/a905917b

The first examples of compounds containing the [SnR₂I₃]^- anion, i.e. [Buⁿ₄N][SnR₂I₃] (R = Me or Ph), have been prepared and characterized by IR, Raman and ¹¹⁹Sn Moessbauer spectroscopies, and X-ray powder diffraction data. The experimental data are consistent with a trigonal bipyramidal structure, with the R groups in equatorial positions, for the [SnR₂I₃]^- anions. Octahedral [SnR₂I₄]^2- anions could not be isolated and it is shown that the previously reported [Buⁿ₄N]₂[SnMe₂I₄] is actually a 1:1 mixture of Buⁿ₄NI and [Buⁿ₄N][SnMe₂I₃]. The X-ray powder structure of Cs₂SnI₆ has been refined by the Rietveld method. The compound crystallises in the cubic Fm3m space group with a = 11.6410(3) A and a Sn-I distance of 2.864(1) A. The Sn-I distances in Cs₂SnI₆, SnI₄(bipy), SnI₄(Ph₃PO)₂ and SnI₄(Ph₂SO)₂ are related to the Moessbauer quadrupole splitting, showing that ligands that are weaker donors than I^- (i.e. have a more positive partial quadrupole splitting) give rise to Sn-I distances that are shorter than for the [SnI₆]^2- anion. The Royal Society of Chemistry 1999.

Full text of DOI:10.1039/a905917b

View Article Online / Journal Homepage / Table of Contents for this issue
Mössbauer spectra of tin(IV) iodide complexes
David Tudela,*a Antonio J. Sánchez-Herencia,a Marcos Díaz,a Ramón Fernández-Ruiz,b
Nieves Menéndezc and Jesús D. Torneroc
a Departamento de Química Inorgánica, Universidad Autónoma de Madrid, 28049-Madrid,
Spain. E-mail: david.tudela@uam.es
b Servicio Interdepartamental de Investigación, Universidad Autónoma de Madrid,
28049-Madrid, Spain
c Departamento de Química Física Aplicada, Universidad Autónoma de Madrid, 28049-Madrid,
Spain
Received 22nd July 1999, Accepted 23rd September 1999
The first examples of compounds containing the [SnR2I3]Ϫ anion, i.e. [Bun N][SnR2I3] (R = Me or Ph), have been
4
prepared and characterized by IR, Raman and 119Sn Mössbauer spectroscopies, and X-ray powder diffraction
data. The experimental data are consistent with a trigonal bipyramidal structure, with the R groups in equatorial
positions, for the [SnR2I3]Ϫ anions. Octahedral [SnR2I4]2Ϫ anions could not be isolated and it is shown that the
previously reported [Bun N]2[SnMe2I4] is actually a 1:1 mixture of Bun NI and [Bun N][SnMe2I3]. The X-ray
4
4
4
powder structure of Cs2SnI6 has been refined by the Rietveld method. The compound crystallises in the cubic
Fm3m space group with a = 11.6410(3) Å and a Sn–I distance of 2.864(1) Å. The Sn–I distances in Cs2SnI6,
SnI4(bipy), SnI4(Ph3PO)2 and SnI4(Ph2SO)2 are related to the Mössbauer quadrupole splitting, showing that
ligands that are weaker donors than IϪ (i.e. have a more positive partial quadrupole splitting) give rise to Sn–I
distances that are shorter than for the [SnI6]2Ϫ anion.
Mössbauer parameters are concerned, the isomer shift (IS) is
Introduction
sensitive to the electronegativity of both the halogen X and the
donor atom of the ligand L. Indeed, a review of the reference
bank of the Mössbauer Effect Data Centre (University of
North Carolina, Asheville, USA) has shown that the average IS
of SnCl4 complexes with O, N, S, P and As donor ligands are
(standard deviations in parentheses) 0.37 (0.08), 0.42 (0.08),
0.66 (0.06), 0.75 (0.10) and 0.78 (0.10) mm sϪ1, respectively.25–27
Furthermore, for the same ligands, the IS of SnF4, SnBr4 and
SnI4 complexes are, respectively, 0.66 (0.02) mm sϪ1 lower, 0.27
(0.07) mm sϪ1 higher, and 0.63 (0.11) mm sϪ1 higher than for
SnCl4 complexes.25 Therefore, IS values can be used to get
information about the atoms bonded to tin, and we have shown
that some literature IS data that did not agree with our expect-
ations were wrong.25–27 This information is particularly useful in
the case of ambidentate ligands.
