Stabilization of 17-electron metal-centered species. Electrochemical study of [WX(CO)3(η5-C5H5)] (X=Cl, Br, I) and [WI(CO)2(PCy3)(η5-C5H5)]

Article abstract of DOI:10.1016/S0022-328X(97)00760-2

The electrochemical study of [WX(CO)₃(η⁵-C₅H₅)] (X=Cl, Br, I) (1-3) is reported. The reductions follow ECE mechanisms, yielding the anion [W(CO)₃(η⁵-C₅H₅)]^- (7^-). The stability of 1-3 towards reduction increases with the increasing electronegativity and decreasing size of the halide. 7^- reoxidizes to the unstable 17-electron radical [W(CO)₃(η⁵-C₅H₅)] (7), which readily dimerizes. The oxidations of 1-3 follow EC mechanisms, leading to the cations [WX(CO)₃(η⁵-C₅H₅)]⁺, which are very unstable and readily decompose. [WI(CO)₂(PCy₃)(η⁵-C₅H₅)] has been prepared and characterized as a mixture of cis (4) and trans (5) isomers (ratio cis:trans 95:5). The electrochemical reduction of the cis isomer (4) is also an ECE process, but takes place at a potential significantly more negative than 3. An anion [W(CO)₂(PCy₃)(η⁵-C₅H₅ )]^- (8^-) is formed which reoxidizes to 8. This new 17-electron radical is considerably more stable than 7 due to the presence of the bulky PCy₃ ligand. A similar effect is observed in the oxidation of 4, where the 17-electron product 4⁺ is significantly more stable than the analogue 3⁺.

Full text of DOI:10.1016/S0022-328X(97)00760-2

Journal of Organometallic Chemistry 555 (1998) 57–66
Stabilization of 17-electron metal-centered species. Electrochemical
study of [WX(CO)3(p5-C5H5)] (X=Cl, Br, I) and
[WI(CO)2(PCy3)(p5-C5H5)]
Maria-Luisa Marcos a, Consuelo Moreno b, Maria-Jose´ Macazaga b, Rosa Medina b,
R. Maderuelo b, Salome´ Delgado b,*, Jaime Gonza´lez-Velasco a
a Departamento de Qu´ımica. Uni6ersidad Auto´noma de Madrid, 28049, Madrid, Spain
b Departamento de Qu´ımica Inorga´nica. Uni6ersidad Auto´noma de Madrid, 28049, Madrid, Spain
Received 10 September 1997
Abstract
The electrochemical study of [WX(CO)3(p5-C5H5)] (X=Cl, Br, I) (1–3) is reported. The reductions follow ECE mechanisms,
yielding the anion [W(CO)3(p5-C5H5)]− (7−). The stability of 1–3 towards reduction increases with the increasing electronegativ-
ity and decreasing size of the halide. 7− reoxidizes to the unstable 17-electron radical [W(CO)3(p5-C5H5)] (7), which readily
dimerizes. The oxidations of 1–3 follow EC mechanisms, leading to the cations [WX(CO)3(p5-C5H5)]+, which are very unstable
and readily decompose. [WI(CO)2(PCy3)(p5-C5H5)] has been prepared and characterized as a mixture of cis (4) and trans (5)
isomers (ratio cis:trans 95:5). The electrochemical reduction of the cis isomer (4) is also an ECE process, but takes place at a
potential significantly more negative than 3. An anion [W(CO)2(PCy3)(p5-C5H5)]− (8−) is formed which reoxidizes to 8. This new
17-electron radical is considerably more stable than 7 due to the presence of the bulky PCy3 ligand. A similar effect is observed
in the oxidation of 4, where the 17-electron product 4+ is significantly more stable than the analogue 3+. © 1998 Elsevier Science
S.A. All rights reserved.
Keywords: Cyclopentadienyl; Tungsten; Carbonyl complexes; Electrochemistry
proposed as intermediates in a variety of reactions
involving organometallic compounds [6]. They are
nonetheless usually very unstable with respect to dimer-
ization. However, there are now several known exam-
ples of 17-electron compounds which are significantly
stabilized towards dimerization to the 18-electron
metal–metal bonded analogues by substitution of small
ligands with more sterically demanding ones (e.g. CO
by tertiary phosphines [7], and p5-C5H5 by p5-C5Me5
[7]h and p5-C5Ph5 [8]).
