Electrochemical behavior of the Cp*(dithiolato)Co(III) complex [(η5-Cp*)Co(1,2-S2C2 B10H10-S,S′)] and its derivatives: The effect of the ligand on half-sandwich cobalt(III) complexes

Article abstract of DOI:10.1016/j.jorganchem.2003.09.019

Reaction of the 16-electron dithiolatocobalt, [(η⁵- Cp*)Co(Cab^S,S′)] (1) (Cab^S,S′=1, 2-S₂C₂B₁₀H₁₀-S,S′), with a ligand L (L=CN^tBu, PMe₃, PEt₃, PHPh₂) affords the 18-electron complexes [(η⁵- Cp*)Co(Cab^S,S′)(L)] (2) in high yield. The structures of 2 in solution were probed by NMR spectroscopy, and the solid-state structure of the diphenylphosphine derivative was established by single-crystal X-ray diffraction. Cyclic voltammetry of 1 shows that the complex undergoes a quasi-reversible one-electron oxidation and reversible reduction. The voltammetry of the corresponding 18-electron complex 2 was investigated rather extensively in terms of coordination and redox chemistry, in which the influence of the enhanced steric interaction between the bulky Cp* and incoming ligand could be observed. Cyclic voltammograms of alkylidiene or acetylene adducts of dithiolatocobalt complex 1, [(η⁵-Cp*)Co{η ³-(S,S′, C′)-SC₂B₁₀ H₁₀S(C′R₂)}] [C′R₂=CHSiMe ₃ (3a), HC=CPh (3b), (COOMe)C=C(COOMe) (3c)], are also included.

Full text of DOI:10.1016/j.jorganchem.2003.09.019

Journal of Organometallic Chemistry 688 (2003) 236ꢃ245
/
www.elsevier.com/locate/jorganchem
Electrochemical behavior of the Cp*(dithiolato)Co(III) complex [(h5-
Cp*)Co(1,2-S2C2B10H10-S,S?)] and its derivatives: the effect of the
ligand on half-sandwich cobalt(III) complexes
Bo Young Kim a, Chongmok Lee a,*, Seung Won Chung b, Young-Joo Lee b,
JaeYoun Pak a, Jaejung Ko b,*, Sang Ook Kang b,*
a Department of Chemistry, Ewha Woman’s University, Seoul 120-750, South Korea
b Department of Chemistry, Korea University, 208 Seochang, Chochiwon, Chung-nam 339-700, South Korea
Received 5 July 2003; received in revised form 4 September 2003; accepted 11 September 2003
Abstract
Reaction of the 16-electron dithiolatocobalt, [(h5-Cp*)Co(CabS,S?)] (1) (CabS ,S?ꢀ
/
1,2-S2C2B10H10-S,S?), with a ligand L (Lꢀ
/
CNtBu, PMe3, PEt3, PHPh2) affords the 18-electron complexes [(h5-Cp*)Co(CabS ,S?)(L)] (2) in high yield. The structures of 2 in
solution were probed by NMR spectroscopy, and the solid-state structure of the diphenylphosphine derivative was established by
single-crystal X-ray diffraction. Cyclic voltammetry of 1 shows that the complex undergoes a quasi-reversible one-electron oxidation
and reversible reduction. The voltammetry of the corresponding 18-electron complex 2 was investigated rather extensively in terms
of coordination and redox chemistry, in which the influence of the enhanced steric interaction between the bulky Cp* and incoming
ligand could be observed. Cyclic voltammograms of alkylidiene or acetylene adducts of dithiolatocobalt complex 1, [(h5-
Cp*)Co{h3-(S,S?,C?)-SC2B10H10S(C?R2)}] [C?R2ꢀ
/
CHSiMe3 (3a), HCꢀ
/
CPh (3b), (COOMe)Cꢀ
/C(COOMe) (3c)], are also
included.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Dithiolatocobalt; 18-Electron dithiolate; Redox process; Cyclic voltammetry
1. Introduction
The redox patterns of mononuclear and dinuclear
complexes are different from each other. Thus, the
electrochemistry of 1 and its derivatives may provide a
clearer view of the redox chemistry, since the mono-
nuclear complexes will be free from dinuclear com-
We recently reported new coordination modes for
coordinatively unsaturated half-sandwich metal com-
plexes [1] possessing an ancillary dithio-o-carboranyl
ligand [2], in which the redox stability of the complexes
was dependent on the ancillary ligand Cp or Cp*. For
plexes. In our previous work with (h5-Cp)Co(CabS,S?
(I) [3] and (h6-arene)Ru(CabS,S?) (II) [4], we also found
that 16-electron complexes had a reversible 0/ꢁ
reduction with a much less reversible 0/ꢂ1 oxidation;
)
example, (h5-Cp)Co(CabS,S?) (I) reacts with BH3×
/THF
/
1
to give a dinuclear complex of [(h5-CpCo)2(CabS,S?)],
while (h5-Cp*)Co(CabS,S?) (1) was suppressed from
making the corresponding dinuclear complex probably
due to the bulkiness of the Cp* ligand (Chart 1) [3].
/
the reversibility of the latter process was improved upon
the addition of a ligand (i.e. it occurs via an 18-electron
complex). However, we had some difficulties in obser-
ving the reversibility of the 0/ꢁ
on coordination in the Ru complexes due to solvent
window, and the 0/ꢂ1 oxidation in the cobalt complexes
/1 reduction, depending
/
due to a lack of electron-donating ability of the Cp
ligand. This redox pattern of 16- versus 18-electron
complexes could be exemplified more clearly by 1 rather
* Corresponding authors. Tel.: ꢂ
5396.
E-mail address: sangok@korea.ac.kr (S.O. Kang).
/
82-41-860-1334; fax: ꢂ82-41-867-
/
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.09.019

B.Y. Kim et al. / Journal of Organometallic Chemistry 688 (2003) 236ꢃ
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237
product [(h5-Cp*)Co(CabS,S?)(CNtBu)] (2a) in 95%
yield. The structure of 2a was determined on the basis
of IR and NMR spectra, as well as elemental analysis.
Thus, complex 2a exhibited a single CN stretching
absorption at 2181 cmꢁ1 in the IR spectrum, which is
quite normal for a terminal CoꢃCNR ligand with
/
significant p-back-bonding of the isocyanide ligand.
1
The H-NMR spectrum of 2a had a singlet at d 1.60
due to a tert-butyl moiety.