In recent years there has been an increasing interest in tin-based
antitumour drugs.1 In particular, many octahedral diorganotin
dihalide complexes, SnR2X2L2, exhibit antitumour activity
against P388 lymphocytic leukaemia,2–4 and recently bis-
(acetone thiosemicarbazone-S)dichlorodiphenyltin() has
shown significant cytotoxicity against several tumours.5 For
that reason, several research groups have been interested in the
synthesis and structural study of SnR2X2L2 complexes.6–13
Some attempts to correlate structural data and antitumour
activity,4,6 have concluded that active complexes have Sn–N
bond lengths ≥2.39 Å. This fact suggests that dissociation of
the ligands is an important step in the mechanism of action,
and points to the importance of getting information about the
tin–ligand bond strength. Therefore, a better understanding
of the antitumour activity of octahedral diorganotin dihalide
complexes can be obtained with the optimization of spectro-
scopic techniques that can provide information about the struc-
ture of the complexes and the tin–ligand bond strength. Several
physical techniques have been used to aid structural elucidation
of SnR2X2L2 complexes.9 In the case of Mössbauer spectro-
scopy the most comprehensive work is that reported by Crowe
and Smith14 in 1982. Although SnR2X2L2 complexes usually
have both R groups in trans positions, Mössbauer spectroscopy
has shown the existence of cis–trans isomerism,15,16 as con-
firmed crystallographically for Sn(C6H4Cl-4)2Cl2ؒ4,4Ј-Me2bipy
(4,4Ј-Me2bipy = 4,4Ј-dimethyl-2,2Ј-bipyridine).17 Furthermore,
in the case of monodentate L ligands, the X and L groups can
be in cis or trans positions, and the structure of SnEt2Cl2-
(OPPh3)2 contains two different isomers in the same unit cell.18
Therefore, the Mössbauer parameters can be affected by several
variables such as the isomer formed (five possible geometrical
isomers, one of them optically active), the C–Sn–C angle, and
the nature of the R, X, and L groups. In order to understand
the effect of the different variables on the Mössbauer param-
eters it is better to study first related simpler compounds such
as SnX4L2 or [SnR2X4]2Ϫ. In the case of SnX4L2 complexes
there is a renewed interest in their structure.19–24 As far as their
The Mössbauer quadrupole splitting (QS) of octahedral
SnX4L2 complexes can give information, not only about the
isomer formed, but also about the Sn–X bond lengths.28–30
Indeed, we have reported linear relationships between the aver-
age tin–halogen distance and the partial quadrupole splitting
28
of the ligands (pqs) for SnCl4L2 and SnBr4L2 complexes.29,30
These correlations can yield information about the Sn–L bond
strengths and the sign of the quadrupole splitting, as well as
rationalize experimental Sn–X distances and QS data in terms
of the donor properties of the ligands. Furthermore, these
correlations can be used to estimate Sn–X distances, which are
expected to agree with experimental values within 0.02 Å, and
discrepancies between calculated and reported distances
enabled us to detect the incorrectness of a published crystal
structure.27 It is important to note that the correlations are
empirical and there seems to be no theoretical basis for linear
correlations between interatomic distances and QS data. For
that reason the validity of the correlations should be checked
for other tin tetrahalide complexes and, in particular, for SnI4L2
complexes, because iodide has a pqs value significantly different
from those of the other halides.31 Furthermore, iodine is the
most polarizable of the halogen atoms, and the Sn–I distances
could be more variable than the other tin–halogen distances.
J. Chem. Soc., Dalton Trans., 1999, 4019–4023
This journal is © The Royal Society of Chemistry 1999
4019

View Article Online
Only a few crystal structures of octahedral tin() iodide com-
plexes are available in the literature,20,32–35 and we have tried to
get accurate Mössbauer data for them. Unfortunately, only one
or two red-orange crystals of cis-[SnI4{MeS(O)(CH2)3SMe}2]
could be isolated, and attempts to prepare it in sufficient quan-
tity to enable spectroscopic characterization were not success-
powder spacings (Cu-Kα, Å) 10.92(53), 9.73(55), 9.41(100),
7.39(21), 6.71(55), 6.30(22), 6.11(32), 5.87(22), 4.85(38),
4.55(24), 4.38(28), 4.05(51) and 3.52(49).
“[Bun N]2[SnPh2I4]”. Orange solid, mp 95–97 ЊC; δH (CDCl3)
4
1.00 (24 H, t, J = 7.3, CH3CH2CH2CH2N), 1.46 (16 H, sextet,
J = 7.3 Hz, CH3CH2CH2CH2N), 1.68 (16 H, m, CH3CH2CH2-
CH2N), 3.35 (16 H, m, CH3CH2CH2CH2N) and 7.40–7.78 (10
ful,20 while SnI4(PPrn )2 35 was too unstable to run its Mössbauer
3
spectrum.36 Indeed, it is really a difficult task to obtain pure
tin() iodide complexes with phosphine ligands.37 An import-
ant point in the correlation of Sn–I distances with QS data is
the perfectly octahedral [SnI6]2Ϫ anion with QS = 0 mm sϪ1. The
X-ray powder crystal structures of cubic Rb2SnI6 and Cs2SnI6
were reported in 1939 to give Sn–I distances of 2.85 Å.38 Bear-
ing in mind that the reported standard deviation of 0.05 Å
is too high for today’s standards, we have refined the X-ray
powder structure of Cs2SnI6. In order to get a significant correl-
ation between Sn–I distances and Mössbauer QS data, a broad
range of Sn–I distances needs to be considered, and the longest
Sn–X distances for SnX4L2 complexes are obtained for
[SnR2X4]2Ϫ anions.28,29,39 While Clark and Wilkins40 could not
isolate [SnMe2I3]Ϫ nor [SnMe2I4]2Ϫ salts, the synthesis of
H, m, Sn(C6H5)2); IR (νmax/cmϪ1) lattice Bun NI (see below)
4
530vw, νasym(SnC) 279s, νsym(SnC) and/or phenyl u 245s, 223m,
200s, ν(SnI) 165s; Raman (νmax/cmϪ1, decomposes in the laser
beam) ν(SnI) 167vs; Mössbauer (mm sϪ1) IS = 1.49, QS = 2.73,
Γ1 = 0.82, Γ2 = 0.81; X-ray powder spacings (Cu-Kα, Å)
11.20(38), 9.37(62), 9.24(50), 8.74(47), 8.27(100), 7.79(84),
6.63(25), 5.98(33), 5.58(22), 4.92(36), 4.12(21), 4.02(42),
3.94(57), 3.87(34) and 3.59(40).