The electrochemistry of the organometallic halides
[MX(CO)m(p5-C5R5)n] (R=H, Me; X=halogen atom)
has received relatively little attention [9], whereas the
electrochemical reactivities of the organometallic
dimers [M(CO)m(p5-C5R5)n]2 have been subjects of ac-
tive research [10]. As part of our ongoing studies on the
1. Introduction
The chemistry of 17- and 19-electron organotransi-
tion-metal radicals has been extensively investigated
during the last decade [1]. The group 6 radicals
[M(CO)3(p5-C5H5)] (M=Cr, Mo, W) and substituted
’
derivatives are some of the most intensely studied neu-
tral organometallic radicals. Such species can be pro-
duced via either photolysis of metal–metal bonded
18-electron dimeric compounds [2] or electron transfer
reactions of 18-electron monomers [3]. They have been
seen to engage in atom abstraction, [2]e; [4] and in the
frequently coupled ligand substitution and dispropor-
tionation reactions [5]. These radicals have also been
* Corresponding author. E-mail: salome.delgado@uam.es
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(97)00760-2

58
M.-L. Marcos et al. / Journal of Organometallic Chemistry 555 (1998) 57–66
redox chemistry of organotransition-metal compounds
[11], we report here the electrochemical study of
[WX(CO)3(p5-C5H5)] (X=Cl (1), Br (2), I (3)). The
electrochemical reduction of 1–3 had been previously
reported [12] using a mercury working electrode. In
that case, the electrode material participates signifi-
cantly in the mechanism, forming an intermediate com-
plex, [W(CO)3(p5-C5H5)]2Hg. The formation of such an
intermediate complex is not possible when using a
platinum working electrode in the present study and,
therefore, the reduction path must be different.
In order to stabilize 17-electron metal-centered radi-
cals, we have prepared and studied elctrochemically the
new compound [WI(CO)2(PCy3)(p5-C5H5)] (4), in which
there is a tricyclohexylphosphine ligand (PCy3) with a
great steric requirement (cone angle=170°).
2.2. Preparation of [WI(CO)2(PCy3)(p5-C5H5)].
Separation of two isomers
A solution of PCy3 (0.05 g, 0.18 mmol) in THF (15
ml) was added to a solution of [WI(CO)3(p5-C5H5)]
(0.05 g, 0.11 mmol) in the same solvent (30 ml). The
solution was heated at 50°C to produce a mixture of cis
(4) and trans (5) isomers. The reaction was monitored
by FTIR spectroscopy. Upon filtration and removal of
the solvent under reduced pressure, a reddish-brown
solid was obtained which was extracted with DCM, and
the cis and trans mixture was separated and purified by
TLC using DCM/hexane (1:2) as eluent. Yield: 95% cis,
5% trans. IR (THF) wCO 1942 (vs), 1859 (s) cm−1
;
(THF-toluene) wCO 1948, 1936 (sh), 1864, 1855 (sh)
cm−1. 1H-NMR (300 MHz, CDCl3): l 5.51 (s, 5H, cis),
5.40 (d, 5H, JPH 1.4Hz, trans), 1.84 (m, 15Hec), 1.24 (m,
18Hax). 31P (CDCl3) 65.24 (s, trans), 51.31 (s, cis). MS:
m/e 684 (M+ –CO). UV (THF) umax 474 nm. Anal.
Calcd. for C25H38WO2PI: C, 42.2; H 5.4. Found: C
42.3; H 5.6.
2. Experimental section
2.1. Reagents and general techniques
All manipulations were carried out by using standard
Schlenk vacuum-line and syringe techniques under an
atmosphere of oxygen-free argon. All solvents for syn-
thetic use were reagent grade. Diglyme, pentane, te-
trahydrofurane (THF), diethyl ether and hexane were
dried and distilled over sodium in the presence of
benzophenone ketyl under an Ar atmosphere. Also
2.3. Preparation of [W(CO)2(PCy3)(p5-C5H5)]2 (6) 6ia
reduction of [WI(CO)2(PCy3)(p5-C5H5)] and oxidation
of resulting Li[W(CO)2(PCy3)(p5-C5H5)]
An 1 M solution of LiEt3BH in THF was added
dropwise to a red solution of [WI(CO)2(PCy3)(p5-
C5H5)] (0.1 g, 0.14 mmol) in THF (30 ml) at 0°C until
the colour turned pale yellow. Gas was evolved dur-
ing the reaction. The mixture was filtered in a Schlenk
tube and an IR spectrum was recorded. Complete
conversion to Li[W(CO)2(PCy3)(p5-C5H5)] (wCO 1920
(vs) and 1831 (s) cm−1) was observed. An aqueous
solution of Fe2(SO4)3 (0.06 g) and glacial acetic acid
(0.1 ml) was added by syringe, the colour turned
orange-red instantly. Degassed hexane (30 ml) was
added via cannula and the two-phase mixture was
shaken vigorously, allowed to separate and then
quickly cooled to −78°C. This caused the aqueous
layer to freeze and the hexane-THF layer was then
decanted via cannula into a Ar-filled, 250 ml flask
containing anhydrous Na2SO4 (20 g). The clear
orange-red solution was decanted via cannula from
the drying agent into a Schlenk flask. Upon removal
of the solvent under reduced pressure, a reddish-
orange solid was obtained which was recrystallized
from hexane-DCM at −15°C. Yield 85%. IR (THF)
under
Ar
atmosphere,
carbon
tetrachloride,
dichloromethane (DCM), dichloroethane (DCE), and
toluene were dried and distilled over MgSO4, CaH2,
and sodium, respectively. Propylene carbonate (PC)
˚
was stored over molecular sieves (5 A) under an Ar
atmosphere. All solvents were bubbled with argon for 1
h after distillation and then stored under Ar, or de-
gassed by means of at least three freeze-pump-thaw
cycles after distillation and before use.