Reaction of 3 with an excess of PR3 in toluene
Chart 1.
afforded red crystals of 2bꢃ
31P{1H}-NMR of 2bꢃ
d had a singlet at around d 7.85ꢃ
26.67. The 1H-NMR data for 2bꢃ
d were consistent with
/
d in 82ꢃ
/
94% yield. The
/
/
than I due to the stronger electron-donating ability of
Cp*; therefore, this is one of the motivations for this
study.
/
the structure of 2d determined by an X-ray structural
study. The ORTEP diagram in Fig. 1 shows the molecular
structure of 2d and confirms the six-coordinate geome-
try about the cobalt atom, assuming that the Cp* ring
serves as a three-coordinate ligand. The Co atom
environment is transformed from a two-legged piano-
stool geometry in 1 to a three-legged version in 2 [6].
Overall, the structure is octahedral with an idealized Cs
molecular symmetry, similar to that observed in other
related complexes such as [(h5-Cp)Co(CabS,S?)(PEt3)]
[3], [(h5-Cp*)Ir(CabS,S?)(PMe3)] [1d] and (h6-p-cym-
To explore the effects of ancillary Cp* ligand in terms
of coordination and redox chemistry, we have synthe-
sized
Cp*)Co(CabS,S?)(L)] (2) [Lꢀ
PEt3 (2c), PHPh2 (2d)] and [(h5-Cp*)Co{h3-(S,S?,C’)-
SC2B10H10S(C’R2)}] [C’R2ꢀCHSiMe3 (3a), HCꢀCPh
(3b), (COOMe)CꢀC(COOMe) (3c)]. The comparison of
a
series of 18-electron dithiolates [(h5-
/
CNtBu (2a), PMe3 (2b),
/
/
/
these two dithiolatocobalts in terms of the ancillary
ligands, Cp and Cp*, is our main interest because they
possess the desired differences in electron-donating
ability and structural bulkiness.
ene)Ru(CabS,S?)(L)
(Lꢀ
/
PPh3,
P(OMe)3,
NH3,
NC5H5, CO, CNtBu, SEt2, SC4H8, CNꢁ, SCNꢁ) [5].
2.2. Electrochemistry of complexes 1 and 2
2. Results and discussion
Electrochemical data obtained in CV experiments are
summarized in Table 1. Fig. 2 shows cyclic voltammo-
2.1. Synthesis and spectroscopic characterization of [(h5-
Cp*)Co(CabS,S?)(L)] LꢀCNtBu (2a), PMe3 (2b),
PEt3 (2c), PHPh2 (2d))
/
grams (CVs) of complexes 1 and 2aꢃd in CH2Cl2
/
containing 0.1 M TBAB with a potential scan rate (v)
of 0.1 V sꢁ1. The CV of 1 shows two major redox
Reaction of the 16-electron complex 1 with 2-electron
donor molecules such as tert-butyl isocyanide and
triethylphosphine gave various 18-electron complexes
(Scheme 1). Our results are closely related to those of the
reported 16-electron cobalt, ruthenium, rhodium, os-
mium, and iridium complexes such as (h5-
Cp)Co(CabS,S?) [3], (h6-p-cymene)Ru(CabS,S?) [4], (h5-
Cp*)Rh(CabS,S?) [3], (h6-p-cymene)Os(CabS,S?) [5], and
(h5-Cp*)Ir(CabS,S?) [1d].
Treatment of the mononuclear 1 with an excess of
CNtBu in toluene gave the reddish-brown addition
Fig. 1. Molecular structure of [(h5-Cp*)Co(CabS ,S ?)(PHPh2)2d×
C7H8)
with atom labeling; ellipsoids show 30% probability levels and
hydrogen atoms have been omitted for clarity.
/
Scheme 1. Addition reaction of 16-electron metal complexes 1 a.

238
B.Y. Kim et al. / Journal of Organometallic Chemistry 688 (2003) 236ꢃ
/245
Table 1
The peak potentials Epcn(red) and Epan(ox) of (h5-Cp*)Co(CabS ,S ?)(L) from CV data
Compound
L
Reduction step
Oxidation step
Epc1 (V)
Epc2 (V)
Epa1 (V)
Epa2 (V)
1
ꢁ
/
1.14
1.66
1.65
1.14
1.04
1.10
1.78
1.37
0.85
0.60
0.66
0.52
0.71
0.24
0.13
0.59
2a
2b
2c
2d
3a
3b
3c
CNt Bu
PMe3
PEt3
ꢁ
/
ꢁ
/
ꢁ
/
ꢁ
/
1.75
1.52
1.91
0.85
0.90
PHPh2
h1-CHSiMe3 ꢃ
ꢁ
/
ꢁ
/
/
S
ꢁ
/
ꢁ/
h1-HCꢀ
/
CPhꢃ
/
S
ꢁ
/
h1-(COOMe)Cꢀ
/
C
ꢁ/
(COOMe)ꢃS
/
Potentials are recorded versus E1/2 (FcꢂjFc).
at E1/2
ꢀ
/
ꢁ/0.59 V and oxidation at Epa
ꢀ0.99 V) and
/
the improved reversibility of the oxidation wave. As the
ligand was changed from Cp to Cp*, both the reduction
and oxidation potentials shifted towards the negative
direction by 0.48 and 0.14 V, respectively; this indicates
the stronger electron-donating ability of Cp* relative to
Cp. The CV of (h5-Cp*)Co(CabS,S?)(CNtBu) (2a) has
two major redox responses, that is, an irreversible 0/ꢁ
wave (Epcꢀ 1.66 V) and a reversible 0/ꢂ1 wave
(E1/2 0.55 V). Another small oxidation peak at ꢁ
/1
/
ꢁ
/
/
ꢀ
/
/
1.08 V is apparently due to the oxidation of the 16-
electon complex 1, which is produced by elimination of
the Lewis base. The CV pattern is similar to that of (h5-
Cp)Co(CabS,S?)(CNtBu) except for an enhancement of
the 0/ꢂ1 reversibility and a lack of other small peaks [3].