[Bun N][SnPh2I3]. Reddish solid, mp 103–104 ЊC; δH (CDCl3)
4
1.02 (12 H, t, J = 7.3, CH3CH2CH2CH2N), 1.48 (8 H, sextet,
J = 7.3 Hz, CH3CH2CH2CH2N), 1.70 (8 H, m, CH3CH2CH2-
CH2N), 3.37 (8 H, m, CH3CH2CH2CH2N) and 7.40–7.78 (10 H,
m, Sn(C6H5)2); IR (νmax/cmϪ1) νasym(SnC) 279s, νsym(SnC) and/or
phenyl u 245s, 223m, 200s, ν(SnI) 165s; Raman (νmax/cmϪ1
,
[Bun N]2[SnMe2I4] was reported later by Harrison and co-
decomposes in the laser beam) ν(SnI) 167vs; Mössbauer (mm
4
workers.41 In order to confirm the existence of [SnMe2I4]2Ϫ
anions, and get X-ray and Mössbauer data for them, we have
s
Ϫ1): IS = 1.50, QS = 2.72, Γ1 = 0.80, Γ2 = 0.79; X-ray powder
spacings (Cu-Kα, Å): 11.17(35), 9.25(54), 8.71(79), 8.24(100),
7.77(62), 6.62(25), 5.98(35), 5.57(17), 4.12(21), 4.02(39),
3.95(20), 3.86(23) and 3.59(28).
repeated the synthesis of [Bun N]2[SnMe2I4], and prepared
4
solids of composition [Bun N][SnMe2I3], [Bun N]2[SnPh2I4] and
4
4
[Bun N][SnPh2I3]. As it will be demonstrated below, it is con-
4
cluded that [SnMe2I4]2Ϫ anions have no real existence.
Physical measurements
1
The H NMR spectra were recorded on a Bruker AMX-300
instrument, infrared spectra between 4000 and 200 cmϪ1 on a
Perkin-Elmer 1650 FT-IR instrument, using Nujol mulls
between CsI windows, and between 550 and 50 cmϪ1 on a
Bruker IFS 66 V by diffuse reflectance. Raman spectra were
measured at room temperature on a Dilor XY spectro-
photometer, using the Raman microscope and an argon laser
(5145 Å), with the polycrystalline samples sealed in capillary
tubes. Mössbauer spectra at liquid N2 temperature (at room
temperature for Cs2SnI6) were obtained using the system and
conditions described previously.44 Sample thickness was 11 mg
of natural tin per cm2. The isomer shift is relative to BaSnO3 at
room temperature, and the reproducibility of the Mössbauer
parameters was 0.02 mm sϪ1. Computer fitting of the spectra
gave χ2 values of 0.98–1.20 (with 249–252 degrees of freedom),
within the range of statistically satisfactory values (0.80–
1.22).45 X-Ray powder diffraction (XRPD) data were obtained
with a Siemens D-5000 diffractometer using Cu-Kα radiation.
Experimental
Preparations
All syntheses were carried out under argon by using standard
Schlenk techniques. The compounds SnI4,42 SnI4(Ph2SO)2,33
SnI4(Ph3PO)2,34 SnI4(bipy),32 and Cs2SnI6 were prepared by
38
literature methods, SnMe2I2 and SnPh2I2 by reaction of aque-
ous HI solutions (57%) with ethanolic suspensions of SnMe2O
(K&K) or SnPh2O (M&T), respectively, in 2:1 molar ratio. The
diorganotin iodides were isolated by vacuum elimination of the
solvent and vacuum sublimation, and characterized by elem-
ental analysis, melting point,43 and IR and H NMR spectro-
1
scopies. The Mössbauer spectrum of SnMe2I2, not previously
reported, gave the following parameters: IS = 1.58, QS = 2.79,
Γ1 = 0.91, Γ2 = 0.97 mm sϪ1. The salts “[Bun N]2[SnR2I4]” and
4
[Bun N][SnR2I3] (R = Me or Ph) were prepared by mixing
4
stoichiometric amounts of SnR2I2 and Bun NI, in CH2Cl2
4
solution, in the absence of light, stirring for ca. 30 min, and
eliminating the solvent in vacuo. Some characterization data
are given below.
Structure refinement of Cs2SnI6
The XRPD data were measured with graphite-monochromated
Cu-Kα radiation (λ = 1.5418 Å) on a Siemens D-5000 diffract-
ometer equipped with a Si(Li) counter. The generator was
operated at 40 kV and 40 mA. The sample was finely ground in
an agate mortar and packed on a flat aluminium sample holder.
Data were collected in the range 6 ≤ 2θ ≤ 100Њ with a step size of
0.02Њ and a counting time of 10 s per step. Starting from the
published unit cell and positional parameters,38 the structure
was refined by the Rietveld method46 in the cubic Fm3m space
group (no. 225) by using the Young–Sakthivel program (version
DBWS-9006PC),47 with a pseudo-Voigt peak shape, back-
ground correction by interpolation between 22 points, asym-
metry correction for angles less than 50Њ, and a correction for
preferred orientation.
“[Bun N]2[SnMe2I4]”. Yellow solid, mp 99–100 ЊC; δH
4
(CDCl3) 1.00 (24 H, t, J = 7.3, CH3CH2CH2CH2N), 1.46 (16 H,
sextet, J = 7.3, CH3CH2CH2CH2N), 1.68 (16 H, m, CH3CH2-
CH2CH2N), 1.72 (6 H, s, J(119SnH) = 65.7 Hz, Sn(CH3)2) and
3.35 (16 H, m, CH3CH2CH2CH2N); IR (νmax/cmϪ1) νasym(SnC)
554s, lattice Bun NI (see below) 529m, νsym(SnC) 502m and
4
ν(SnI) 178s; Raman (νmax/cmϪ1) νasym(SnC) 556w, νsym(SnC)
502vs and ν(SnI) 178vs; Mössbauer (mm sϪ1) IS = 1.63, QS =
3.33, Γ1 = 0.74, Γ2 = 0.76; X-ray powder spacings (Cu-Kα, Å)
10.91(32), 9.75(45), 9.39(100), 7.38(30), 6.72(18), 6.30(15),
6.11(23), 5.86(16), 4.92(19), 4.85(21), 4.55(14), 4.37(21),
4.04(25), 3.94(32) and 3.51(21).