The compounds [W(CO)3(p5-C5H5)]2, [WH(CO)3(p5-
C5H5)] and [WX(CO)3(p5-C5H5)] (X=Cl, Br, I) were
prepared according to literature procedures [13–15].
The halo-derivatives were purified by thin-layer chro-
matography (TLC) using CH2Cl2/hexane (1:2) as elu-
1
ent. The H-NMR spectra were recorded on a Bruker
AMX-300 instrument. Chemical shifts were measured
relative to an internal reference of tetramethylsilane or
1
to residual protons of the solvents for H and H3PO4
for 31P. Infrared spectra were measured on a Perkin-
Elmer 1650 infrared spectrometer. Elemental analyses
were performed by the Microanalytical Laboratory of
the University Auto´noma of Madrid on a Perkin-Elmer
240 B microanalyser. Electronic spectra were recorded
on a Pye Unicam SP 8-100 UV-visible spectrophotome-
ter. Mass spectra were measured on a VG-Autospec
mass spectrometer for FAB or AIE by the Mass Labo-
ratory of the University Auto´noma of Madrid.
wCO 1912 (m), 1859 (sh), 1824 (vs) cm−1 1H-NMR
.
(300 MHz, CDCl3): l 5.02 (s, 5H), 1.86 (m, 15Hec),
1.25 (m, 18Hax). 31P (CDCl3) 52.1 (s). MS: m/e 585
(1/2 M+). UV (THF) umax 500, 363 nm. Anal. Calcd.
for C50H76W2O4P2: C, 51.3; H 6.5. Found: C 51.1; H
6.6.

M.-L. Marcos et al. / Journal of Organometallic Chemistry 555 (1998) 57–66
59
2.4. Electrochemical measurements
The 31P NMR spectrum at room temperature ex-
hibits two singlets at 51.31 ppm (cis) and 65.24 ppm
(trans). The ratio of cis:trans isomers obtained from the
integrals of these resonances agreed reasonably well
A computer-driven PAR Mo. 273 electrochemistry
system with positive feedback iR compensation was
employed. All experiments were carried out in a three
electrode cell under N2 atmosphere in anhydrous de-
oxygenated solvent. Temperature was maintained con-
1
with that obtained from H-NMR data.
The IR spectrum in THF exhibits two absorption
bands at 1942 (vs) and 1859 (s) cm−1 associated with
terminal W–CO stretching (wCO) modes, the higher
frecuency (sym) more intense than the lower one (anti-
sym), whilst the IR spectrum in THF-toluene shows
stant at 25°C. Solutions contained 0.2
M
tetrabutylammonium hexafluorophosphate (TBAPF6)
as supporting electrolyte, and either DCE or PC as
solvent.
four bands at 1948, 1936 (sh), 1864, 1855 (sh) cm−1
This supports the assignment made from the H-NMR
data [20]a.
.
1
Cyclic voltammetry studies were made on a pc–Pt
working electrode (0.05 cm2 real surface area, as calcu-
lated in a separate experiment through the H-adatom
charge in 0.5 M H2SO4 solution [16]). The counter
electrode was a Pt gauze. The reference electrode was
an Ag wire quasi-reference electrode (QRE), pretreated
by immersion in 10 M HNO3 for 5 min before use.
After a series of experiments, the QRE potential was
calibrated by adding ferrocene (Fc) to the solution and
taking voltammograms of the ferrocenium/ferrocene
system. Potentials in this paper are then referred to this
couple, Fc+/Fc, which has been recommeded as a
standard reference in nonaqueous solvents [17]. It can
be related to a SCE reference electrode by using a value
of +0.307 V vs SCE for Fc+/Fc [18]. Controlled
potential coulometry studies were made on a large area
Pt working electrode.
The separation of this isomeric mixture into their
components is essential to the electrochemical study
and is described in the Experimental Section. The com-
plexes are stable with respect to isomerization both in
the solid state and in solution at low temperature.
3.2. Synthesis and characterization of
[W(CO)2(PCy3)(p5-C5H5)]2 (6)
The photochemical reactions of [W(CO)3(p5-C5H5)]2
with phosphines lead to disproportionation and/or sub-
stitution products [1,21]. The ratio of substitution to
disproportionation products depends on the size and
concentration of the ligand. When the size of the ligand
increases, the monosubstituted dimer is formed prefer-
entially to the disubstituted dimer or disproportiona-
tion products. Thus, bulky ligands (e.g. PCy3, q=170°)
yield only monosubstituted products [22]. We therefore
prepared 6 from reduction of the PCy3-substituted
halide with LiEt3BH to yield the Li+ salt, Li
[W(CO)2(PCy3)(p5-C5H5)]. The IR spectrum of this
compound (1920 and 1831 cm−1) is similar to that of
Li[W(CO)3(p5-C5H5)] [23]. Acidification of the salt with
glacial acetic acid and oxidation by Fe2(SO4)3 produce
the disubstituted dimer 6, isolated as a light reddish-or-
3. Results and discussion
3.1. Synthesis and characterization of cis- and
trans-[WI(CO)2(PCy3)(p5-C5H5)] (4 and 5)
The reaction of 3 with the unidentate phosphorus
ligand PCy3 in refluxing THF gives the monosubsti-
tuted derivative [WI(CO)2(PCy3)(p5-C5H5)] in quantita-
tive yield (98%) as a mixture of cis and trans isomers.
