Coordination chemistry of 2,5,8-trithia[9],(2,9)-1,10-phenanthrolinophane (L) toward rhodium(III) at the polarised water/1,2-dichloroethane interface - A possible new approach to the problem of separating RhIII from chloride media

Article abstract of DOI:10.1002/1099-0682(200207)2002:7<1816::AID-EJIC1816>3.0.CO;2-Y

The coordination chemistry involved in the assisted transfer of Rh^III by interfacial coordination with 2,5,8-trithia[9],(2,9)-1,10-phenanthrolinophane (L) has been studied at the water/1,2-dichloroethane interface by cyclic voltammetry. The dependence of the half-wave transfer potential on the ligand and metal concentrations suggests the [Rh(L)Cl₂]⁺ formulation for the complex cation involved in the assisted transfer. In contrast, from the reaction of L in refluxing MeCN/water solution the [Rh(L)Cl][BF₄]₂.2MeCN compound has been isolated and structurally characterised. In the [Rh(L)Cl]²⁺ complex cation L acts as an N₂S₃ donor encapsulating the metal centre within a cavity having a square-based pyramidal geometry. A chloride ligand completes the distorted octahedral coordination sphere around the Rh^III ion. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0682(200207)2002:7<1816::AID-EJIC1816>3.0.CO;2-Y

FULL PAPER
Coordination Chemistry of 2,5,8-Trithia[9],(2,9)-1,10-phenanthrolinophane (L)
toward Rhodium(III) at the Polarised Water/1,2-Dichloroethane Interface ؊
A Possible New Approach to the Problem of Separating RhIII from
Chloride Media
Alexander J. Blake,[a] M. Helena M. Cacote,[b] Francesco A. Devillanova,[c]
¸
Alessandra Garau,[c] Francesco Isaia,[c] Vito Lippolis,*[c] Carlos M. Pereira,[b]
Fernando Silva,[b] and Lorenzo Tei[c]
Keywords: Rhodium / Macrocyclic ligands / Interfaces / Cyclic voltammetry
The coordination chemistry involved in the assisted transfer
of RhIII by interfacial coordination with 2,5,8-trithia[9],(2,9)-
solution the [Rh(L)Cl][BF4]2·2MeCN compound has been
isolated and structurally characterised. In the [Rh(L)Cl]2+
1,10-phenanthrolinophane (L) has been studied at the water/ complex cation L acts as an N2S3 donor encapsulating
1,2-dichloroethane interface by cyclic voltammetry. The de-
pendence of the half-wave transfer potential on the ligand
and metal concentrations suggests the [Rh(L)Cl2]+ formula-
tion for the complex cation involved in the assisted transfer.
In contrast, from the reaction of L in refluxing MeCN/water
the metal centre within a cavity having a square-based
pyramidal geometry. A chloride ligand completes the dis-
torted octahedral coordination sphere around the RhIII ion.
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
2002)
Introduction
aquachloro complexes of the type [RhCl6Ϫn(H2O)n]nϪ3
(0 Ͻ n Ͻ 6) are formed in different proportions depending
The quest for new macrocyclic ligands capable of specific primarily on the chloride concentration.[7] These species are
and effective molecular recognition of metal ions in carrier- highly hydrophilic and not easily isolated by direct solvent
assisted membranes or solid-state ion-selective electrodes is extraction. Some reagents such as SnBr2, SnCl2,[8,9] or
a topic of current interest.[1,2] Attention has mainly been SCNϪ [10] can be used to enhance the extractability of the
focussed on the assisted transfer of group I and group II metal by forming RhIII complexes with better solvent ex-
ions between two immiscible electrolyte solutions; different traction properties, but once the metal is in the organic
crown ether derivatives have been used as neutral carriers phase, the stripping process is not that easy. Despite these
for the construction of ion-selective electrodes and for per- difficulties, the separation of RhIII has been carried out by