/
Both the reduction and oxidation potentials of 2a shift
towards the negative direction compared with those of 1
Fig. 2. Cyclic voltammograms (CVs) of (h5-Cp*)Co(CabS ,S ?) (1) (A)
and [(h5-Cp*)Co(CabS ,S ?)(L)] [LꢀCNt Bu (2a) (B), PMe3 (2b) (C),
PEt3 (2c) (D), PHPh2 (2d) (E)] at a platinum disk electrode with
/
negative initial scan direction; in CH2Cl2 containing 0.1 M TBAB with
the scan rate (v) of 0.1 V sꢁ1
.
responses, that is, a reversible 0/ꢁ
(Co3ꢂ/Co2ꢂ) at E1/2
1.07 V [3,6,7] and a quasi-
reversible 0/ꢂ1 oxidation wave of the cobaltadithiolene
ring at E1/2
/
1 reduction wave of
ꢀ
/
ꢁ
/
/
ꢀ0.79 V. The general pattern of the CV is
/
Fig. 3. Effect of the scan rate (v) on the CVs of (h5-
the same as (h5-Cp)Co(CabS,S?) (I) except for the
corresponding potential values (for I, reduction occurs
Cp*)Co(CabS ,S ?)(CNt Bu) (2a); vꢀ1.0 (A); 0.10 (B); 0.020 V sꢁ1
(C). Other conditions are the same as in Fig. 1.
/

B.Y. Kim et al. / Journal of Organometallic Chemistry 688 (2003) 236ꢃ
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239
by 0.52 and 0.24 V, respectively; this implies that both
the metal and sulfur atoms become more electron rich
by the addition of Lewis base.
The CVs of 2a as a function of v are illustrated in Fig.
3. At vꢀ
(ipa) at E :
at E : 1.66 V is 0.48. The same current ratio becomes
smaller as the scan rate decreases, that is, 0.25 at vꢀ
0.10 V sꢁ1 and virtually zero at vꢀ0.020 V sꢁ1. The
oxidation peak at E : 1.08 V is not observed in the
CV with a potential scan range between ꢁ0.15 and
1.35 V; however, it does appear in the CV of the first
cycle with a potential scan range between ꢁ0.15 and
1.85 V but with only a vague hint of the correspond-
ing re-reduction peak at the next cycle even with vꢀ1.0
/
1.0 Vsꢁ1, the ratio of the anodic peak current
/
ꢁ1.08 V to the cathodic peak current (ipc)
/
/
ꢁ
/
/
/
/
ꢁ
/
/
ꢁ
/
/
ꢁ
/
/
V sꢁ1 (data not shown). Thus, this behavior can be
explained as follows: the CNtBu ligand is eliminated
upon reduction of the electronically stable 18-electron
complex 2a due to the excess electron density, but the
ligand remains near the coordination sphere to re-
coordinate and therefore regenerate 2a (Eq. (1); see
more detailed mechanism is expected as in Refs. [7,8]).
Judging from the potential scan rate, the re-coordina-
tion process is completed within a few seconds.
Fig. 4. Effect of the scan rate (v) on the CVs of (h5-
Cp*)Co(CabS ,S ?)(PEt3) (2c); vꢀ1.0 (A); 0.10 (B); 0.020 V sꢁ1 (C).
Other conditions are the same as in Fig. 1.
/
observed. To understand the CV responses better, we
also investigated the effect of v on the CV of 2c (Fig. 4).
By analyzing the CV obtained at vꢀ
1.0 V sꢁ1 (Fig.
4A), we can clearly separate it into the sum of two CVs:
/
ð1Þ
one is a CV with a reversible reduction and quasi-
reversible oxidation wave near E1/2
respectively, which is the same as the CV of 1 (the 16-
ꢀ
/
ꢁ1.08 and 0.79 V,
/
electon complex); the other is a CV with an irreversible
The CV of complex 2b resembles that of 2a, that is, it
reduction and quasi-reversible oxidation wave at Epcꢀ
/
has an irreversible 0/ꢁ
reversible 0/ꢂ1 wave (E1/2
oxidation peak at ꢁ0.98 V. Both the reduction and
/
1 wave (Epcꢀ
/
ꢁ
/
1.65 V), a
ꢁ/1.75 and near 0.52 V, respectively. If we subtract the
/
ꢀ
/
0.62 V), and a small
former CV from the CV in Fig. 4A, the resulting wave is
expected to be a pure CV of the 18-electon complex 2c
free from other reactions. The shape of the resulting
wave is also expected to be similar to that of 2a or 2b.
/
oxidation potentials of 2b are shifted towards the
negative direction compared with those of 1 by 0.51
and 0.17 V, respectively. The CVs of 2b as a function of
v, which are also similar to the CVs given in Fig. 3 (not
Thus, we can say that both the 0/ꢁ/1 reduction and the
0/ꢂ1 oxidation potentials of 2c shift towards the
/
shown). However, the current ratios of ipa at E :
V to ipc at E : 1.65 V are 0.54 at vꢀ
1.0 V sꢁ1, 0.33
at vꢀ .
0.10 V sꢁ1, and virtually zero at vꢀ0.020 V sꢁ1
/
ꢁ0.98
/
negative direction compared with those of 1 by approxi-
mately 0.61 and 0.33 V, respectively, as in 2a or 2b but
with larger amounts of shift.
/
ꢁ
/
/
/
/
Thus, a similar explanation for the CVs of 2a can hold
for 2b, except that the current ratios for the reduction of
the 18-electron complex indicate that a somewhat easier
elimination of PMe3 relative to CNtBu occurs.
Interestingly, the ratio of ipc at E :
at E : 1.75 V increases as v decreases. The ratios of
the two ipc values are 0.54, 1.1, and 2.0 at vꢀ1.0, 0.1,
/
ꢁ1.14 V to the ipc
/
/
ꢁ
/
/
and 0.02 V sꢁ1, respectively. Thus, we suspect that the
PEt3 ligand is eliminated as the applied potential goes
towards the negative direction and the ligand is too far
from the coordination sphere to re-coordinate to the
cobalt atom due to the enhanced steric interaction
between the bulky Cp* and PEt3 ligands. Such deso-
rption of phosphine ligands is also reported for (h5-
Cp)Co(S2C2X2)(L) complexes previously [7,8].