The relative proportion of these isomers in related
compounds depends markedly on the nature of phos-
phorus ligands, the method of preparation as well as
the solvent [19]. The identity of the isomers was proven
according to well-established infrared and NMR crite-
1
ange powder. This new compound exhibits IR, H and
31P NMR spectra very similar to those of the disubsti-
tuted dimer [W(CO)2L(p5-C5H5)]2 (M=Mo, W) (L=
P-donor ligands) [24]. These data can only be explained
in terms of the existence of two isomers (cis and trans)
in solution.
1
ria [19]a; [20]. The H-NMR spectrum shows that the
3.3. Electrochemical studies
monosubstituted complex exists as a mixture of these
two isomers, the resonances due to the cyclopentadienyl
protons of cis isomer are a singlet at 5.51 ppm, while
those due to the trans isomer consist of 1:1 doublet at
5.40 ppm (JPH=1.4 Hz). Integrals of the cis and trans
resonances suggest a cis:trans ratio of ca. 95:5. This
ratio also depends on the solvent, thus when the polar-
ity of the solvent increases the percentage of cis isomer
increases (CDCl3 cis:trans 95:5; acetone-d6 cis:trans
98:2).
3.3.1. Reduction of 1, 2 and 4
The voltammetric reductions of 1, 2 and 3 show
similar characteristics. An irreversible cathodic wave is
observed for each of the compounds, and the solvent
dependent peak potentials, Epc, are assembled in Table
1. In the reduction process, a reduced species is formed
which reoxidizes upon scan reversal at a much more
positive potential, Ereox (Fig. 1, Table 1). When the
anodic scan following the reduction of 1–3 is extended

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M.-L. Marcos et al. / Journal of Organometallic Chemistry 555 (1998) 57–66
Table 1
Electrochemical data for the reduction of 1, 2 and 3 in solvents DCE and PC
1
2
3
DCE
PC
PC
PC
Epc (V vs Fc+/Fc)a
pc/C (mA cm−2 M−1 a, b
−1.73
208
—
—
—
—
−0.68
32
−1.53
162
65
0.46
100
0.48
−0.47
65
−1.49
150
75
0.40
110
0.43
−0.45
72
−1.46
145
74
0.41
115
0.41
−0.46
73
i
)
Epc vs log 6 slope (mV)
h (from Epc vs log 6)
ꢀEpc−Ep/2ꢀ (mV)
h (from ꢀEp−Ep/2ꢀ)
Ereox (V vs Fc+/Fc)a
i
pa/C (mA cm−2 M−1 a, c
)
Eadd(V vs Fc+/Fc)a, d
0.49
0.07; 0.38; 0.49
0.04; 0.37; 0.49
a Data at 6=0.1 Vs−1
.
b Reduction peak current density/C.
c Reoxidation peak current density/C.
d Additional peaks in the anodic scan. Peak for [WX(CO)3(p5-C5H5)] oxidation is omitted.
until 1 V vs Fc+/Fc, additional oxidation peaks can be
observed in the voltammograms of the three com-
pounds (Fig. 2, solid line). One of them, at around 0.65
V, corresponds to the oxidation of 1–3 and will be
further discussed in the following section. The other
peaks do not appear when the cell is switched at −0.6
V (a potential value less negative than that necessary to
reduce 1–3) and scanned in the positive direction (Fig.
2, dashed line); therefore, they must be due to the
oxidation of species produced by the reduction of 1–3
or by the subsequent reoxidation at ca. 0.46 V. One of
these peaks, at +0.49 V, is common to the three
compounds; the remaining ones are dependent on the
nature of the halogen atom (Eadd in Table 1).
dissolved in 0.2 M TBAPF6 in PC (yielding red solu-
tions) and reduced at −2.15 V vs Fc+/Fc, a potential
well within the diffusion-limited region of the reduction
waves. The amount of electricity consumed when the
current decayed to background (after ca. 1 h) (the
colours of the end solutions were very pale), indicated
that all the reductions were two-electron processes.
Cyclic voltammograms recorded inmediately after the
electrolyses did not show any reduction peaks when the
sweeps started at −0.6 V in the cathodic direction,
indicating that 1–3 had disappeared from the solutions.
However, when the scans began at −0.8 V in the
positive direction, an anodic peak at −0.46 V (6=0.1
Vs−1) appeared, with a current density higher than that
for the starting solutions of 1–3.