forming selective separations.[1Ϫ4] A smaller number of using different techniques such as solvent extraction,[11] ion
studies have been performed with transition and heavy exchange[9,12] and liquid membrane separation,[13Ϫ16] and
metal ions, particularly precious metal ions, probably be- particular attention is paid to research in this area is very
cause of their greater inertness.[5,6] However, the recovery guarded. Recently we have reported the synthesis of new
and removal of precious metals from aqueous solutions is mixed aza-thioether crowns containing the 1,10-phen-
of considerable economic and environmental concern. In anthroline (phen) unit as an integral part of the macrocyclic
particular, the separation and purification of rhodium from structure.[17Ϫ19] These ligands have shown a great affinity
other platinum group metals in aqueous chloride solutions for heavier transition metal ions such as PdII, PtII and RhIII
,
is made difficult by the complex chemistry that RhIII ex- with coordination properties strictly dependent upon the
hibits in these media, which are largely employed because conformational constraints imposed by the heteroaromatic
platinum group metals are usefully soluble in them. Mixed moiety on the S-donor thioether linker of the ring. Herein
we report a study by cyclic voltammetry of the assisted
[a]
transfer of RhIII by interfacial complexation with 2,5,8-tri-
School of Chemistry, The University of Nottingham,
Nottingham, NG7 2RD, UK
thia[9],(2,9)-1,10-phenanthrolinophane (L) at the polarised
[b]
´
Departamento de Quımica, Universidade do Porto, Faculdade
water/1,2-dichloroethane interface. The principal target of
this work has been to study the coordination chemistry of
L towards RhIII at the water/1,2-dichloroethane junction;
however, the results obtained could represent a useful start-
ˆ
de Ciencias do Porto,
R. Campo Alegre 687, 4169Ϫ007 Porto, Portugal
Dipartimento di Chimica Inorganica ed Analitica, Complesso
Universitario di Monserrato, SS
[c]
554 Bivio per Sestu, 09042 Monserrato (CA), Italy
1816
 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ1948/02/0707Ϫ1816 $ 20.00ϩ.50/0 Eur. J. Inorg. Chem. 2002, 1816Ϫ1822

A Possible New Approach to the Problem of Separating RhIII from Chloride Media
FULL PAPER
˚
ing point for a novel way of approaching the problem of
RhIII separation and purification from aqueous solutions.
Table 1. Selected bond lengths (A) and angles (°) for
[Rh(L)Cl][BF4]2·2MeCN
Rh(1)ϪCl(1) 2.3456(8)
Rh(1)ϪS(2) 2.3219(9)
Rh(1)ϪN(1) 2.001(3)
Rh(1)ϪS(1) 2.3361(9)
Rh(1)ϪS(3) 2.3033(9)
Rh(1)ϪN(2) 1.996(3)
N(1)ϪRh(1)ϪN(2) 82.35(11)
N(1)ϪRh(1)ϪS(2) 166.43(8)
N(1)ϪRh(1)ϪCl(1) 89.16(8)
N(2)ϪRh(1)ϪS(2) 84.36(8)
N(2)ϪRh(1)ϪCl(1) 90.31(8)
S(1)ϪRh(1)ϪS(3) 89.27(3)
S(2)ϪRh(1)ϪS(3) 89.94(3)
S(3)ϪRh(1)ϪCl(1) 178.10(3)
N(1)ϪRh(1)ϪS(1) 82.62(8)
N(1)ϪRh(1)ϪS(3) 92.74(8)
N(2)ϪRh(1)ϪS(1) 164.88(8)
N(2)ϪRh(1)ϪS(3) 89.80(3)
S(1)ϪRh(1)ϪS(2) 110.72(3)
S(1)ϪRh(1)ϪCl(1) 91.12(3)
S(2)ϪRh(1)ϪCl(1) 88.18(3)
Results and Discussion
Table 1). The octahedral coordination sphere is completed
by a ClϪ ion [Rh(1)ϪCl(1): 2.3456(8) A]. The phenanthro-
˚
We reported previously[18] the reaction of L with RhCl3
in refluxing MeCN/water to afford a yellow microcrystal-
line powder after addition of an excess of NH4PF6.
line moiety and the two adjacent sulfur atoms S(1) and S(2)
of the thioether linker lie approximately in an equatorial
plane of the octahedron, with the third sulfur atom S(3)
and the ClϪ ligand occupying the two apical positions
[S(3)ϪRh(1)ϪCl(1): 178.10(3)°]. This structure demon-
strates the ability of L to host the RhIII ion within its ring
cavity without any particular constraint of the organic
framework; therefore, L is a possible candidate as a seques-
tering agent for the recovery of this precious metal ion from
aqueous solutions.