[Bun N][SnMe2I3]. Yellow solid, mp 119–120 ЊC; δH (CDCl3)
4
1.02 (12 H, t, J = 7.3, CH3CH2CH2CH2N), 1.48 (8 H, sextet,
J = 7.3, CH3CH2CH2CH2N), 1.69 (8 H, m, CH3CH2CH2-
CH2N), 1.72 (6 H, s, J(119SnH) = 65.3 Hz, Sn(CH3)2) and
3.36 (8 H, m, CH3CH2CH2CH2N); IR (νmax/cmϪ1) νasym(SnC)
Results and discussion
Diorganotin iodide anions
While octahedral [SnR2X4]2Ϫ (R = Me, Et, Ph or vinyl; X = F,
Cl or Br) anions are well known, and several crystal structures
554s, νsym(SnC) 502m and ν(SnI) 178s; Raman (νmax/cmϪ1) νasym
-
(SnC) 556w, νsym(SnC) 503vs and ν(SnI) 178vs; Mössbauer
(mm sϪ1): IS = 1.64, QS = 3.32, Γ1 = 0.77, Γ2 = 0.77; X-ray
4020
J. Chem. Soc., Dalton Trans., 1999, 4019–4023

View Article Online
have been reported,29,39,48–59 the only report on the analogous
Table 1 Crystallographic data for Cs2SnI6
[SnR2I4]2Ϫ anions deals with the synthesis and IR spectrum of
[Bun N]2[SnMe2I4].41 On the other hand, Clark and Wilkins40
Empirical formula
M
Crystal system
Space group
Cs2I6Sn
1145.95
cubic
Fm3m (no. 225)
11.6410(3)
4
4.825
0.103(2)
4
concluded that the capacity to form both [SnMe2X3]Ϫ
and [SnMe2X4]2Ϫ ions extends over the fluoride, chloride and
bromide, but neither type of iodo-complex could be isolated.
In order to clarify the situation, we have repeated the synthesis
a/Å
Z
of [Bun N]2[SnMe2I4]. While only one ν(SnC) would be IR
Dc/g cmϪ3
4
active for octahedral [SnMe2I4]2Ϫ, the IR spectrum shows three
Preferred orientation, G(111)
No. data points
No. contributing reflections
No. refined variables
Rexp
Rp
Rwp
RBragg
χ2
bands (at 554, 529 and 502 cmϪ1) between 500 and 600 cmϪ1
.
4700
128 (64 Kα doublets)
14
0.085
0.087
0.114
0.050
1.80
Furthermore, the concordance of activities between IR and
Raman spectra precludes a centrosymmetric octahedral struc-
ture. In order to check whether the composition [Bun N]2-
4
[SnMe2I4] corresponds actually to a 1:1 mixture of Bun NI and
4
[Bun N][SnMe2I3], we have prepared this last compound.
4
Indeed, both [Bun N]2[SnMe2I4] and [Bun N][SnMe2I3] have
4
4
Rp = Σ|yi,o Ϫ yi,c|/Σyi,o, Rwp = [Σwi(yi,o Ϫ yi,c)2/Σwiyi,o
]
1/2, RBragg = Σ|Ii,o Ϫ
2
essentially the same Mössbauer parameters, and the IR spec-
Ii,c|/ΣIi,o, Rexp = Rwp/(χ2)1/2, χ2 = Σwi(yi,o Ϫ yi,c)2/(Nobs Ϫ Nvar), where yi
and Ii are the (observed or calculated) profile and integrated intensities,
respectively, wi is a weighting factor (taken as 1/yi,o), and Nobs and Nvar
are the number of observations and variables, respectively.
trum of [Bun N]2[SnMe2I4] agrees with the sum of the spectra
4
corresponding to Bun NI and [Bun N][SnMe2I3] (a finely
4
4
ground 1:1 mixture of Bun NI and [Bun N][SnMe2I3] has the
4
4
same melting point and IR spectra as those of [Bun N]2-
4
[SnMe2I4]). The IR spectrum of Bun NI shows a band at 529
4
cmϪ1 that does not appear in the spectra of other Bun Nϩ salts,
4
and could correspond to a lattice band, thus leaving the bands
at 554 and 502 cmϪ1 as the antisymmetric and symmetric,
respectively, tin–carbon stretching vibrations of a bent SnMe2
arrangement. The non-existence of [Bun N]2[SnMe2I4] is
4
unambiguously confirmed by the X-ray powder diffraction
patterns because all the peaks corresponding to [Bun N]-
4
[SnMe2I3] are present in the pattern of [Bun N]2[SnMe2I4],
4
that also shows the peaks corresponding to Bun NI. Anions
4
[SnMe2I4]2Ϫ are not formed either in CDCl3 solutions because
even 7:1 mixtures of Bun NI and SnMe2I2 gave a J(119SnH)
4
coupling constant as low as 76.5 Hz, that corresponds to a
C–Sn–C angle of about 127Њ.60 In order to check whether other
[SnR2I4]2Ϫ anions can be isolated in the solid state we have pre-
pared solids of composition [Bun N]2[SnPh2I4] and [Bun N]-
4
4
Fig. 1 Rietveld refinement plot for polycrystalline Cs2SnI6. Reflection
markers and the difference plot are also included.