The overall number of electrons transferred in the
reduction process was determined by controlled poten-
tial coulometry. Thus, weighted amounts of 1–3 were
Fig. 1. Cyclic voltammogram for the reduction of a 2.1×10−3
M
Fig. 2. Cyclic voltammogram of a 2.1×10−3 M solution of 2 in PC
containing 0.2 M TBAPF6 at 0.1 Vs−1 and 25°C. Scans started at
−0.6 V in the cathodic direction (——) and in the anodic direction
(– – –).
solution of 2 in PC containing 0.2 M TBAPF6 at 0.1 Vs−1 and 25°C.
The first (——) and second (– – –) scans are shown. An asterisk
indicates the start of a cycle.

M.-L. Marcos et al. / Journal of Organometallic Chemistry 555 (1998) 57–66
61
Reoxidations of the previously formed reduced spe-
cies were also attempted by controlled potential cou-
lometry. However, the current intensity decayed to the
background value with less than 1 F mol−1 (ca. 0.75)
being consumed, although the colour of the solutions
turned red. Cyclic voltammograms recorded inmedi-
ately after such reoxidations were significatively differ-
ent from the original ones, showing several cathodic
peaks that are poorly defined and difficult to identify.
The voltammetric study at different sweep rates (6)
reveals that, for the three compounds, the reduction
peak current density (ipc) is a linear function of 61/2 in
the range 0.01B6B10 Vs−1. A systematic shift in the
peak potentials with scan rate was observed, and the
plots of Epc vs log 6 are linear for 6 up to 1 Vs−1. The
slopes of these plots are assembled in Table 1. The
width of the waves were also studied. Experimentally
observed ꢀEp−Ep/2ꢀ values, where Ep/2 is the potential
at half-height, are also shown in Table 1 for the reduc-
tion processes. They are always greater than 30 mV, the
value which would correspond to a reversible, diffu-
sion-controlled, two-electron transfer.
an unpaired electron in a W-centered orbital of less
energy than the starting W–X antibonding orbitals.
Therefore, the introduction of a new electron is now
more favourable than in 1–3, and reduction takes place
more easily; that is, M· is readily electroreducible at the
reduction potential of M–X and an overall two elec-
tron transfer is observed, yielding the anion M−
([W(CO)3(p5-C5H5)]− (7−)).
7− is also the final product of the reduction of the
dimer [W(CO)3(p5-C5H5)]2, in which there is a W–W
bond, and which has been voltammetrically studied by
Anderson et al. [10]d. Reoxidation of the anion was
found to follow Eqs. (6) and (7):
[W(CO)3(h5-C5H5)]− ?[W(CO)3(h5-C5H5)]· +e− (6)
2[W(CO)3(h5-C5H5)]· “[W(CO)3(h5-C5H5)]2
(7)
Thus, the final product is [W(CO)3(p5-C5H5)]2. This
process was studied in DCM, THF and acetonitrile
[10]e, and both the potential for dimer reduction and
the potential for anion reoxidation were found to de-
pend on the solvent. In order to have exactly the same
experimental conditions, we have also studied voltam-
metrically [W(CO)3(p5-C5H5)]2 in PC, finding that it is
reduced at −1.61 V vs Fc+/Fc, and that the resulting
anion is reoxidized at Ereox= −0.46 V. This Ereox is
practically the same as that obtained for 1, 2 and 3
(Table 1), in agreement with the proposed mechanism.
On the other hand, we have found that [W(CO)3(p5-
C5H5)]2 is oxidized at +0.49 V in PC, the potential
value of the additional peak found in the voltam-
mograms of the three compounds when reduction of
1–3 was initially performed. This corroborates that
reduction of 1–3 leads to 7−, and that reoxidation of
this anion yields the dimer [W(CO)3(p5-C5H5)]2, which
is absent in the starting solution.
The linear dependency of Ep on log 6, with slopes
greater than 60 mV per decade, as well as the width of
the waves, are indicative of irreversible charge transfer
processes. The transfer coefficient, h, can be calculated
from the Ep vs log 6 plot according to [25]:
Ep=(2.3RT/2hnhF) log 6+constant,
(1)
where nh is the number of electrons transferred in the
rate limiting step. Calculated h values assuming nh=1
are assembled in Table 1. Alternatively, h can be calcu-
lated from the width of the wave [25]:
ꢀEp−Ep/2ꢀ=1.857(RT/hnhF)
(2)
Table 1 also lists h values calculated by this method,
which are consistent with those obtained from the Ep vs
log 6 slope.
All the above data, the shapes of the voltam-
mograms, and the reported studies on related com-
pounds [26], suggest that the reduction follows an ECE
type mechanism according to Eqs. (3)–(5), where M=
W(CO)3(p5-C5H5).