In order to test the ability of L to assist the electrochem-
ical extraction of RhIII at the water/1,2-dichloroethane (1,2-
DCE) polarised interface, we have made preliminary studies
by cyclic voltammetry (CV) with an electrochemical cell as
shown schematically below (Cell 1).
The
resulting
complex
was
formulated
as
[Rh(L)Cl][PF6]2·MeCN on the basis of elemental analysis
data and IR and FAB mass spectrometry. 13C NMR spec-
troscopic measurements in CD3CN confirmed this formula-
tion and indicated an octahedral stereochemistry at the
RhIII centre similar to that observed around NiII in
[Ni(L)Cl]ϩ,[19] with all the five donor atoms of the macro-
cyclic ligand strongly coordinated and with a chloride com-
pleting the octahedral environment. After many attempts
we
have
been
able
to
grow
crystals
of
[Rh(L)Cl][BF4]2·2MeCN suitable for X-ray diffraction stud-
ies by diffusion of Et2O vapour into a MeCN solution of
the complex. The structure determination confirms the rho-
dium(III) ion to be bound to the pentadentate macrocycle
through two N-donors of the phen unit [Rh(1)ϪN: 2.001(3)
˚
and 1.996(3) A] and three thioether S-donors of the thio-
ether linker of the ligand [Rh(1)ϪS: 2.3361(9), 2.3219(9)
˚
(trans to phen) and 2.3033(9) A (trans to Cl)] (Figure 1,
This type of cell has already been successfully employed
to study the transfer of CuII assisted by 6,7-dimethyl-2,3-
di(2-pyridyl)quinoxaline (DMDPQ),[20] and the transfer of
ZnII, CdII and PbII assisted by cyclic thioether ligands.[21]
Previous work by Homolka,[22] Matsuda,[23] Kakiuchi and
Senda,[24] and Reymond et al.[25,26] has provided a theoret-
ical framework for the determination of parameters such as
the complex stoichiometry involved in the transfer process.
In the absence of L, there is no visible wave in the cyclic
voltammogram within the potential window available, dem-
onstrating the absence of any transfer process of RhIII
across the organic phase under these conditions (Figure 2a,
dashed line). When the ligand L is present in the organic
phase, the CV responses exhibit a well-defined single revers-
ible wave (Figure 2a, full line) related to the transfer of an
ionic species with ∆wo ␾1/2 ϭ 30 mV and a peak-to-peak sep-
aration of 60 mV; this is consistent with the assisted transfer
of RhIII by L across the water/1,2-dichloroethane interface.
Measurement of the pH of the aqueous solution containing
RhCl3 (electrodic compartment on the right in cell 1)
Figure 1. View of the [Rh(L)Cl]2ϩ cation with the numbering
scheme adopted; displacement ellipsoids are drawn at the 50%
probability level and hydrogen atoms have been omitted for clarity showed that the medium was acidic due to the hydrolysis of
Eur. J. Inorg. Chem. 2002, 1816Ϫ1822
1817

V. Lippolis et al.
FULL PAPER
Figure 2. (a) Voltammetric responses in the absence (dashed line)
and in the presence (full line) of L in the organic phase for the cell
1 system; (b) voltammetric responses for the cell 1 system in the
case of acidic aqueous phase (dashed line) and neutralised aqueous
phase by addition of LiOH (full line)
Figure 3. Voltammetric responses for the cell 2 system showing the
effect of pH on the assisted transfer of Hϩ by L across the water/
1,2-DCE interface
RhIII aquo/chloro complexes[7] and therefore the assisted
transfer of Hϩ by the ligand could also occur under these
experimental conditions. The peak observed could then in-
dicate either the facilitated transfer of the proton or the
RhIII ion by the ligand. In order to identify clearly the
transfer of RhIII assisted by L, LiOH was then added to the
acidic aqueous phase containing RhIII in order to neutralise
it. The cyclic voltammogram recorded under neutral condi-
tions resembles that obtained when the aqueous phase is
acidic, with a single reversible wave observed at a slightly (pKa ϭ 5.85)[29] as determined potentiometrically in water
more positive ∆wo ␾ (65 mV) with a peak-to-peak separa- solution. A direct pH-potentiometric determination of the
1/2
tion of 60 mV (Figure 2b). pKa of L was not possible because of the very low solubility
We also decided to study the protonated ligand transfer of the ligand in water.
across the water/1,2-DCE interface upon changing the pH
From these results it can be concluded that the rhodi-
of the aqueous phase. Figure 3 includes the voltammograms um(III)-assisted transfer by L at a polarised water/1,2-DCE
observed when the pH of the aqueous solution (electrodic interface is possible and that pH control of the aqueous
compartment on the right in cell 2) changes from 2 to 12.
