Evidence of intramolecular electron transfer between two metallic atoms in a bimetallic complex by electrochemical methods

Article abstract of DOI:10.1039/b507665j

The electrochemical properties of the monomeric complex [(η⁵-C₅H₅)(μ-η⁵: η¹-C₅H⁴(CH₂)₂P(C ₆H₅)₂TiCl₂] 1 and the heterobimetallic complex [(η⁵-C₅H₅)(μ- η⁵:η¹-C₅H₄(CH ₂)₂P(C₆H₅)₂TiCl ₂][RuCl₂(C₆H₄(CH₃)(C ₃H₇))] 2 have been studied by cyclic voltammetry, controlled potential electrolysis and rotating disk electrode voltammetry. An unexpected electron transfer between the two heterobimetallic atoms has been observed. This transfer takes place via an intramolecular interaction, hence via a chloride bridge. Electrochemical simulation has been carried out to verify experimental results and to obtain the kinetic constant of the proposed square scheme. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2005.

Full text of DOI:10.1039/b507665j

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P A P E R
Evidence of intramolecular electron transfer between two metallic
atoms in a bimetallic complex by electrochemical methodsw
´
Helene Cattey,* Alain Vallat, Pierre Le Gendre, David Evrard, Claude Moıse and
Yves Mugnier
¨
` ` ´ ´
Laboratoire de Synthese et Electrosynthese Organometalliques LSEO-UMR 5188, Universite de
´
Bourgogne, Faculte des Sciences Mirande, 9, Allee Alain Savary, 21000 Dijon, France.
E-mail: hcattey@u-bourgogne.fr; Fax: 33 380 39 6096; Tel: 33 380 39 3665
´
Received (in Montpellier, France) 1st June 2005, Accepted 12th July 2005
First published as an Advance Article on the web 31st August 2005
The electrochemical properties of the monomeric complex [(Z5-C5H5)(m-Z5:Z1-C5H4(CH2)2P(C6H5)2TiCl2] 1
and the heterobimetallic complex [(Z5-C5H5)(m-Z5:Z1-C5H4(CH2)2P(C6H5)2TiCl2][RuCl2(C6H4(CH3)(C3H7))]
2 have been studied by cyclic voltammetry, controlled potential electrolysis and rotating disk electrode
voltammetry. An unexpected electron transfer between the two heterobimetallic atoms has been observed.
This transfer takes place via an intramolecular interaction, hence via a chloride bridge. Electrochemical
simulation has been carried out to verify experimental results and to obtain the kinetic constant of the
proposed square scheme.
to better understand the catalytic results already obtained.1
The electrochemical mechanism of this bimetallic complex will
be extensively described. The kinetics of its electrochemical
reactions are also investigated.
Introduction
Organometallic chemistry abounds with examples of dinuclear
compounds in which the two metallic centers are held together
by a variety of bridging ligands. Such compounds provide
good models for the study of cooperative effects between
adjacent metal atoms in catalysis. The recent surge of interest
in the chemistry of early-late heterobimetallic compounds is
motivated by the aptitude of the compound to confine two
metallic fragments within a chloride bridge, thus yielding
complexes with new properties which are interesting to explore.
The catalytic behaviour1 of the titanium–ruthenium heterobi-
metallic complexes have shown surprising differences accord-
ing to the ligands used. Since such transformations proceed
mostly through electron transfer on the metal, it is of primary
importance to gain insight into the nature of the species formed
by the stepwise electron addition or abstraction. Electroche-
mical methods can therefore be of great help in the under-
standing of such mechanisms.
Experimental
Synthesis and catalysis behaviour of the complex 2
The heterobimetallic compound 2 was prepared according to
an earlier published procedure25 by reaction between titano-
cene monophosphine with the ruthenium dimer, as described
as below.
Pioneering work in the field may be found in the reduction of
3ꢀ 2
] and in the electrochemical study of organome-
A preliminary assessment of the catalytic performance of the
bimetallic complex 2 in ring closing metathesis (RCM) has
already been carried out.1 First attempts in neutral catalysis
were conducted in CH2Cl2 under neon light with a solution of
N,N-diallyltosylamide and a catalytic amount of bimetallic
complexes. These latter, unlike the monometallic complex
[RuCl2P(C6H11)3(C6H4(CH3)(C3H7))], showed almost no cat-
alytic activity. A second set of experiments, which met with
more success, required changing the neutral complex 2 into
cationic Ti–Ru-allenylidene precatalyst. In this case, a cationic
site generated on ruthenium by a chloride ion extraction is
needed. Cationic complexes thus obtained have as a result a
vacant site in which alkyne can be coordinated to initiate the
RCM reaction. Such complexes can be generated in situ or not
by successive addition of silver triflate and propargylic alcohol.