X− is also a product of the reduction process, and
can be oxidized in the subsequent anodic scan. In order
to clearly identify the peaks due to this process, small
amounts of KX were added to the solutions, and scans
performed from −0.6 V in the positive direction. For
X=Br, a complex oxidation peak at ca. 0.38 V is
observed. For X=I two new peaks appear at 0.04 V
and at 0.37 V. All these halide oxidation peaks are
observed in the voltammograms of the corresponding
solutions of 2 and 3 after initial reduction (see Eadd in
Table 1). No extra peaks appear for X=Cl in the
potential range studied, in accordance with the higher
value of E0 for the Cl2/Cl− couple.
The anodic peak at 0.07 V, which is seen after the
reduction of 2, is not due to the oxidation of Br− or
[W(CO)3(p5-C5H5)]2. It could be due to the oxidation of
the product of a different kind of reaction of the
radicals formed after reoxidation of 7−, probably with
solvent molecules involved [1]m.
M–X+e− “[M–X]−
[M–X]− ?M· +X−
M· +e− ?M−
(3)
(4)
(5)
In the first electrochemical step, an electron is trans-
ferred to the LUMO of the compound, which is proba-
bly an antibonding W-halide orbital. Eq. (3) is rate
determining and yields an anion radical with a very
weak W–X bond [26]. As a result of this, an homo-
geneus chemical reaction readily takes place Eq. (4) in
which the weakened W–X bond is broken, yielding X−
anions and M· radicals ([W(CO)3(p5-C5H5)] (7)). 7 has

62
M.-L. Marcos et al. / Journal of Organometallic Chemistry 555 (1998) 57–66
The diffusion coefficients, D, of 1–3 were obtained in
PC from the chronoamperometric i–t transients ac-
cording to [27]:
it1/2=nFAC(D/y)1/2
,
(8)
where n is the overall number of electrons transferred
per molecule (n=2), A is the electrode area and C is
the bulk concentration of electroactive species. Several
of these transients were recorded at different constant
potentials in the range of −1.8 to −2.1 V vs Fc+/Fc,
i.e. where the charge transfer is diffusion controlled.
The plots of i vs t−1/2 were always linear and the slopes
almost coincident, but best results were obtained at ca.
−2.0 V vs Fc+/Fc. From the slope of these plots, D is
1.50×10−6, 1.45×10−6 and 1.43×10−6 cm2 s−1, for
1, 2 and 3, respectively. These values are in very good
agreement with those expected from the direct applica-
tion of the following equation [18]:
Fig. 3. Cyclic voltammogram for the oxidation of a 1.4×10−3
M
solution of 3 in PC containing 0.2 M TBAPF6 at 0.1 Vs−1 and 25°C.
An asterisk indicates the start of the run.
ip=(2.99×105)n(hnh)1/2ACD1/261/2
(9)
both the intermediate anion radical and the anion
formed after total reduction, making this process easier.
The change in ipc when the solvent is DCE or PC is in
agreement with the different viscosities of both media
(2.53 cP for PC and 0.785 cP for DCE). According to
Walden’s (Dp=D%p%), DDCE ca. 3.22DCP and, conse-
quently, ipc is higher in DCE, as observed.
which is valid for irreversible processes. Considering
n=2 and nhh=0.47 for 1 and 0.41 for 2 and 3 (Table
1), and averaging the results in the 0.01B6B1 Vs−1
range, (9) yields D=1.52×10−6, 1.50×10−6 and
1.40×10−6 cm2 s−1 for 1, 2 and 3, respectively.
When the values of Epc in PC are compared for 1, 2
and 3, a trend to less negative Epc is found from the Cl
to the I compound. A more negative Epc is indicative of
a greater difficulty to reduce the compound, i.e. a
greater stability towards this process as the electrons
transferred occupy orbitals of a higher energy. As dis-
cussed above, in [WX(CO)3(p5-C5H5)] these LUMO are
probably W–X antibonding orbitals. A higher energy
W–X antibonding orbital corresponds to a more stable
W–X bonding orbital when W=Cl, as a result of the
stronger and more efficient interaction between W and
Cl. Thus, the stability of 1–3 towards reduction in-
creases with the increasing electronegativity and de-
creasing size of the halide. A previous work on the
reduction of [WX(CO)3(p5-C5H5)] was reported by
Denisovich et al. [12]c in 1968. The working electrode
material, Hg, actively participated in the reduction
mechanism, as the intermediate radical [W(CO)3(p5-
C5H5)], formed after electron transfer and W–X bond
breaking, reacted with Hg to yield [W(CO)3(p5-
C5H5)]2Hg. This latter species was reduced in a second
wave to [W(CO)3(p5-C5H5)]−. Although the conditions
were different from those of the present study, the same
trend in the reduction potential with the nature of X
was found.
3.3.2. Oxidation of 1, 2 and 3
The electrochemical oxidation of 1, 2 and 3 was
studied in PC. An irreversible anodic wave with two
associated cathodic processes (at ca. 0.1 V and ca.
−0.6 V) is found for the three compounds (Fig. 3).
Peak potentials and peak currents are assembled in
Table 2. Coulometry of the oxidation wave was at-
tempted, but the results were not conclusive. The cur-
rents decayed to background long before 1 F mol−1
had been transferred. This kind of problem seems to be
common in the coulometric determination for the oxi-
dation of related compounds [11,28]. Poisoning of the
electrode and compound decomposition are probably
taking place.