While a single well-defined peak appears in the voltam-
phase is necessary to study the process.
The evaluation of the charge and stoichiometry of the
mogram at pH 2, no transfer processes are observed at pH complex involved in the transfer was carried out by study-
7 and 12, indicating that the proton transfer facilitated by L ing the voltammetric behaviour of the transfer under neu-
across the water/1,2-DCE interface occurs only when acidic tral pH conditions for the aqueous phase, in the presence
solutions are used. From the transfer potential at different of excess metal with respect to the ligand in the organic
pH of the aqueous phase, obtained by cyclic voltammetry phase. Cell 3 was used to carry out these studies: bis(tri-
using cell 2, an ionic partition diagram of the ligand was phenylphosphoranylidene)ammonium
tetrakis(4-chloro-
constructed as described by Reymond et al.[27] (Figure 4a). phenyl)borate (BTPPATPBCl) was used as the organic elec-
This diagram can be used to predict qualitatively the influ- trolyte salt because it allows a larger potential window (and
ence of the variation of the Galvani potential difference it has no effect on the voltammetric signal).
(∆wo ␾1/2) and/or aqueous pH on the nature of the trans-
Figure 5a shows the change in mid-peak potential
ferring species present at the water/1,2-DCE interface. Fur- (∆wo ␾1/2) when the concentration of excess metal is varied.
thermore, the behaviour of the peak current on changing From the slope of the line (Ϫ58 Ϯ 6 mV) a charge of ϩ1
the pH of the aqueous phase in cell 2 (Figure 4b) allowed for the complex is deduced.[20] The peak current (ipc) is in-
the assessment of the acidic constant (pKa) in water for the dependent of the concentration of the metal. The effect on
ligand L, the value of which was estimated to be 4.3. This ∆wo ␾ of the change of the concentration of the ligand
1/2
value is lower than those reported for 1,10-phenanthroline (in the organic phase) in the presence of excess metal (in
(pKa
ϭ
4.97)[28] and 2,9-dimethyl-1,10-phenanthroline the aqueous phase) has also been monitored in order to
1818
Eur. J. Inorg. Chem. 2002, 1816Ϫ1822

A Possible New Approach to the Problem of Separating RhIII from Chloride Media
FULL PAPER
Figure 4. (a) Ionic partition diagram of L at room temperature; (b)
change of the peak current upon variation of the pH of the aqueous
phase in cell 2
evaluate the stoichiometry of the complex.[20] Since there is
no dependence of the half-wave transfer potential on log CL
(Figure 5b), according to Reymond et al.,[25,26] the stoichi-
ometry of the complex must be 1:1. From the variation of
the peak current intensity on changing the concentration of
L on excess metal ion concentration (Figure 5c), and using
the modified RandlesϪSevcik expression,[26] it was possible
to estimate the diffusion coefficient of the ligand in the or-
ganic phase (in fact, due to the metal excess, the current is
limited by the diffusion of the ligand): Do ϭ 5.1 ϫ 10Ϫ6
cm2/s. This value is in good agreement with those reported
Figure 5. (a) Change in mid-peak transfer potential (∆wo ␾1/2) when
the concentration of excess metal in the aqueous phase is varied
(log CL ϭ Ϫ4); (b) effect on ∆wo ␾1/2 of the change of the concentra-
tion of L in the organic phase in the presence of excess metal in
the aqueous phase (log CM ϭ Ϫ2.3); (c) current forward peak in-
tensity vs. the log of ligand concentration (CL) (log CM ϭ Ϫ2.3,
slope ϭ 30 µA/m); the system cell considered is cell 3
in the literature for 1,10-phenanthroline (phen) and its either the metal or the ligand concentration but keeping the
bulky derivatives in 1,2-dichloroethane (Do[phen] ϭ 9.6 ϫ metal-ion concentration in excess. If excess ligand is used
10Ϫ6, Do[4,7-diphenyl-phen] ϭ 4.8 ϫ 10Ϫ6 cm2/s).[30]
and the metal concentration is varied, another voltam-
As already pointed out, the studies reported so far have metric signal appears when CL Ͼ CM (Figure 6), which can
been conducted by cyclic voltammetry in cell 3, varying be assigned to the assisted transfer of a 1:2 Rh/L complex.