It is worth noting that the rate of formation of the cumulene
species is very different according to the bimetallic complex.
The reactivity of 2 is very mixed: the reaction occurred but
required a long time and was not quantitative. Thus, the ease
of access to the allenylidene complex and therefore to an
effective precatalyst was thoroughly studied. At this point,
[Cr(CN)6
tallic complexes.3–12 The electrochemical properties of mono-
nuclear titanium13–16 or ruthenium17–18 complexes have been
widely investigated but, in contrast with the abundant work in
this area and to our best knowledge, only a few electron-
transfer studies19–22 have been published on bimetallic com-
plexes. A chloride bridge between two metallic atoms has
already been observed,21–24 but it is the first electrochemical
study which gives evidence of an electron transfer via this
bridge.
In this article, we report an electrochemical study of a
monometallic compound [(Z5-C5H5)(m-Z5:Z1-C5H4(CH2)2
P(C6H5)2TiCl2] 1 and the corresponding heterobimetallic
complex
[RuCl2(C6H4(CH3)(C3H7))] 2, giving evidence of the chloride
bridge between the two metallic centers and providing elements
[(Z5-C5H5)(m-Z5:Z1-C5H4(CH2)2P(C6H5)2TiCl2]
w Electronic supplementary information (ESI) available: Electronic
supplementary information available: ESR spectra of 10 and 30. See
http://dx.doi.org/10.1039/b507665j
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
1302
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electrochemistry seems to be the methodology of choice to
study the subtle reactivity of 2 and more generally to obtain
insight into the influence of the titanocene fragment on its
metal neighbour.
Electrochemistry
All manipulations were performed using Schlenk techniques in
an atmosphere of dry oxygen-free argon gas and using dry
solvents. The main supporting electrolyte used was the tetra-
butylammonium hexafluorophosphate (NBu4PF6), obtained
by adding 100 mL of tetrabutylammonium hydroxide and 15
mL of hexafluorophosphate acid to 1 L of water. The white
solid obtained was filtered, recrystallized three times in ethanol
and dried at 80 1C for at least 2 days before use. A second
electrolyte used was sodium tetraphenylborate (NaBPh4) pur-
chased from Avocado. Theses electrolytes were degassed under
vacuum before use and then solubilized at a concentration of
0.2 mol Lꢀ1. Tetrahydrofuran (THF) was distilled under argon
over sodium and benzophenone. Throughout our work, a
three-compartment cell was used with a platinum wire as a
counter electrode. The reference electrode was a saturated
calomel electrode (SCE) separated from the solution by a
sintered glass disk. For all voltammetric measurements carried
out with a Taccussel UAP4 unit cell, the working electrode was
a vitreous carbon electrode (+ ¼ 3 mm). The controlled
potential electrolyses were performed with an Amel 721 elec-
tronic integrator in a cell with three compartments separated
by fritted glass of medium porosity. A carbon gauze was used
as the cathode, a platinum plate as the anode and a saturated
calomel electrode as the reference electrode.
Fig. 1 Voltammograms of complex 1 on carbon electrode in THF
containing 0.2 mol Lꢀ1 NBu4PF6 by CV (a, scan rate: 100 mV sꢀ1) or
by RDE voltammetry (b–c, scan rate: 20 mV sꢀ1); (a, b) initial; (c) after
electrolysis at ꢀ1.20 V (n ¼ 0.88eꢀ).
of mechanism was first introduced by Jacq26 and developed
afterwards.27–33 By RDE voltammetry, a second reduction
wave was assigned to TiIII reduction at ꢀ2.13 V (Fig. 1b).
When the electrolysis was performed at ꢀ1.20 V on the A
reduction plateau (Fig. 1c) with one electron consumption (n ¼
0.88eꢀ), the TiIII oxidation wave B0 was obtained (see reaction
(1)) and an ESR signal characteristic of the TiIII–P complex10
10 was detected in the resulting solution (see Fig. 1S in ESIw). It
consists of an intense doublet due to the interaction of the
unpaired electron with one 31P nucleus (nuclear spin I ¼ 1/2).