Lau et al. [9]f,g reported the oxidation of
[MoX(CO)3(p5-C5H5)] (X=Cl, Br, I) in DCM and
acetonitrile on a Pt working electrode. They observed
monoelectronic irreversible waves (the oxidation of the
Table 2
Electrochemical data for the oxidation of 1, 2 and 3 in PC
Changing the solvent significantly influences the
value of Epc. For 1, the reduction peak is ca. 0.2 V
more negative in DCE than in PC. The direction of the
shift agrees with what might be expected from the
higher dipole moment and dielectric constant of PC
respect to DCE. The polar molecules of PC stabilize
Compound
1
2
3
Eox (V vs Fc+/Fc)a
0.62
150
0.64
90
0.66
88
i
pa/C (mAcm−2 M−1 a, b
)
a Data at 6=0.1 Vs−1
b Oxidation peak current density/C.
.

M.-L. Marcos et al. / Journal of Organometallic Chemistry 555 (1998) 57–66
63
iodine compound was reversible at 5 Vs−1 in DCM).
The process follows Eqs. (10) and (11):
[MoX(CO)3(h5−C5H5)]
“[MoX(CO)3(h5−C5H5)]+ +e−
(10)
(11)
[MoX(CO)3(h5−C5H5)]+ “decomposition
Based on the above results with compounds closely
related to 1–3, together with the similarity of the
reported waves, we think that the oxidations of 1, 2 and
3 are also one electron processes, leading to the cations
[WX(CO)3(p5-C5H5)]+, which are very unstable and
readily decompose. The assignment of n=1 is in agree-
ment with the current density of the oxidation peak as
compared with that of the reduction peak (where n=2)
for 2 and 3. However, for 1 (ip)ox is higher than
expected for n=1, and the process may be more com-
plicated. One possibility is that the follow-up chemical
reaction of 1+ produces a species that can be further
oxidized at Eox(1), thus yielding an ECE global mecha-
nism.
In the present study, none of the compounds showed
any sign of coupled cathodic processes on the reverse
scan with sweep rates as high as 80 Vs−1, which
establishes the existence of follow-up chemical reactions
that are fast on the time scale of the cyclic voltammetry
experiment.
Fig. 4. Cyclic voltammogram for the oxidation of a 1.5×10−3
M
solution of 1 in DCE containing 0.2 M TBAPF6 at 0.1 Vs−1 and
25°C. An asterisk indicates the start of the run.
3.3.3. Electrochemistry of 4
The voltammetric reduction of 4 in PC gives rise to
an irreversible cathodic wave (Fig. 5) with Epc signifi-
cantly more negative than that of 3 (−1.94 V as
compared to −1.46 V). Upon scan reversal, a reoxida-
A comparison of Eox values in Table 2 reveals that
the oxidation potential increases when X changes
from Cl to Br to I. The observed differences are
small, but significant and in agreement with the trend
found for the above mentioned [MoX(CO)3(p5-C5H5)]
[9]f,g (in DCM at 6=0.2 Vs−1: +0.63, +0.64 and
+0.68 V vs Fc+/Fc for X=Cl, Br, I, respectively).
The order is opposite to what would be expected if
only the halogen electronegativity (decreasing from Cl
to I) is considered. Decreasing X electronegativity
would involve increasing electron density on the metal
atom, thus decreasing Eox. The opposed trend, called
‘inverse halide order’ by Zietlow et al. [29], was at-
tributed to metal-d to halogen-d backbonding. Many
other similar systems have been reported in the litera-
ture [30]. Using such explanation, iodine is the best
y-aceptor and allows the most effective backbonding.
This gives the order ClBBrBI, the same as found in
Eox.
The oxidation of 1 has also been studied in DCE. An
anodic peak at +0.66 V vs Fc+/Fc (ip/C=166 mA
cm−2 M−1 at 0.1 Vs−1), analogous to that found in
PC, is obtained (Fig. 4). However, it is followed by a
second peak at +0.96 V (71 mA cm−2 M−1 at 0.1
Vs−1), which could be a further oxidation of a product
of the chemical reaction following the first charge trans-
fer.
Fig. 5. Cyclic voltammograms for the reduction of a 8.5×10−4
solution of 4 in PC containing 0.2 M TBAPF6 at 25°C. The first
(——) and second (– – –) scans are shown in a. The sweep rate is 0.1
Vs−1 in a, and 1 Vs−1 in b. An asterisk indicates the start of a cycle.
M

64
M.-L. Marcos et al. / Journal of Organometallic Chemistry 555 (1998) 57–66
Table 3
them takes place at +0.04 V and is observed as a
shoulder of the main oxidation peak at +0.23 V. This
peak is due to the oxidation of I− produced in the
reduction of 4, as confirmed by in situ addition of a
small amount of KI to the solution. No clear peak for
the oxidation of 6 is observed, but it could easily be
masked by the peaks at 0.04 and 0.23 V. Therefore,
there is no evidence of dimerization taking place, but it
cannot be discarded.