[SnPh2I3]. Again, melting point, vibrational and Mössbauer
spectra, and X-ray powder diffraction data confirm that the
solid of composition [Bun N]2[SnPh2I4] is actually a 1:1 mix-
4
iodine should have a weaker tendency than chlorine to form
bridges, it would be reasonable to expect monomeric trigonal
bipyramidal structures for the [SnR2I3]Ϫ anions. Indeed, the
C–Sn–C angles calculated from the Mössbauer QS values31
ture of Bun NI and [Bun N][SnPh2I3]. Therefore, we have dis-
4
4
proven previous claims on the existence of [SnR2I4]2Ϫ anions,41
and characterized the first examples of compounds containing
[SnR2I3]Ϫ anions, i.e. [Bun N][SnR2I3] (R = Me or Ph). The
are 137Њ for [Bun N][SnMe2I3] (compare with 140(2)Њ for
4
Mössbauer parameters of [Bun N][SnMe2I3] and [Bun N]-
4
[SnMe2Cl(terpy)][SnMe2Cl3]69) and 127Њ for [Bun N][SnPh2I3]
4
4
[SnPh2I3] are reasonable. So, the IS of [Bun N][SnPh2I3]
4
(compare with 127.51(8)Њ for [Et4N][SnPh2Cl3]63 and 135.78(8)Њ
for [C10H9ONؒHؒNOH9C10][SnPh2Cl3]59). The fact that the
C–Sn–C angles are larger than the ideal value of 120Њ, and
larger for [SnMe2X3]Ϫ than for [SnPh2X3]Ϫ anions, is in agree-
ment with Bent’s isovalent rehybridization principle.70 The tin
atom concentrates greater s character in the orbitals directed to
the less electronegative Me groups, thus increasing the C–Sn–C
angle. The electronegativity considerations also agree with the
bond angles according to the VSEPR model.71 The absence of
data for compounds containing [SnR2I4]2Ϫ anions will severely
restrict the range for any possible corrrelation between Sn–I
distances and Mössbauer QS data for SnI4L2 complexes, but it
is shown below that a significant trend can still be observed.
4
(IS = 1.50 mm sϪ1) is lower than that of [Bun N][SnMe2I3]
4
(IS = 1.64 mm sϪ1), in agreement with the higher electro-
negativity of the Ph group as compared with Me. Furthermore,
a comparison of the Mössbauer parameters of [Bun N]-
4
[SnMe2I3] (IS = 1.64, QS = 3.32 mm sϪ1) with those correspond-
61
ing to [Et4N][SnMe2Br3] (IS = 1.52, QS = 3.41 mm mm sϪ1
)
62
and [Bun N][SnMe2Cl3] (IS = 1.39, QS = 3.40 mm sϪ1
)
shows
4
similar QS values, and IS values increasing as the electro-
negativity of the halogen diminishes. A similar trend is
observed when comparing the Mössbauer parameters of
[Bun N][SnPh2I3] (IS = 1.50, QS = 2.72 mm sϪ1) and [Bun N]-
4
4
[SnPh2Cl3] (IS = 1.20, QS = 2.56 mm sϪ1).62 Finally, on going
from SnMe2I2 (IS = 1.58, QS = 2.79 mm sϪ1) and SnPh2I2
(IS = 1.51, QS = 2.38 mm sϪ1
)
61 to [Bun N][SnR2I3] (R = Me or
4
Powder structure of Cs2SnI6 and Mössbauer spectra of SnI4L2
complexes
Ph) the IS remains essentially unchanged and the QS increases
by ca. 0.3–0.5 mm sϪ1. It is surprising that the Mössbauer
parameters of such a simple compound as SnMe2I2 had not
been reported previously. While the crystal structures reported
for [SnPh2Cl3]Ϫ anions show isolated ions with the phenyl
groups in the equatorial positions of a trigonal bipyramidal
structure,59,63 [SnMe2Cl3]Ϫ anions are usually associated into
dimers through more or less strong chlorine bridges.50,64–68
Nevertheless, the structure of [SnMe2Cl(terpy)][SnMe2Cl3]
contains isolated [SnMe2Cl3]Ϫ anions.69 Bearing in mind that
Crystal data and refined values for the structural parameters of
Cs2SnI6 are given in Table 1. A plot of the experimental X-ray
diffraction pattern and difference data after Rietveld refinement
are shown in Fig. 1, and final positional and thermal param-
eters are collected in Table 2. The [SnI6]2Ϫ ion has a crystallo-
graphically imposed regular octahedral structure (and, hence,
a zero value for the Mössbauer QS) with a Sn–I distance of
2.864(1) Å. That distance is longer than the average distances
J. Chem. Soc., Dalton Trans., 1999, 4019–4023
4021

View Article Online
Table 2 Atomic coordinates and isotropic thermal parameters for
Cs2SnI6 with estimated standard deviations in parentheses
Table 3 Mössbauer data (mm sϪ1) and crystallographic average Sn–I
distances (Å) for SnI4L2 complexes
B/Å2
Cs2SnI6
SnI4(bipy)
SnI4(Ph3PO)2
SnI4(Ph2SO)2
a
X/a
Y/b
Z/c
Sn
Cs
I
0
0.25
0.2460(1)
0
0.25
0
0
0.25
0
2.00(7)
4.51(7)
3.96(4)
IS
QS
Γ1
1.37
0
0.80
—
4
0
1.09
0.28
1.02
0.55
0.94
0.84
19
1.10
2.798(2)
1.15
0.62
0.89
0.85
17
1.24
2.784(1)
1.01
Γ2
1.01b
23
% effect
4 ∆pqsc
d(Sn–I)
0.56
2.802(2)
2.864(1)
a At room temperature. b The two lines were constrained to have iden-
tical width. c ∆pqs = pqs(L) Ϫ pqs(IϪ).