The CV of 2c displays rather complicated redox
responses (two major reduction and two major oxida-
tion waves), which were not observed with (h5-
Cp)Co(CabS,S?)(PEt3) [3]. Two major reductions, that
is, a reversible wave at E1/2
sible one at Epcꢀ 1.75 V, and two major oxidations,
that is, an irreversible wave near Epa 0.52 V and a
quasi-reversible one at Epa 0.85 V, respectively, are
ꢀ
/
ꢁ/1.08 V and an irrever-
/
ꢁ
/
ꢀ
/
ꢀ
/

240
B.Y. Kim et al. / Journal of Organometallic Chemistry 688 (2003) 236ꢃ245
/
The CV of 2d (Fig. 2E) exhibits similar redox pattern
as in 2c. A reversible reduction wave at E1/2 0.99 V,
an irreversible one at Epcꢀ 1.52 V, and two quasi-
reversible oxidation waves near Epa 0.71 and 0.90 V,
respectively, are observed. If we follow the earlier
explanation, both the 0/ꢁ1 reduction and the 0/ꢂ1
more pronounced for the bulky Cp* ancillary ligand
than for Cp. However, it becomes easier for complex 2
ꢀ
/
ꢁ
/
to eliminate L by reduction in the order of CNtBuB
/
/
ꢁ
/
ꢀ
/
PMe3B
more pronounced than steric effects especially for PEt3
and PHPh2. The CV results of 2aꢃd nicely illustrate the
/
PEt3B
/
PHPh2; here, electronic effects [10] are
/
/
/
oxidation potentials of 2d shift towards the negative
direction compared with those of 3 by approximately
0.37 and 0.14 V, respectively, as in 2a or 2b but with
smaller amounts of shift. These small shifts of redox
potentials imply that both the metal and sulfur atoms
become less electron-rich on the addition of PHPh2 than
control of electrochemical behavior not only by the
coordinating strength, but also by the cone angle of the
ligands and the direction of redox. Again, such an effect
was hard to be observed with the more spacious
coordination complex of [(h5-Cp)Co(CabS,S?)(L)]; the
earlier report about the elimination/addition of the
halides in [CpCoX(S2C2(COOMe)2)(CHR2)] [11a] is
also compatible with our argument.
those of other ligands in 2aꢃc due to the electron-
/
withdrawing ability of the two phenyl moieties.
The CVs of 2d as a function of v are also similar to
Fig. 4 (not shown). Thus, the same explanation for the
CVs of 2c can be applied to the CVs of 2d. The current
This part of our argument is also supported by the
results shown in Fig. 5. The CV of (h5-
Cp*)Co(CabS,S?)(PHPh2) (2d) changes to that of (h5-
Cp*)Co(CabS,S?)(PMe3) (2b) depending on the amount
of PMe3 added. In contrast, the CV of 2b is independent
of the amount of PHPh2 added. However, no change of
the peak positions was observed in the NMR spectra
after mixing 2d and PMe3. These results convinced us
that redox-induced ligand substitution reactions of (h5-
Cp*)Co(CabS,S?)(L) might be applied to an electroche-
mical in situ synthesis.
ratios of ipc at E :
0.86, 2.2, and 3.5 at vꢀ
/
ꢁ
/
1.04 V to ipc at E :
/
ꢁ1.45 V are
/
/
1.0, 0.1 and 0.02 V sꢁ1
,
respectively. The cathodic current ratios imply that
PHPh2 is more easily eliminated than PEt3 as the
applied potential goes towards the negative direction.
However, it is noteworthy that the first 0/ꢂ
wave (near Epa 0.71 V) is larger than the second 0/ꢂ
one (near Epa 0.90 V) (not shown), while both the 0/ꢂ
1 oxidation waves (near Epa 0.52 and 0.85 V) are
almost the same in Fig. 4A. Judging from the relative
size of the 0/ꢂ1 oxidation waves, this behavior indicates
/1 oxidation
ꢀ
/
/
1
ꢀ
/
/
ꢀ
/
/
that PHPh2 is more difficult to eliminate than PEt3 as
the applied potential goes towards the positive direction.
Analyzing the CV results of 2 allows the following
major conclusion to be drawn: the relatively large
difference (0.19 V) between E1/2 oxidation potentials
(some are calibrated with E1/2
ꢀ
/
Epa
ꢁ
(DEp, Fcꢂ/Fc)/2)
/
for 2bꢃd suggests that the alkyl or aryl substituent on
/
the phosphine moiety does influence the net electron
donor ability of PR3 to a significant extent; note that the
substituent is far from the oxidation site (S atom) at
three times the atom-to-atom distance (P atom to C
atom of R group). The order of magnitude of the shifts
of oxidation potentials of 2bꢃ
/
d from 1 is PHPh2B
/
PMe3BPEt3, which parallels the basicity or the electro-
/
nic effect of the PR3 ligand. However, in fact our CV
results imply that it becomes easier for 2 to eliminate/re-
coordinate the L by oxidation to 1.0 V in the order
PMe3BPHPh2, PEt3, in which the discrepant order of
/
PHPh2 and PMe3 seems to be ascribed to a steric factor.
Indeed, our observation agrees well with the increasing
order of cone angle: PMe3 (1188)B
/
PHPh2 (1288)BPEt3
/
(1328) [9]. The failure of the synthesis of (h5-
Cp*)Co(CabS,S?)(PPh3) also supports our argument.
For example, in 2, eight atoms are directly bonded to
the metal. Inter-ligand repulsions are therefore common
and relief of these repulsions on ligand dissociation
favors ligand loss and makes the Co-P re-coordination
more difficult. In particular, this effect may be even
Fig. 5. The CVs of 1 mM (h5-Cp*)Co(CabS ,S ?)(PHPh2) (2d) (A) in the
presence of 0.4 mM (B) and 1 mM of PMe3 (C), and 1 mM (h5-
Cp*)Co(CabS ,S ?)( PMe3) (2b) (D) in the presence of 0.4 mM (E) and 1
mM of PHPh2 (F). Other conditions are the same as in Fig. 1.

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241
2.3. Addition reactions to the CoꢃS bonds of 1
/
Recently, we reported alkylidene or acetylene addi-
tion reactions to the CoꢃS bonds of dithiolate I [4].
/
Consequently, we examined the electrochemical beha-
vior of these alkylidene- or acetylene-bridged complexes
and attempted to draw a comparison with their chemical
behavior. The electrochemical results concerning the
alkylidene- or acetylene-bridged complexes 3 are now
described. Indeed, complex 1 was found to be a good
precursor for such addition reactions as shown in
Scheme 2 [11,12].