Other possibilities apart from dimerization can ac-
count for the chemical reaction in the EC mechanism.
Seemingly, the introduction of a solvent molecule
would yield a 19-electron species which could be re-
sponsible for the small oxidation peaks that appear at
−0.47 and at −0.23 V in the voltammogram (Fig. 6).
The voltammetric oxidation of 4 gives rise to two
peaks, at +0.23 V and at ca. +0.5 V (6=0.1 Vs−1).
The second peak is completely irreversible even at 80
Vs−1. However, a coupled cathodic peak is obtained
for the first one at 6]0.02 Vs−1. The reversibility is
more clearly appreciated when the scan is reversed
before the second peak (Fig. 7). The peak current ratio
ipc/ipa, although not accurately determined due to the
vicinity of the peak at +0.5 V, clearly approaches 1 as
6 increases (Fig. 7b). DEp parallels that of ferrocene
under the same conditions.
Electrochemical data for the reduction and oxidation of 4 in PC
4“8−
8−“8
4“4+
Ep (V vs Fc+/Fc)a
−1.94
130
75
−1.24c
30
0.23d
110
ip/C (mA cm−2 M−1 a, b
Ep vs log 6 slope (mV)
h (from Ep vs log 6)
)
0.40
a Data at 6=0.1 Vs−1
.
b Peak current density/C.
c Anodic peak potential, E1/2=−1.27 V.
d Anodic peak potential, E1/2=0.20 V.
tion peak at −1.24 V (6=0.1 Vs−1) is found. The
general shape of the waves is similar to those obtained
with 1–3 solutions. Peak potentials, peak currents (ipc is
a linear function of 61/2 in the range 0.01B6B10
Vs−1) and wave parameters are assembled in Table 3.
All these data imply that the reduction process must
follow a path analogous to that of Eqs. (3)–(5).
The negative shift in Epc when a CO ligand is substi-
tuted by PCy3 is in agreement with the greater basicity
of PCy3, as reflected by the decrease of the value of the
higher frequency wCO in 4 (1942 cm−1) with respect to
3 (2032 cm−1). The substitution increases the electron
density on the W atom and stabilizes 4 towards reduc-
tion. The final product of the reduction process is the
anion [W(CO)2(PCy3)(p5-C5H5)]− (8−), which is oxi-
dized to the radical species [W(CO)2(PCy3)(p5-C5H5)]
(8) at −1.24 V (6=0.1 Vs−1). In the related com-
pound 3, the radical 7 readily dimerizes and no ca-
thodic peak is found upon scan reversal, indicating that
the rate constant of the dimerization reaction Eq. (7) is
large. However, when the scan is reversed at −0.6 V in
a solution of 4 (Fig. 5a) a reduction peak is observed
even at a sweep rate as slow as 0.05 Vs−1. The peak
current ratio ipc/ipa for the 8/8− system approaches 1 as
v increases (Fig. 5b). On the other hand DEp=Epa−
Epc parallels that of ferrocene under the same condi-
tions (it is 58 mV at 0.1 Vs−1, but increases slightly
with scan rate; the behaviour of ferrocene was
analogous). All these data indicate that the oxidation of
8− follows an EC scheme. A reversible charge transfer
yielding the radical 8 is followed by a homogeneous
chemical reaction whose rate constant is rather small.
Therefore, the presence of the bulky PCy3 ligand
greatly stabilizes the 17-electron radical species 8.
The chemical reaction could be a dimerization to
[W(CO)2(PCy3)(p5-C5H5)]2 (6), and an independent ex-
periment was carried out in order to check this idea.
Thus, a solution of 6 was prepared, and cyclic voltam-
metry performed. 6 is oxidized at +0.1 V by an EC
process (at 6=10 Vs−1, a cathodic peak is obtained
upon scan reversal). When the anodic sweep after re-
duction of 4 was extended to +1 V (Fig. 6), some
additional peaks appear in the voltammogram; one of
The irreversible peak at ca. 0.5 V seems to have at
least two components. The more anodic one becomes
comparatively more important as 6 increases (compare
Fig. 7a and b). It has two associated reduction waves,
at −0.1 V and at −0.75 V. The latter eventually
disappears as 6 increases, indicating that it is related to
the less anodic component of the complex irreversible
oxidation peak.
Fig. 6. Cyclic voltammogram of a 8.5×10−4 M solution of 4 in PC
containing 0.2 M TBAPF6 at 0.1 Vs−1 and 25°C. Scans started at ca.
−1.0 V in the cathodic direction (——) and in the anodic direction
(– – –).

M.-L. Marcos et al. / Journal of Organometallic Chemistry 555 (1998) 57–66
65
Acknowledgements
We express our great appreciation of financial sup-
port from the DGICYT (Project PB 91-0003); Spain.
The authors whish to thank Dr Ying Zheng for careful
revision of the text.
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