and the IS values are in reasonable agreement with ours. While
the spectra of the SnI4L2 complexes were collected at liquid N2
temperature, the spectrum of Cs2SnI6 was registered at room
temperature because we know from the XRPD data that it is
cubic at that temperature and, therefore, its Mössbauer QS is
exactly zero (we do not know if it may have a phase transition at
lower temperatures). The lower percent effect in the room tem-
perature spectrum of Cs2SnI6 is reflected in the appearance of
the spectrum (see Fig. 2). The Mössbauer parameters are
collected in Table 3 with the Sn–I distances. The IS values of
the SnI4L2 complexes are consistent with those expected for
octahedral SnI4 complexes with N and O donor ligands,25 while
the QS and pqs data† can be related to the Sn–I distances by
means of the correlation d(Sn–I) = Ϫ0.058 (4 ∆pqs) ϩ 2.854 Å.
Although, the range of Sn–I distances in Table 3 is very narrow,
the range for SnI4L2 complexes that can be obtained in the
future is not much broader for the following reasons. (a) If
complexes containing [SnR2I4]2Ϫ anions cannot be obtained
(see above), the next type of ligands that give rise to long Sn–X
distances are phosphines,28 and the average Sn–I distance in
SnI4(PPrn )2 is 2.867 Å,35 only 0.003 Å longer than for Cs2SnI6.
3
(b) The shortest Sn–I distances for SnI4L2 complexes will be
obtained for the weakest donor ligands that can form isolable
complexes with SnI4. The crystal structures of SnX4(thf)2
(X = Cl or Br) have been reported recently,75 but no complex
between SnI4 and thf could be isolated. Bearing in mind that
Fig. 2 Mössbauer spectra of Cs2SnI6 at room temperature (a), and
SnI4(bipy) (b), SnI4(Ph3PO)2 (c) and SnI4(Ph2SO)2 (d) at liquid N2
temperature.
reported for SnI4(bipy) (2.802 Å),32 SnI4(Ph3PO)2 (2.798 Å),34
and SnI4(Ph2SO)2 (2.784 Å).33 According to the relationship
between crystallographic and Mössbauer data reported for
SnX4L2 (X = Cl or Br),28–30 ligands that are stronger donors
than XϪ (i.e. have a negative pqs) weaken the Sn–X bond and
give rise to longer Sn–X distances, while ligands that are weaker
donors than XϪ (i.e. have a positive pqs) give rise to shorter
Sn–X distances. In both cases the more positive or negative the
pqs is, the more different the Sn–X distances from those found
in the [SnX6]2Ϫ anion. If a similar relationship also holds for
SnI4L2 complexes, the ligands bipy, Ph3PO and Ph2SO are
weaker donors than IϪ, have a more positive pqs, and the QS
should increase in the order Cs2SnI6 < SnI4(bipy) < SnI4(Ph3-
PO)2 < SnI4(Ph2SO)2 (all three complexes have a cis structure).
The four Mössbauer spectra are displayed in Fig. 2, where it is
shown that the linewidth increases from Cs2SnI6 to SnI4(bipy),
while an incipient doublet is starting to be resolved for
SnI4(Ph3PO)2, and it is clearly seen for SnI4(Ph2SO)2. Bearing in
mind that the average Sn–I bond length for SnI4(bipy) (2.802
Å)32 is significantly shorter than for Cs2SnI6 (2.864 Å), the QS
for the former complex should not be exactly zero. Indeed,
going from a single line fit to a doublet fit reduces the linewidth
from 1.11 to 1.01 mm sϪ1, and χ2 from 1.68 (with 252 degrees of
freedom) to 0.98 (with 250 degrees of freedom), while the statis-
tically acceptable values of χ2 are in the range 0.80–1.22.45
Therefore, although the computed QS value of 0.28 mm sϪ1
is probably subject to a higher error than for the other
compounds reported in this paper, it is certainly more accurate
than a QS value of 0 mm sϪ1. The spectra of SnI4(bipy)72,73 and
75
the average Sn–Br distances for SnBr4(thf)2 and SnBr4-
76
(Ph2SO)2 are quite similar, we may conclude that SnI4-
(Ph2SO)2 is close to the limit of shortest Sn–I distances that
can be obtained for SnI4L2 complexes.‡ Therefore, although the
correlation is probably not very significant, it can be a useful
guide to relate Mössbauer and crystallographic data within the
short range of Sn–I distances expected for SnI4L2 complexes. In
any case, the principal point is that SnI4L2 complexes follow the
same trend as SnX4L2 (X = Cl or Br), showing that ligands that
are weaker donors than IϪ (i.e. have a more positive pqs) give
rise to Sn–I distances that are shorter than for the [SnI6]2Ϫ
anion.
† ∆pqs = pqs(L) Ϫ pqs(IϪ). QS = 2 ∆pqs for cis complexes, and Ϫ4
∆pqs for trans complexes.
‡ Note added at proof: quite recently, the structures of three octahedral
tin(IV) iodide complexes with O amd As donor ligands have been
reported.77 Average Sn–I distances range from 2.784 to 2.814 Å.
Acknowledgements
Financial support from the Spanish Dirección General de
Enseñanza Superior (Projects PB97-0067 and APC1998-0029)
is gratefully acknowledged.