Treatment of 1 with the diazo compound SiMe3CHN2
afforded the soluble product [(h5-Cp*)Co{h3-(S,S?,C?)-
SC2B10H10(CHSiMe3)}] (3a) in excellent yield (96%)
after recrystallization (Scheme 2). Complex 3a was
isolated as an air-stable microcrystalline solid and was
characterized by 1H- and 13C-NMR spectroscopy. Thus,
1
the H-NMR spectrum of complex 3a had resonances
for a trimethylsilyl group at d 0.29 and a methine proton
at d 2.26. The 13C-NMR spectrum had signals for the
two types of carbons of the Cp* ring and the carbon
atoms of the Coꢃ
unit.
The CV of 3a shows a well-defined reversible oxida-
tion wave at E1/2 0.19 V, that is, the magnitude of
potential shift is more than twice of that of 2 (Fig. 6B).
The greater stability of the ꢂ1 oxidation state of 3a
/
S-substituted trimethylsilyl methine
Fig. 6. The CVs of (h5-Cp*)Co(CabS ,S ?) (1) (A: for purpose of
comparison) and [(h5-Cp*)Co(CabS ,S ?)(L)] [Lꢀh1-CHSiMe3 ꢃ
S (3a)
(B), h1-HCꢀ (3b) (C), h1-(COOMe)Cꢀ
CPhꢃ C(COOMe)ꢃ (3c)
(D)]. Other conditions are the same as in Fig. 1.
/
/
ꢀ
/
/
/
S
/
/
S
/
tion is restored upon re-oxidation. A similar effect is
also reported in the literature for the CVs of [(h5-
versus
compared to that of 2 seems to be ascribed to the
coordination of the tridentate S,S,C-ligand as we
Cp)Co(S2C2(COOMe)2)(h2-C,C-CHSiMe3)]
reported
for
[(h5-Cp)Co{h3-(S,S?,C?)-SC2B10H10-
[(h5-Cp)Co(S2C2(COOMe)2)(h2-C,C ꢃ
The cobaltadithia-o-carborane ring in 1 also under-
/CPh2)] [11c].
(CHSiMe3)}]. In contrast to the oxidation, the reduction
responses of 3a are more complicated and have three
peaks, that is, a quasi-reversible reduction wave at
goes addition to an acetylene unit between the Co and S
atoms in its reaction with alkynes [1aꢃc,3,4,13]. Con-
/
E1/2
ꢀ
/
ꢁ
/
1.06 V and two irreversible ones near Epc of
1.91 V. The first reduction wave near that
version of 1 to the corresponding metallabicyclic com-
plexes 3b and 3c (Scheme 2) was expected to be favored
by an electron-rich cobalt complex; therefore, we
decided to investigate the reactivity of 1 towards a
reactive phenyl acetylene and acetylenedicarboxylate.
Treatment of 1 with one to two equivalents of the alkyne
in refluxing toluene for 2 h indeed gave the alkyne
adduct complexes, [(h5-Cp*)Co{h3-(S,S,C?)-SC2B10-
ꢁ
/
1.62 and ꢁ
/
of complex 1 becomes irreversible at the slower scan rate
of 0.02 V sꢁ1 (data not shown). In contrast to 3a, a
simple irreversible oxidation wave was observed for the
analogous complex with Cp [3]. However, the electronic
and steric effects of Cp* versus Cp allows us to speculate
that CoꢃC coordination, or at least one coordination of
the S,S,C-ligand, becomes loose as the applied potential
/
goes towards the negative direction and the coordina-
H10S(C?R2)}] [C?R2ꢀ
/
HCꢀ
/
CPh (3b), (COOMe)Cꢀ
/
Scheme 2. Addition reaction of 16-electron metal complex 1 a.

242
B.Y. Kim et al. / Journal of Organometallic Chemistry 688 (2003) 236ꢃ245
/
C(COOMe) (3c)], in moderate yield (Scheme 2). During
the reaction, the color of the solution changed from red
demonstrated clearly although we could not address
them in detail.
to orange yellow. Pure 3b and 3c were isolated in 35ꢃ
/
89% yield after recrystallization. The complexes were
characterized by a combination of elemental analysis
and IR and NMR (1H and 13C) spectroscopy. The IR
3. Conclusion
We have examined the electrochemical behavior of
the 16-electron species 1 and its 18-electron derivative 2,
in which some of the complexes exhibit elimination/re-
coordination of Lewis bases. This may be applied to the
in situ syntheses of new complexes through redox
reactions starting from one of the aforementioned
complexes. Moreover, we have demonstrated some
possible CV patterns of 18-electron complexes depend-
ing on the nature of the complex. Thus, a rationale for
the addition reaction with respect to the steric effects of
the phosphine at the cobalt center based on the cone
angle of the ligand has been established. The CV results
of 2 may serve as a nice example to illustrate the control
of electrochemical behavior not only by the coordinat-
ing strength, but also by the cone angle of the ligands
and the direction of redox.
spectrum of 3b has one Cꢀ
/
C stretching band, and the
1H-NMR spectrum of 3b has the characteristic reso-
nances of a coordinated olefin. Thus, the 1H-NMR
spectrum of 3b shows the terminal proton at d 2.36 and
multiplet signals of the phenyl group of the acetylene at
d 7.53. Similarly, for 3c, the OMe groups of the ester
give rise to two singlets centered at d 3.89 and 3.76,
respectively. For the 13C{1H}-NMR spectra of 3c, it is
sufficient to point out that the resonances due to the p-
bound olefin carbons are characteristically shifted up-
field. The spectroscopic data of the cobalt complexes are
in complete agreement with the proposed structure of
3c.
The CV pattern of 3b is similar to that of the 18-
electron complex 2a except for a difference in the
potential of the 0/ꢂ
and a small irreversible re-oxidation peak at Epa
0.50 V (Fig. 6C). The two small irreversible oxidation
peaks at Epa 1.10 and ꢁ0.50 V both disappear at
the slower scan rate of 0.02 Vsꢁ1 (data not shown). We
also speculate that the CoꢃC bond of a coordinated
/
1 oxidation wave at E1/2
ꢀ
/
0.09 V
ꢀ
/
ꢁ
/
4. Experimental
ꢀ
/
ꢁ
/
/
4.1. General procedures
/
olefin, and/or at least one coordination of the S,S,C-
ligand, becomes loose as the applied potential goes
All manipulations were performed under a dry,
oxygen-free, nitrogen or argon atmosphere using stan-
dard Schlenk techniques or in a Vacuum Atmosphere
HE-493 dry box. THF was freshly distilled over
potassium benzophenone. Toluene was dried and dis-
tilled from sodium benzophenone. Dichloromethane
and hexane were dried and distilled over CaH2. 13C-,
1H- and 31P-NMR spectra were recorded on a Varian
Mercury 300 spectrometer operating at 75.4, 300.0 and
121.4 MHz, respectively. All proton and carbon chemi-
cal shifts were measured relative to internal residual
chloroform from the lock solvent (99.5% CDCl3) and
then referenced to Me4Si (0.00 ppm). The 31P-NMR
spectra were recorded with 85% H3PO4 as an external
standard. IR spectra were recorded on a Biorad FTS-
165 spectrophotometer. Elemental analyses were per-
formed with a Carlo Erba Instruments CHNS-O
EA1108 analyzer. All melting points were uncorrected.