Each of the doublet lines is symmetrically flanked by an
hyperfine sextet resulting from the 47Ti nucleus (I ¼ 5/2,
natural abundance 7.4%) and a hyperfine octet caused by the
49Ti nucleus (I ¼ 7/2, natural abundance 5.4%). However, the
intense ‘‘31P doublet’’ prevents the observation of the inner
lines of the 47,49Ti hyperfine pattern. The ESR parameters were
found equal to: g ¼ 1.9896, ap ¼ 19.8 G, aTi ¼ 9.3 G.
All the electrochemical results will be presented versus the
saturated calomel electrode (SCE) and the complex concentra-
tion was generally close to 2 ꢁ 10ꢀ3 mol Lꢀ1
.
Simulation
Digital simulations were obtained with the program Digisim
(Bianalytical Systems). The potential step size was 5 mV. The
uncompensated resistance, here Ru ¼ 800 O, was determined
with a PAR 373A potentiostat using the same experimental
conditions as for the CV measurements. The double layer
capacitance value was estimated as Cdl/A ¼ 1.71 mF cmꢀ2
using a blank solution prior to voltammetric measurements.
The experimental curves were corrected for residual current,
also determined using voltammetric measurements of blank
solutions.
ð1Þ
Other measurements
ESR spectra were obtained using a Bruker ESP 300 spectro-
meter and at 295 K, in the electrochemical study conditions
(THF and electrolyte salt 0.2 mol Lꢀ1).
Results and discussion
The electrochemical characteristics of the complexes were
studied in THF, 0.2 mol Lꢀ1 of NBu4PF6 solution by cyclic
voltammetry (CV) and rotating disk electrode voltammetry
(RDE). The monomeric species used in the bimetallic synthesis,
such as Cp2TiCl2 or [RuCl2(C6H4(CH3)(C3H7))]2, have already
been extensively investigated.13,14,18
Electrochemical studies
The monometallic complex 1. The monometallic complex 1
studied by CV revealed one reduction peak A at ꢀ0.70 V and
one oxidation peak B0 at ꢀ0.15 V (Fig. 1a). This electroche-
mical system was characteristic of a square scheme13,14
(Scheme 1), since the same voltammogram is obtained with
the second potential cycle, as described previously.16 This type
Scheme 1 Mechanism scheme of the bimetallic complex 2. This
complex is symbolized by its oxidation degree, for example the initial
product as Ti(IV)Ru(II).
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For low scan rate (20 mV sꢀ1, Fig. 2b), a new reduction
peak A1 appears at ꢀ0.91 V. Its intensity increases as the scan
rate is reduced and may be attributed to the reduction of
transient TiIV species. Simultaneously, by RDE, the second
reduction wave RuII/Ru0 decreases as the first one increases.
The ratio id/id without electronic transfer of the wave A
decreases and tends toward 1 when the rotation scan is
increased (1.21 for 20.94 rad sꢀ1 and 1.08 for 209.43 rad
s
ꢀ1). When the experiments were carried out at low tempera-
ture, this A1 peak was missing. Moreover, this peak A1 was
obtained whatever the complex concentration (with a concen-
tration range investigated from 7.5 ꢁ 10ꢀ4 to 4 ꢁ 10ꢀ3 mol
L
ꢀ1), which tends to prove the existence of intramolecular
interactions.
The existence of a second reduction peak (A1), situated
in the potential area of TiIV complex reduction revealed by
CV and the diffusion current variation with the rotation rate
could be explained by an electronic transfer between the two
metallic atoms to regenerate a TiIV complex. This electronic
transfer is intramolecular and the only way for it to take place
is via a chloride bridge. Moreover, previous studies have
already pointed out a chloride bridge between two heterobi-
metallic atoms. Numerous examples are available: a choice
with few of them concerning titanium or ruthenium are cited as
references.20–24 In the complex 3, the chloride atom is then
transferred from Ru to Ti; Ti0(III)Ru(II) thus becomes Ti(IV)
Ru0(I) (see reaction (3)). This is in accordance with the fact that
the second reduction wave decreases as A1 increases, since in
these conditions, this process corresponds to a Ru(I)/Ru(0)
reduction.