References
1 M. Gielen, Coord. Chem. Rev., 1996, 151, 41.
2 A. J. Crowe, P. J. Smith and G. Atassi, Chem.-Biol. Interact., 1980,
32, 171.
74
SnI4(Ph3PO)2 have been previously reported as single lines
4022
J. Chem. Soc., Dalton Trans., 1999, 4019–4023

View Article Online
3 A. J. Crowe, P. J . Smith and G. Atassi, Inorg. Chim. Acta, 1984, 93,
179.
4 A. J. Crowe, P. J. Smith, C. J. Cardin, H. E. Parge and F. E. Smith,
Cancer Lett., 1984, 24, 45.
39 D. Tudela, J. Organomet. Chem., 1994, 471, 63.
40 J. P. Clark and C. J. Wilkins, J. Chem. Soc. A, 1966, 871.
41 M. K. Das, J. Buckle and P. G. Harrison, Inorg. Chim. Acta, 1972, 6,
17.
5 S. G. Teoh, S. H. Ang, S. B. Teo, H. K. Fun, K. L. Khew and C. W.
Ong, J. Chem. Soc., Dalton Trans., 1997, 465.
6 T. A. Kabanos, A. D. Keramidas, D. Mentzafos, U. Russo, A. Terzis
and J. M. Tsangaris, J. Chem. Soc., Dalton Trans., 1992, 2729.
7 G. Bandoli, A. Dolmella, V. Peruzzo and G. Plazzogna,
J. Organomet. Chem., 1993, 452, 47.
8 J. McManus, D. Cunningham and M. J. Hynes, J. Organomet.
Chem., 1994, 468, 87.
9 N. A. Davies, K. B. Dillon, R. K. Harris, G. F. Hewitson and
L. Toms, Polyhedron, 1994, 13, 19.
42 G. Brauer, Química Inorgánica Preparativa, Reverté, Barcelona,
1968.
43 A. G. Davies and P. J. Smith, in Comprehensive Organometallic
Chemistry, eds. G. Wilkinson, F. G. A. Stone and E. W. Abel,
Pergamon, Oxford, 1982, vol. 2, p. 519.
44 D. Tudela, V. Fernández and J. D. Tornero, Z. Anorg. Allg. Chem.,
1984, 509, 174; Inorg. Chem., 1985, 24, 3892.
45 G. M. Bancroft, Mössbauer Spectroscopy. An Introduction for
Inorganic Chemists and Geochemists, McGraw Hill, London, 1973,
p. 60.
10 D. Kovala-Demerzi, P. Tauridou, A. Moukarika, J. M. Tsangaris,
C. P. Raptopoulou and A. Terzis, J. Chem. Soc., Dalton Trans., 1995,
123.
11 F. Caruso, M. Giomini, A. M. Giuliani and E. Rivarola,
J. Organomet. Chem., 1996, 506, 67.
12 P. Alvarez Boo, J. S. Casas, U. Casellato, M. D. Couce, E. Freijanes,
R. Graziani, B. Salgado, U. Russo and J. Sordo, J. Organomet.
Chem., 1997, 530, 141.
46 H. M. Rietveld, J. Appl. Crystallogr., 1969, 2, 65.
47 D. W. Wiles and R. A. Young, J. Appl. Crystallogr., 1982, 15, 430.
48 L. E. Smart and M. Webster, J. Chem. Soc., Dalton Trans., 1976,
1924.
49 F. A. K. Nasser, M. B. Hossain, D. van der Helm and J. J.
Zuckerman, Inorg. Chem., 1984, 23, 606.
50 G. E. Matsubayashi, K. Ueyama and T. Tanaka, J. Chem. Soc.,
Dalton Trans., 1985, 465.
13 C. Pettinari, M. Pellei, M. Miliani, A. Cingolani, A. Cassetta,
L. Barba, A. Pifferi and E. Rivarola, J. Organomet. Chem., 1998,
553, 345.
14 A. J. Crowe and P. J. Smith, J. Organomet. Chem., 1982, 224, 223.
15 V. G. Kumar Das, C. K. Yap and P. J. Smith, J. Organomet. Chem.,
1985, 291, C17.
51 K. Ueyama, G. E. Matsubayashi, R. Shimizu and T. Tanaka,
Polyhedron, 1985, 4, 1783.
52 G. Valle, A. Sánchez-Gonzalez, R. Ettorre and G. Plazzogna,
J. Organomet. Chem., 1988, 348, 49.
53 S. G. Teoh, S. B. Teo, G. Y. Yeap and J. P. Declercq, Polyhedron,
1992, 11, 2351.
16 B. N. Biddle, J. S. Gray and A. J. Crowe, J. Chem. Soc., Dalton
Trans., 1990, 419.
17 V. G. Kumar Das, C. K. Yap, W. Chen, P. J. Smith and T. C. W. Mak,
J. Chem. Soc., Dalton Trans., 1987, 129.
18 A. I. Tursina, L. A. Aslanov, V. V. Chernyshev, S. V. Medvedev and
A. V. Yatsenko, Koord. Khim., 1985, 11, 1420.
19 P. Sobota, S. Szafert and T. Glowiak, J. Chem. Soc., Dalton Trans.,
1996, 439.
54 U. Casellato, R. Graziani, M. Martelli and G. Plazzogna, Acta
Crystallogr., Sect. C, 1995, 51, 2293.
55 J. S. Casas, A. Castiñeiras, G. Martinez, J. Sordo, J. M. Varela and
M. D. Couce, Acta Crystallogr., Sect. C, 1995, 51, 2561.
56 J. S. Casas, A. Castiñeiras, M. D. Couce, G. Martinez, J. Sordo and
J. M. Varela, J. Organomet. Chem., 1996, 517, 165.
57 V. J. Hall and E. R. T. Tiekink, Acta Crystallogr., Sect. C, 1996, 52,
2143.