All the electrochemical measurements were carried out
in CH2Cl2 solutions containing 1 mM metal complex
and 0.1 M tetrabutylammonium fluoroborate (TBAB),
otherwise specified, at room temperature (r.t.) using a
BAS 100B electrochemical analyzer. A platinum disk
(dia. 1.6 mm) and platinum wire were used as a working
and counter electrode. The reference electrode used was
Ag j AgNO3 and all the potential values shown were
calibrated versus Fc/Fcꢂ redox couple, unless otherwise
specified. Epa and Epc denotes anodic and cathodic peak
towards the negative direction and leaves the SꢃC bond
/
of a coordinated olefin intact. This may be indirect
evidence that the reduction process takes place at the
metal site.
Finally, the CV of 3c has a quasi-reversible 0/ꢁ
reduction wave at E1/2 1.31 V and an irreversible 0/
1 oxidation wave at Epa 0.59 V with a small
reduction peak at Epcꢀ 0.14 V (Fig. 6D). The
/
1
ꢀ
/
ꢁ
/
ꢂ
/
ꢀ
/
/
ꢁ
/
difference in oxidation potential between 3b and 3c is
0.46 V and the corresponding difference in reduction
potential is 0.43 V; this indicates that the coordinated
olefin in 3b behaves as a stronger ligand than that in 3c
as was observed in (h6-arene)Ru(CabS,S?)(L) complexes
[4]. For the analogue with Cp, no clear redox features
were observed [3], which might be ascribed to the redox
instability. By changing the ligand from Cp to Cp*, we
could obtain a well-defined CV; however, the CV
pattern of 3c does not belong to the general redox
pattern of 18-electron complexes so far, that is, a
reduced reversibility in both the reduction and oxidation
waves was found. The irreversible oxidation peak seems
to be somewhat larger than the reduction peak, which
may suggest that the redox chemistry of 3c involves
some chemical reaction such as a self-electrocatalytic
reaction. Overall, the changes of the redox properties of
3 with the ancillary ligands, Cp and Cp*, have been

B.Y. Kim et al. / Journal of Organometallic Chemistry 688 (2003) 236ꢃ
/245
243
potential, respectively, and E1/2 value indicated is the
average of Epa and Epc for the reversible redox waves.
o-Carborane was purchased from the Katechem and
used without purification. Complex Li2CabS,S? [14] and
Cp*Co(CO)I2 [15] were prepared according to the
literature methods.
H, 7.25%. IR (KBr, cmꢁ1): n(Bꢃ
/
H) 2911, n(Bꢃ
/
H) 2557,
1
2585, n(Pꢃ
9H, PMe3, 2JPꢃH
1.8 Hz). 13C{1H}-NMR (ppm, CDCl3): 97.21 (s, Cp*),
/C) 1016. H-NMR (ppm, CDCl3): 1.50 (d,
ꢀ/10.2 Hz), 1.50 (d, 15H, Cp* JPꢃH
ꢀ
/
10.94 (d, P(CH3)3, JPꢃC
Cp*),. 31P{1H}-NMR (ppm, CDCl3): 7.85 (s, PMe3).
ꢀ44.59 Hz), 9.11 (s, Me of
/
4.1.1. Preparation of [(h5-Cp*)Co(CabS,S?)] (1)
Representative procedure: a 3.2 mmol solution of
Li2CabS,S? in THF (20 ml) was added to a stirred
solution of Cp*Co(CO)I2 (1.43 g, 3.0 mmol) in THF (30
4.2.2. Analytical data for 2c
Complex 2c was prepared from 0.40 g (1.0 mmol) of 1.
After recrystallization with hexaneꢃCH2Cl2 (90/10), 2c
/
ml) cooled to ꢁ78 8C. The reaction mixture was then
/
(82%) was isolated as a yellow solid. Data for 2c. Anal.
Calc. for C18H40B10S2CoP: C, 41.68; H, 7.77. Found: C,
allowed to react at 0 8C for 1 h, and the solution was
stirred for another 2 h at r.t. The solution gradually
turned dark red, suggesting the formation of an o-
carboranyl dithiolato metal complex. The solution was
reduced in vacuo to about half its orginal volume, and
some insoluble material was removed by filtration. The
solution was removed under vacuum, and the resulting
residue was taken up in a minimum of methylene
chloride and then transferred to a column of silica gel.
The crude residue was purified by column chromato-
41.80; H, 7.82%. IR (KBr, cmꢁ1): n(Cꢃ
/
H) 2970, 2907,
2878, n(Bꢃ
CDCl3): 1.97 (dq, 6H, PCH2, JPꢃH
7.5 Hz), 1.47 (s, 15H, Cp*), 1.03 (dt, 9H, PCH2Me,
/
H) 2576, 2559, n(Pꢃ
/
C) 1032. 1H-NMR (ppm,
3
1
ꢀ
/
9.2 Hz, JHꢃH
ꢀ
/
1
3JPꢃH
ꢀ
/
14.4 Hz, JHꢃH
ꢀ
7.7 Hz). 13C{1H}-NMR
(ppm, CDCl3): 96.72 (s, Cp*), 30.22 (s, P(CH2Me)3),
/
1
17.65 (d, JPꢃC
ꢀ26 Hz, P(CH2Me)3), 9.86 (s, Me of
/
Cp*). 31P{1H}-NMR (ppm, CDCl3): 20.43 (s, PEt3).
graphy with hexaneꢃ
/
CHCl3 (90/10) affording ꢀ98%
/
pure complex as red crystals. Red crystals of 1 were
formed in 78% yield (0.94 g, 2.3 mmol). Data for 1.
Anal. Calc. for C12H25B10S2Co: C, 35.99; H, 6.29.