Fig. 2 Voltammograms of complex 2 on carbon electrode in THF
containing 0.2 mol Lꢀ1 NBu4PF6 by RDE voltammetry (a, scan rate:
20 mV sꢀ1) or by CV (b–e) at scan rates: (b): 20 mV sꢀ1; (c): 50 mV sꢀ1
;
(d) 100 mV sꢀ1 and (e): 200 mV sꢀ1
.
Otherwise, by adding
a
half equivalent of the
[RuCl2(C6H4(CH3)(C3H7))]2 dimer on the TiIII generated com-
plex, no changes were observed in this oxidation wave B0.
This means that there were no spontaneous interactions
between TiIII and RuII which is due to the fact that the
phosphorus atom coordinated then protected the TiIII from
the RuII interaction.
The bimetallic complex 2. The bimetallic complex 2, studied
by RDE voltammetry, exhibited two reduction waves at ꢀ0.79
V and ꢀ1.57 V (Fig. 2a): the first one was due to a TiIV
reduction. The second wave observed is attributed to the RuII
reduction. Its height is nearly twice that of the first one, hence it
might be explained by the reduction6,7 of RuII to Ru0.
Using CV (Fig. 2b–2e), a pseudo-reversible system TiIV/TiIII
A/A0 was observed at ꢀ0.76 V, following a classical electro-
chemical square scheme.3,13,14 The initial complex 2, symbo-
lized as Ti(IV)Ru(II), is reduced according to the A/A0 square
scheme to the complex Ti0(III)Ru(II), formulated as 3, where the
chemical reaction corresponds to the elimination of a chloride
atom (see reaction (2)).
ð3Þ
This new TiIV complex is then reduced at ꢀ0.91 V (A1 peak,
see reaction (4)). When the scan rate is increased, this interac-
tion does not have enough time to take place, hence the A/A0
system only is observable. At low temperature, the peak A1 is
once again not apparent since at this temperature, the chemical
reactions are slowed and so the chloride atom transfer can no
longer occur.
ð2Þ
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ð6Þ
Scheme 1 summarizes all the results obtained for the reac-
tivity study of the complex 2.
ð4Þ
When the experiments were carried out with three equiva-
lents of P(C6H5)2(CH3), the peak A1 disappeared. In these
conditions and after an electrolysis at ꢀ0.95 V, an oxidation
wave was obtained at ꢀ0.27 V. The ESR spectrum (see Fig. 2S
in ESIw) showed as for the monometallic complex (see above)
that the Ti signal was split into a doublet (ap ¼ 20 G) owing to
the interaction of the unpaired electron10 of TiIII species with
31P (g ¼ 1.9885, aTi ¼ 10 G) and the formation of the complex
30 (see reaction (5)). These results prove that when the phos-
phorus atom was coordinated to the Ti atom, no interaction
between the Ti and the Ru atoms was able to take place. The
complex is then stopped in the Ti0(III)Ru(II) stage, as complex
30 and may be oxidized to Ti0(IV)Ru(II).
Could the electronic transfer observed in a bimetallic complex be
obtained with only the corresponding monometallic complexes?
The interest of the bimetallic complex, compared to the
corresponding separated monometallic complexes was verified.
Cp2TiCl2 was reduced electrochemically with consumption of
one electron at ꢀ1.2 V and the corresponding Cp2TiIIICl was
obtained, which presents its oxidation wave at ꢀ0.71 V.
One equivalent of RuIICl2(p-cymene)(PPh2(CH3)) was intro-
duced to this solution. After ten minutes, the TiIII oxidation
wave was nearly unchanged. Twenty-five minutes were needed
to begin to observe a decrease in this wave. Simultaneously, the
corresponding reduction wave of Cp2TiCl2 increases only
weakly (10% max. according to the wave height).
These results indicated that an electronic transfer between
the two monometallic complexes TiIII and RuII may be ob-
served after a relatively long time. This electronic transfer may
only be explained by the formation of an intermediate (inter-
molecular inner-sphere electronic transfer), since, as the differ-
ence of potential between the TiIII oxidation potential and the
RuII reduction potential is equal to 0.65 V (potentials, respec-
tively, equal to ꢀ0.71 V and ꢀ1.36 V), an outer-sphere
electronic transfer could be eliminated.