Eur. J. Inorg. Chem. 2002, 1816Ϫ1822
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V. Lippolis et al.
FULL PAPER
able to coordinate the octahedral metal centre and that the
two chloride ligands are mutually trans.[31]
The unassisted transfer of RhIII as RhCl2 occurs at
ϩ
∆wo ␾0Ј [RhCl2]ϩ ϭ 514 mV and was determined by applying
the methodology proposed by Shao et al.[32] for determining
the half-wave potential of the species limiting the potential
window. The value of K0f , corresponding to the com-
plexation of one [RhCl2]ϩ ion with one molecule of L, can
therefore be estimated from the dependence of ∆wo ␾ on
1/2
the metal concentration (Figure 5a) using the methodology
proposed by Reymond et al.,[26] which in the case of a metal
ion excess is:
For the case of a 1:1 complex between the [RhCl2]ϩ ion
and L this expression can be rewritten as:
Figure 6. Cyclic voltammograms obtained for the RhIII transfer
assisted by L (CL ϭ 1.30 m) for different metal concentrations
(CM ranging from 0.05 to 9.70 m)
The plot of forward peak current (ipc) vs. CM for the vol-
tammograms in Figure 6 clearly demonstrates the presence
of a transition point between two limiting diffusion regimes
(Figure 7). However, a study similar to that performed for
the transfer of the 1:1 complex was not possible due to the
poor definition of the two voltammetric signals.
where j is the number of ligands in the complex, z the
charge of the complex species involved in the assisted trans-
fer, ∆wo ␾
the half-wave Galvani potential for the
zϩ
1/2,ML
j
transfer of the complex MLZj ϩ, ∆ow␾
the formal Gal-
0Ј zϩ
M
vani transfer potential for the metal ion, β0j the stability
constant for the complex MLzj ϩ, and CM , and CL the
init
init
initial concentrations of metal ion and ligand, respectively.
0Ј
Taking ∆wo ␾ [RhCl2]ϩ to be 514 mV and using the inter-
cept of Figure 5a (Ϫ69.7 mV), the value calculated for the
formation constant of [Rh(L)Cl2]ϩ is log K0f ϭ 9.7. This
value, when compared with those obtained by cyclic vol-
tammetry at the water/1,2-DCE interface for some 1:3
metal-phenanthroline complexes {log β03 [Ca(phen)3]2ϩ
ϭ
15.8, log β03 [Sr(phen)3]2ϩ ϭ14.0},[33] clearly shows that, des-
pite its steric hindrance, the thiacrown ring in L has an
overall stabilising effect (in comparison with complexes hav-
ing higher stoichiometries). This should be due to the donor
properties of the S-donors in the ring.
Conclusions
The assisted transfer of RhIII ion with 2,5,8-tri-
thia[9],(2,9)-1,10-phenanthrolinophane (L) at the water/1,2-
DCE interface has been demonstrated, thus pointing to-
wards a new approach to the problem of separating RhIII
from chloride media. The nature of the RhIII complex in-
volved in the assisted transfer has been elucidated by cyclic
voltammetry and it corresponds to the formulation
Figure 7. Plot of the forward peak current vs. CM for the results
in Figure 6
The 1:1 complex involved in the assisted transfer seems [Rh(L)Cl2]ϩ. This complex cation is different from that ob-
to have the formulation [Rh(L)Cl2]ϩ. An analogous stoichi- tained by refluxing L with RhCl3 in MeCN/water solution,
ometry has been observed for the RhIII complex with a which has the formulation [Rh(L)Cl]2ϩ, and has the same
structural analogue of L where S(3) (Figure 1) has been re- stoichiometry as the RhIII complex obtained with a struc-
placed by an oxygen atom. In this case, it has been shown tural analogue of L having an O-donor instead of the S(3)
by NMR spectroscopy that the O-donor of the ligand is not atom.