4.2.3. Analytical data for 2d
Complex 2d was prepared from 0.40 g (1.0 mmol) of
1. After recrystallization with hexane/toluene (90/10), 2d
(94%) was isolated as a yellow solid. Data for 2d. Anal.
Calc. for C24H36B10S2CoP: C, 49.13; H, 6.18. Found: C,
Found: C, 36.63; H, 6.35%. IR (KBr, cmꢁ1): n(Bꢃ
H)
/
2590. 1H-NMR (ppm, CDCl3): 1.65 (s, 15H, Cp*).
13C{1H}-NMR (ppm, CDCl3): 97.86 (s, Cp*).
49.55; H, 6.23%. IR (KBr, cmꢁ1): n(Cꢃ
/
H) 2905, n(Bꢃ
/
4.1.2. Reaction of 1 with CNtBu
1
C) 1096. H-NMR (ppm, CDCl3): 7.66
H) 2560, n(Pꢃ
/
Under argon, CNtBu (0.23 ml, 2.0 mmol) was added
to a stirred solution of complex 1 (0.40 g, 1.0 mmol) in
toluene (10 ml) cooled to 0 8C. The red solution
immediately turned yellow. After stirring for 30 min,
the solution was evaporated to dryness. The yellow
residue of 2a was dried in vacuo. Yield: 0.46 g (0.95
mmol, 95%). Data for 2a. Anal. Calc. for
C17H34B10S2CoN: C, 42.22; H, 7.09; N, 2.89. Found:
(m, PC6H5), 7.47 (m, PC6H5), 7.32 (m, PC6H5), 2.5 (br,
1H, PHPh2), 1.34 (s, 15H, Cp*). 13C{1H}-NMR (ppm,
CDCl3): 133.85, 129.18, 128.74, 128.37 (HPPh2), 97.09
(s, Cp*), 9.91 (s, Me of Cp*). 31P-NMR (ppm, CDCl3):
1
21.67 (d, JPꢃH
ꢀ432.7 Hz, PHPh2).
/
C, 43.00; H, 7.11; N, 2.92%. IR (KBr, cmꢁ1): n(Cꢃ
/
H)
4.2.4. Reaction of 1 with Me3SiCHN2
N) 2181. 1H-NMR
A solution of complex 1 (0.40 g, 1.0 mmol) in toluene
(20 ml) was treated with a 2.0 M hexane solution of
(trimethylsilyl)diazomethane (0.6 ml, 1.2 mmol) at r.t.
for 5 min. The red color of the solution quickly faded to
give a yellow solution. The volatile substances were then
removed in vacuo, and the resulting solid was extracted
with CH2Cl2. Addition of hexane to the concentrated
2985, 2915, n(Bꢃ
/
H) 2560, n(Cꢀ
/
(ppm, CDCl3): 1.60 (s, 9H, NtBu), 1.49 (s, 15H, Cp*).
13C{1H}-NMR (ppm, CDCl3): 97.99 (s, Cp*), 29.96 (s,
CNtBu), 9.96 (s, Me of Cp*), 8.43 (s, CNtBu).
4.2. General synthesis of [(h5-Cp*)Co(CabS,S?)(L)]
[LꢀPMe3 (2b), PEt3 (2c), PHPh2 (2d)]
/
extract gave complex 3a as yellowꢃorange crystals.
/
Yield: 0.47 g (0.96 mmol, 96%). Data for 3a. Anal.
Calc. for C16H35B10S2CoSi: C, 39.48; H, 7.25. Found: C,
4.2.1. Reaction of 1 with PMe3
Under argon, PMe3 (2.0 mmol) was added to a red
solution of complex 1 (0.40 g, 1.0 mmol) in 10 ml of
methylene chloride. The resultant yellow solution was
stirred for 30 min and evaporated to dryness. The yellow
residue of 2b was dried in vacuo. Yield: 0.40 g (0.85
mmol, 85%). Data for 2b. Anal. Calc. for
C15H34B10S2CoP: C, 37.80; H, 7.19. Found: C, 38.00;
39.66; H, 7.32%. IR (KBr, cmꢁ1): n(Cꢃ
/
H) 2955, 2910,
H) 2586. 1H-NMR (ppm, CDCl3): 2.26 (s, 1H,
n(Bꢃ
/
Me3SiCH), 1.65 (s, 15H, Cp*), 0.29 (s, 9H, SiMe3).
13C{1H}-NMR (ppm, CDCl3): 92.93 (s, Cp*), 39.24 (s,
Me3SiCH), 10.59 (s, Me of Cp*), 1.43 (s, SiMe3), 1.17
(s, SiMe3).

244
B.Y. Kim et al. / Journal of Organometallic Chemistry 688 (2003) 236ꢃ245
/
4.3. General synthesis of [(h5-Cp*)Co{h3-(S,S,C?)-
SC2B10H10S(C?R2)}] [C?R2ꢀHCꢀCPh (3b),
(COOMe)CꢀC(COOMe) (3c)]
Table 2
Crystal data and structure refinement for 2d×
/
C7H8
/
/
/
Identification code
Empirical formula
Formula weight
Temperature (K)
kor291
31H44B10PS2Co
678.84
C
In a typical run 1 (1.0 mmol) and alkynes were
dissolved in toluene (15 ml) under argon and the
solution was stirred for 8 h at r.t. The red color of the
solution slowly faded to give a yellow solution, suggest-
ing the formation of an adduct. The completion of the
reaction was monitored by TLC. The solution was
reduced in vacuo to about half its original volume,
and some insoluble material was removed by filtration.
Addition of hexane to the resulting solution afforded
293(2)
˚
Wavelength (A)
0.71070
Orthorhombic
Pbca
Crystal system
Space group
Unit cell dimensions
˚
a (A)
15.863(1)
20.534(1)
21.868(2)
7123.1(8)
8
˚
b (A)
˚
c (A)
3
V (A )
˚
Z
Dcalc (g cmꢁ3
)
1.266
0.666
2768
3bꢃc as a dark yellow precipitate.