ð5Þ
When the complex 2 was studied in the presence of NaBPh4,
the A, A1 and B peaks were obtained as before. However, the
first system becomes totally irreversible for scan rates varying
from 20 to 200 mV sꢀ1 (reaction (20)). At 500 mV sꢀ1, a peak
with low intensity appears at ꢀ0.34 V.
In conclusion, we have showed that an electronic transfer
between two metallic atoms in the case of heterobimetallic
derivatives was observed at the cyclic voltammetry time scale,
but was not observed with the two separated monometallic
complexes at the same time scale. This last electronic transfer
was all the same observed but with a much longer time scale
and a weaker yield.
ð20Þ
In this medium, NaBPh4 trapped the chloride anion released
from reaction (2) to yield to a NaCl precipitate; this reaction
then becomes irreversible and consequently, 3 can be oxidized
at –0.34 V to form the cationic species 20 (reaction (6)).
Electrochemical simulation
Since the complex 2 presented an unexpected reactivity with an
electron transfer occurring via a chloride bridge between two
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Fig. 3 RDE voltammograms of complex 3 on carbon electrode in
THF containing 0.2 mol Lꢀ1 NBu4PF6: experimental curves (—);
simulated curves (’ ’ ’); simulated curves without electronic
transfer (nnn) at a scan rate of 20 mV sꢀ1 and a rotation rate of
104.72 rad sꢀ1
.
metallic atoms, we tried to verify all the experimental results
obtained for it by using the Digisim simulation software and to
complete them by determining all the thermodynamic and
kinetic parameters defined in Scheme 1.
To first determine the square scheme parameters,34,35 the
results obtained with the complex 2 in the presence of NaBPh4
were analyzed. In this case, the precipitation of NaCl occurs
during the Ti(IV)Ru(II) reduction, the initial complex is reduced
and for a relatively fast scan rate, the reoxidation pathway of
Ti0(III)Ru(II) to Ti0(IV)Ru(II) was observed at ꢀ0.34 V (reaction
(6)). No chloride atom being available, the reverse reduction
peak Ti0(IV)Ru(II)/Ti0(III)Ru(II) should have been obtained at
the second scan. Unlikely, this reduction peak was not ob-
tained but E11, the corresponding standard potential, may also
be approximated as equal to E(6) ꢀ (0.06/2), hence (ꢀ0.34,
ꢀ0.03) or (ꢀ0.37) V, if the ohmic drop is neglected.
Fig. 4 Cyclic voltammograms of complex 3 on carbon electrode in
THF containing 0.2 mol Lꢀ1 NBu4PF6: experimental curves (—);
simulated curves (’ ’ ’); simulated curves without electronic
transfer (nnn) at scan rates: (a: 200 mV sꢀ1; b: 20 mV sꢀ1).
imply that the reduction and the oxidation are taking place by
the same pathway: Ti(IV)Ru(II) to Ti(III)Ru(II) to Ti0(III)Ru(II)
(see reaction (2)).
The electron transfer from the titanium to the ruthenium is
expressed by the A wave increase in RDE voltammetry (Fig. 3)
and the A1 peak appearance in CV (Fig. 4). The corresponding
constants K3 and kf3, as the K4 and kf4 constants, linked to the
formed Ti(IV)Ru0(I) reduction are evaluated by numeric simu-
lation (Figs. 3 and 4). K3 and kf3 influenced the A1 peak and
slightly modified the A0 peak height, K4 and kf4 applying the
essential part in this height. Lastly, the normal potential of the
global system: Ti0(III)Ru(II)/Ti0(III)Ru(0), Er ¼ 1.57 V was
calculated, assuming that this reaction is a bielectronic system
including two successive monoelectronic steps, which is only an
hypothesis (a disproportionation reaction could also be con-
sidered).
All the electrochemical and chemical constants of Scheme 1
were then determined, as described in Table 1.
It is interesting to note that the initial square scheme behaves
like a simple Electrochemical–Chemical scheme and that the
rate of the electron transfer between the titanium to the
ruthenium is not so fast.
Otherwise, the potential of the peaks A/A0 is also function of
E12 (the Ti(IV)Ru(II)/Ti(III)Ru(II) standard potential) and K2.