1820
Eur. J. Inorg. Chem. 2002, 1816Ϫ1822

A Possible New Approach to the Problem of Separating RhIII from Chloride Media
FULL PAPER
ˆ
to Fundac¸a˜o para a Ciencia e Tecnologia for a PRAXIS XXI Ph.D.
grant. Work was carried out under INOVELREC Ϫ EU contract
no. BRPR-CT95Ϫ010. L. Tei is grateful to the University of Cagli-
ari for financial support.
Experimental Section
General Procedures and Chemicals: Cyclic voltammetry was per-
formed using a four-electrode potentiostat (Solartron 1287) with
ohmic drop compensation. The electrochemical cell was made from
borosilicate glass and was of the same design as used previously;[21]
with two Luggin reference probes and a geometrical area for the
water/1,2-DCE interface of 0.22 cm2. All the measurements were
carried out at laboratory temperature (20 Ϯ 3 °C), with scan rates
ranging from 10 to 100 mV/s. For each experiment the addition of
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[2]
[3]
Ϫ
a fixed amount of ClO4 (LiClO4 Ͼ 99.99% purity from Aldrich)
338Ϫ345.
provided a reference for all half-wave potential measurements The
values measured were easily transposed to the Galvani potential
scale taking into account the following relationship
[4]
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[5]
A. T. Yordanov, D. M. Roundhill, Coord. Chem. Rev. 1998,
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Ϫ
E1/2 Ϫ ∆wo ␾ ϭ E1/2(ClO4Ϫ) Ϫ ∆wo ␾0(ClO4
)
170, 93Ϫ124.
[6]
G. Khayatian, M. Shamsipur, Sep. Purif. Technol. 1999, 16,
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235Ϫ241.
[7]
E. Benguerel, G. P. Demopoulos, G. B. Harris, Hydrometal-
The organic electrolyte salts used in this study were tetraoctylam-
monium tetrakis(phenyl)borate (TOATPB), prepared as reported
previously,[35] and bis(triphenylphosphoranylidene)ammonium
tetrakis(4-chlorophenyl)borate (BTPPATPBCl), which was pre-
pared by the metathesis of BTPPACl (Aldrich) and KTPBCl
(Fluka). Rhodium(III) unassisted transfer in the absence of L was
studied using 10 m BTPPATPBCl as the supporting electrolyte in
the organic phase. 2,5,8-Trithia[9],(2,9)-1,10-phenanthrolinophane
(L) was synthesised according to the literature.[17] LiCl (Ͼ99% pur-
ity) and RhCl3·3H2O (38% Rh) were obtained from Merck. Aque-
ous solutions were prepared with purified water from Milli Q 185
system (Millipore). 1,2-Dichloroethane (1,2-DCE) (Ͼ99.5% purity,
Merck) was used without further purification and was handled
with all necessary precautions.
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[8]
M. S. Alam, K. Inoue, Hydrometallurgy 1997, 46, 373Ϫ382.
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Crystal
Structure
Determination:
Crystal
data
for
C22H24B2ClF8N4RhS3, M ϭ 752.61, orthorhombic, Pccn, a ϭ
3
˚
˚
14.2317(13), b ϭ 31.974(3), c ϭ 12.4458(11) A, V ϭ 5663.4(9) A ,
[18]
[19]
F. Contu, F. Demartin, F. A. Devillanova, A. Garau, F. Isaia,
V. Lippolis, A. Salis, G. Verani, J. Chem. Soc., Dalton Trans.
1997, 4401Ϫ4405.
Mo-Kα radiation λ ϭ 0.71073 A, Z ϭ 8, Dcalcd. ϭ 1.765 g cmϪ3
,
˚
F(000) ϭ 3008. T ϭ 150(2) K, µ(Mo-Kα) ϭ 0.994 mmϪ1. Yellow
plate, 0.21 ϫ 0.16 ϫ 0.12 mm3. Bruker SMART1000 CCD area
detector diffractometer with Oxford Cryosystems open-flow cryo-
stat,[36] ω scans, 6808 absorption-corrected reflections (SAD-
ABS,[37] T ϭ 0.80Ϫ1.00), 5132 reflections with I Ͼ 2σ(I). The struc-
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Acknowledgments
The authors wish to thank Dr. Pietro A. Vigato (Istituto di Chim-
ica e Tecnologie Inorganiche e dei Materiali Avanzati, CNR, Pa-
dova, Italy) since without his contribution the realisation of this
work would not have been possible. M. H. M. Cac¸ote is grateful
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