20 S. E. Dann, A. R. J. Genge, W. Levason and G. Reid, J. Chem. Soc.,
Dalton Trans., 1996, 4471.
21 S. E. Dann, A. R. J. Genge, W. Levason and G. Reid, J. Chem. Soc.,
Dalton Trans., 1997, 2207.
58 A. Hazell, L. E. Khoo, J. Ouyang, B. J. Rausch and Z. M. Tavares,
Acta Crystallogr., Sect. C, 1998, 54, 728.
59 J. Ouyang, Y. Xu and L. E. Khoo, J. Organomet. Chem., 1998, 561,
143.
22 A. R. J. Genge, W. Levason and G. Reid, J. Chem. Soc., Dalton
Trans., 1997, 4479.
23 A. R. J. Genge, W. Levason and G. Reid, J. Chem. Soc., Dalton
Trans., 1997, 4549.
60 T. P. Lockhart and W. F. Manders, Inorg. Chem., 1986, 25, 892.
61 R. V. Parish and R. H. Platt, Inorg. Chim. Acta, 1970, 4, 65.
62 J. W. Nicholson, J. A. Douek and A. J. Crowe, J. Organomet. Chem.,
1981, 219, 309.
24 K. G. Rasmussen, R. G. Hazell and K. A. Jørgensen, Chem.
Commun., 1997, 1103.
25 D. Tudela, A. J. Maira, D. Cunningham and B. Timmins, Inorg.
Chim. Acta, 1995, 232, 195.
63 E. García-Martinez, A. Sánchez-Gonzalez, A. Castiñeiras, J. S.
Casas and J. Sordo, J. Organomet. Chem., 1994, 469, 41.
64 A. J. Buttenshaw, M. Duchene and M. Webster, J. Chem. Soc.,
Dalton Trans., 1975, 2230.
26 D. Tudela, L. M. Rodríguez-Lorenzo, N. Menéndez and J. D.
Tornero, Inorg. Chim Acta, 1993, 206, 83.
27 D. Tudela, M. A. Khan and J. J. Zuckerman, J. Chem. Soc., Dalton
Trans., 1991, 999.
65 P. B. Hitchcock, S. I. Klein and J. F. Nixon, J. Organomet. Chem.,
1983, 241, C9.
66 M. Lanfranchi, M. A. Pellinghelli, G. Vasapollo and C. F. Nobile,
J. Crystallogr. Spectrosc. Res., 1986, 16, 863.
28 D. Tudela, M. A. Khan and J. J. Zuckerman, J. Chem. Soc., Chem.
Commun., 1989, 558.
29 D. Tudela and M. A. Khan, J. Chem. Soc., Dalton Trans., 1991,
1003.
67 R. Jones, C. P. Warrens, D. J. Williams and J. D. Woollins, J. Chem.
Soc., Dalton Trans., 1987, 907.
68 S. G. Teoh, S. B. Teo, G. Y. Yeap and H. K. Fun, J. Organomet.
Chem., 1992, 439, 139.
30 D. Tudela, J. D. Tornero, A. Monge and A. J. Sánchez-Herencia,
Inorg. Chem., 1993, 32, 3928.
31 R. V. Parish, in Mössbauer Spectroscopy Applied to Inorganic
Chemistry, ed. G. J. Long, Plenum, New York, 1984, pp. 542–544.
32 K. A. Paseshnitchenko, L. A. Aslanov, A. V. Yatsenko and
S. V. Medvedev, Koord. Khim., 1984, 10, 1279.
69 F. W. B. Einstein and B. R. Penfold, J. Chem. Soc. A, 1968, 3019.
70 H. A. Bent, Chem. Rev., 1961, 61, 275.
71 R. J. Gillespie and E. A. Robinson, Angew. Chem., Int. Ed. Engl.,
1996, 35, 495.
72 K. G. Huggins, F. W. Parrett and H. A. Patel, J. Inorg. Nucl. Chem.,
1969, 31, 1209.
33 A. V. Yatsenko, S. V. Medvedev, K. A. Paseshnitchenko and
L. A. Aslanov, J. Organomet. Chem., 1985, 284, 181.
34 A. I. Tursina, L. A. Aslanov, V. V. Chenysev, S. V. Medvedev and
A. V. Yatsenko, Koord. Khim., 1986, 12, 420.
35 N. Bricklebank, S. M. Godfrey, C. A. McAuliffe and R. G.
Pritchard, J. Chem. Soc., Chem. Commun., 1994, 695.
36 C. A. McAuliffe, personal communication.
37 N. Bricklebank, S. M. Godfrey, C. A. McAuliffe and K. C. Molloy,
J. Chem. Soc., Dalton Trans., 1995, 1593.
38 W. Werker, Recl. Trav. Chim. Pays-Bas, 1939, 58, 257.
73 P. A. Cusack and B. N. Patel, Inorg. Chim. Acta, 1984, 86, 1.
74 F. P. Mullins and C. Curran, J. Inorg. Nucl. Chem., 1977, 39, 377.
75 G. R. Willey, T. J. Woodman, R. J. Deeth and W. Errington,
Main Group Met. Chem., 1998, 21, 583.
76 A. V. Yatsenko, S. V. Medvedev, A. I. Tursina and L. A. Aslanov,
Zh. Obshch. Khim., 1986, 56, 2330.
77 A. R. J. Genge, W. Levason and G. Reid, Inorg. Chim. Acta, 1999,
288, 142.
Paper 9/05917B
J. Chem. Soc., Dalton Trans., 1999, 4019–4023
4023