K2 is defined an equilibrium constant and is then equal to kf2/
kb2 ¼ [Ti0(III)Ru(II)] ꢁ [Clꢀ]/[Ti(III)Ru(II)]. Theses two values
have been evaluated by numeric simulation at different scan
rates by RDE voltammetry and CV: E21 ¼ ꢀ0.80 V; K2 ¼ 6 ꢁ
10ꢀ3 mol Lꢀ1. The K1 constant (equal to kf1/kb1 ¼ [Ti0(IV)
Ru(II)] ꢁ [Clꢀ]/[Ti(IV)Ru(II)]) is deducted from the relation34
K2/K1 ¼ exp[F/RT(E11ꢀE21)]. We also verified that the rate
constant of the charge transfer khi, equal to 0.025 cm sꢀ1, and
the charge transfer coefficient ai, equal to 0.5, previously
determined35 for the di(propylthiotetramethylcyclopentadi-
enyl) titanium dichloride (Cp02TiCl2) under the same experi-
mental conditions, were suitable for the simulation. Better
results were obtained for khi equal to 0.01 cm sꢀ1 by RDE
fitting.
On the contrary, the chemical reaction constants are rather
far from the already mentioned case35: kf1 ¼ 10ꢀ4 sꢀ1 hence kb1
¼ 1.78 ꢁ 105 molꢀ1 L sꢀ1 (instead of kb1 ¼ 1011 molꢀ1 L sꢀ1);
kf2 ¼ 107 sꢀ1 (instead of kf2 ¼ 103 sꢀ1), since the square scheme
is pseudo-reversible and no second oxidation peak was ob-
served in this explored scan rate field. The results therefore
Conclusion
The unexpected electrochemical activity of the complex 2 was
well-studied by cyclic voltammetry, controlled potential elec-
trolysis and RDE. In fact, an intramolecular electron transfer
between the two metallic atoms via a chloride atom transfer
Table 1 Electrochemical and chemical constants for the complex 2 in Scheme 1a
EI1/V
khi/cm sꢀ1
ai
Ki
kf/sꢀ1
E11 ¼ ꢀ0.38
E21 ¼ ꢀ0.80
kh1 ¼ 0.01
kh2 ¼ 0.01
a1 ¼ 0.5
a2 ¼ 0.5
K1 ¼ 5.6 ꢁ 10ꢀ10 mol Lꢀ1
K2 ¼ 6 ꢁ 10ꢀ3 mol Lꢀ 1
K3 ¼ 10ꢀ2
kf1 ¼ 10ꢀ4
kf2 ¼ 1017
kf3 ¼ 10
E41 ¼ ꢀ0.76
Er ¼ ꢀ1.57
a
kh4 ¼ 0.01
a4 ¼ 0.5
K4 4 102
kf4 ¼ 10ꢀ1
khi ¼ heterogeneous charge transfer rate constant; ai ¼ charge transfer coefficient; Ki ¼ equilibrium constant, respectively equal to: K1 ¼ kf1/kb1
¼
[Ti0(IV)Ru(II)] ꢁ [Clꢀ]/[Ti(IV)Ru(II)]; K2 ¼ kf2/kb2 ¼ [Ti0(III)Ru(II)] ꢁ [Clꢀ]/[Ti(III)Ru(II)]; K3 ¼ kf3/kb3 ¼ [Ti(IV)Ru0(I)]/[Ti0(III)Ru(II)]; kbi and
kfi ¼ chemical reaction constants.
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was revealed for this complex. The experimental results were
verified by simulation and all the chemical and electrochemical
constants were defined.
14 Y. Mugnier, A. Fakhr, M. Fauconet, C. Moıse and E. Laviron,
¨
Acta Chem. Scand., 1983, B37, 423.
15 R. J. Enemaerke, J. Larsen, T. Skrydstrup and K. Daasbjerg,
Organometallics, 2004, 23, 1866.
16 I. M. M. Fussing, D. Pletcher and R. J. Whitby, J. Organomet.
Chem., 1994, 470, 119.
Acknowledgements
17 F. Estevan, P. Lahuerta, J. Latorre, A. Sanchez and C. Sieiro,
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18 P. Lahuerta, J. Latorre, J. Soto and Y. Mugnier, Polyhedron,
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The authors are grateful to E. Pousson for her technical
assistance and to the CNRS and the French Ministry of
Research and Education for financial support.
19 K. Lu, J. F. Ojo and J. E. Earley, Inorg. Chem., 1988, 27, 2325.
20 J. C. Kotz, W. Vining, W. Coco, R. Rosen, A. R. Dias and M. H.
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