/
m (mmꢁ1
F(000)
)
4.3.1. Analytical data for 3b
Crystal size (mm3)
0.3ꢄ
1.86ꢃ
05h 5
l 527
6981/6981 [Rint
/
0.45ꢄ
25.97
19, 05
/
0.45
Complex 3b was prepared from 2.0 mmol (0.22 ml) of
phenylacetylene. After recrystallization from toluene, 3b
(0.18 g, 0.35 mmol, 35% yield) was isolated as a yellow
solid. Anal. Calc. for C20H31B10S2Co: C, 47.79; H, 6.22.
u Range for data collection (8)
Index ranges
/
/
/
/k 5
/
25, 05
/
/
Reflections collected/unique
Refinement method
ꢀ
/
0.0000]
Full-matrix least-squares on
F2
Found: C, 47.84; H, 6.30%. IR (KBr, cmꢁ1): n(Bꢃ
2591, n(CꢀC) 1590, n(Cꢀ
C) 1486. 1H-NMR (ppm,
CDCl3): 7.53 (m, 5H, Ph), 2.36 (s, 1H, CꢀCH), 1.24
(s, 15H, Cp*). 13C{1H}-NMR (ppm, CDCl3): 193.07 (s,
HCꢀCPh), 176.66 (s, HCꢀCPh), 130.10 (m, Ph), 93.36
/H)
/
/
Data/restraints/parameters
Goodness-of-fit on F2
6978/0/400
1.039
/
Final R indices [Iꢀ/2s(I)]
R1 aꢀ
/
0.0653, wR2 bꢀ
0.1713, wR2 bꢀ
0.339
/0.1603
R indices (all data)
R1 aꢀ
/
/0.2117
/
/
Largest difference peak and hole (e. 0.676 and ꢁ
/
(s, Cp*), 9.92 (Me of Cp*).
ꢁ3
˚
A
)
a
R1ꢀ
/
a½½Fo½ꢁ
wR2ꢀ
[a[w(F2oꢁ
[max(F2o, 0)ꢂ2Fc2]/3 (also with F2oꢀ
/
½Fc½½ (based on reflections with F2oꢀ
F2c)2]/a[w(Fo2)2]]1/2 1/[s2(Fo2)ꢂ
wꢀ
2sF2).
/
2sF2).
4.3.2. Analytical data for 3c
b
/
/
;
/
/
(0.095P)2];
Complex 3c was prepared from 2.0 mmol (0.25 ml) of
dimethyl acetylenedicarboxylate. After recrystallization
from toluene, 3c (0.48 g, 0.89 mmol, 89% yield) was
Pꢀ
/
/
/
Table 3
Selected bond lengths (A) and bond angles (8) in 2d×
isolated as
C18H31B10S2CoO4: C, 39.84; H, 5.76. Found: C, 40.02;
H, 5.82%. IR (KBr, cmꢁ1): n(Cꢃ
H) 2951, 2908, 2847,
n(BꢃH) 2584, n(CO) 1710, n(Cꢀ
C) 1573. 1H-NMR
a
yellow solid. Anal. Calc. for
˚
/
C7H8
Bond lengths
/
Co(1)ꢃ
Co(1)ꢃ
S(2)ꢃ
P(1)ꢃ
/
S(1)
2.253(2)
2.204(2)
1.777(6)
1.833(7)
Co(1)ꢃ
S(1)ꢃC(1)
P(1)ꢃC(13)
C(1)ꢃC(2)
/
S(2)
2.258(2)
1.791(6)
1.830(6)
1.666(8)
/
/
/P(1)
/
(ppm, CDCl3): 3.89 (s, 3H, OMe), 3.76 (s, 3H, OMe),
1.48 (s, 15H, Cp*). 13C{1H}-NMR (ppm, CDCl3):
/
C(2)
/
/
C(19)
/
185.74 (s, MeOCO), 119.78 (s, MeOCOCꢀ), 98.91 (s,
Cp*), 95.48 (s, Cp*), 78.82 (s, OMe), 9.43 (Me of Cp*),
9.34 (Me of Cp*).
/
Bond angles
P(1)ꢃCo(1)ꢃ
S(1)ꢃCo(1)ꢃ
C(2)ꢃS(2)ꢃCo(1)
C(13)ꢃP(1)ꢃCo(1)
/
/
S(1)
87.56(6) P(1)ꢃ
93.23(6) C(1)ꢃ
105.1(2)
121.5(2)
/
Co(1)ꢃ
S(1)ꢃCo(1)
C(13)ꢃP(1)ꢃC(19)
C(19)ꢃP(1)ꢃCo(1)
/
S(2)
87.24(7)
104.9(2)
103.4(3)
117.9(2)
/
/S(2)
/
/
/
/
/
/
/
/
/
/
4.4. X-ray crystallography
Suitable crystals of 2d were obtained by slow diffusion
of hexane into a toluene solution of the complexes at r.t.
and were mounted on a glass fiber. Crystal data and
experimental details are given in Table 2. The data set
5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 214276 for compound 2d.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
for 2d were collected on an Enraf CAD4 automated
˚
0.7107 A) was
diffractometer. Moꢃ
/
Ka radiation (lꢀ
/
used for all structures. Each structure was solved by the
application of direct methods using the SHELXS-96
program [16a] and least-squares refinement using
SHELXL-97 [16b]. All non-hydrogen atoms in compound
2d were refined anisotropically. All other hydrogen
atoms were included in calculated positions (Table 3).
Cambridge CB2 1EZ, UK (Fax: ꢂ44-1223-336033; e-
mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
/

B.Y. Kim et al. / Journal of Organometallic Chemistry 688 (2003) 236ꢃ
/245
245
[9] (a) P.W.N.M. van Leeuwen, P.C.J. Kamer, J.N.H. Reek, P.
Dierkes, Chem. Rev. 100 (2000) 2741;
(b) C.A. Tolman, Chem. Rev. 77 (1977) 313.
Acknowledgements
We thank Prof. Sukbok Chang at the Korean
Advanced Institute of Science and Technology (KAIST)
for helpful discussions. Financial support from the
Korean Science & Engineering Foundation (R01-2000-
00050) and the Korea Research Foundation (2001-041-
D00119) is gratefully acknowledged.
[10] (a) M.N. Golovin, M.M. Rahman, J.E. Belmonte, W.P. Giering,
Organometallics 4 (1985) 1981;
(b) C.A. Tolman, J. Am. Chem. Soc. 92 (1970) 2953.
[11] (a) C. Takayama, K. Takeuchi, S. Ohokoshi, M. Kajitani, T.
Sugiyama, A. Sugimori, Organometallics 18 (1999) 4032;
(b) C. Takayama, K. Takeuchi, S. Ohokoshi, G.C. Janairo, T.
Sugiyama, M. Kajitani, A. Sugimori, Organometallics 18 (1999)
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(c) C. Takayama, M. Kajitani, T. Sugiyama, A. Sugimori, J.
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