Synthesis and characterization of d2-molybdenum imido complexes containing coordinated dithio-ligands. MO rationalization of the preferential isomer formation of Mo(N-2,4,6-Me3C6H2)(S 2COMe)Cl(PMe3)2

Article abstract of DOI:10.1016/S0022-328X(99)00314-9

Treatment of the complex Mo(Nmes)Cl₂(PMe₃)₃ (1) (mes=2,4,6-Me₃C₆H₂, 2,4,6-trimethylphenyl) with two equivalents of the potassium salt of methyl xanthate, KS₂COMe, or the potassium 1-pyrrole-carbodithioate, KS₂C(NC₄H₄), gave Mo(Nmes)(S-S)₂(PMe₃) (S-S=S₂COMe 2; S₂C(NC₄H₄) 3). When the reaction of 1 with KS₂COMe is carried out in a 1:1 stoichiometry, the complex Mo(Nmes)(S₂COMe)Cl(PMe₃)₂ (4) is obtained. The chlorine atom in 4 occupies the trans position with respect to the organoimido ligand. MO calculations of the EH type rationalize the preferential formation of 4 as the thermodynamically preferred isomer in this metathesis reaction. The reaction of 4 with CO produces Mo(Nmes)(S₂COMe)Cl(CO)(PMe₃) (5). Reaction of 1 with carbon disulfide yields the complex Mo(Nmes)Cl₂(S₂CPMe₃) (6), containing a trihapto-(S,S′,C) phosphoniumdithiocarboxylate ligand.

Full text of DOI:10.1016/S0022-328X(99)00314-9

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 587 (1999) 127–131
Synthesis and characterization of d2-molybdenum imido complexes
containing coordinated dithio-ligands. MO rationalization of the
preferential isomer formation of
Mo(N-2,4,6-Me3C6H2)(S2COMe)Cl(PMe3)2
Francisco Montilla, Antonio Pastor, Agust´ın Galindo *
Departamento de Qu´ımica Inorga´nica, Uni6ersidad de Se6illa, Aptdo 553, E-41071 Se6ille, Spain
Received 15 March 1999; received in revised form 19 May 1999
Abstract
Treatment of the complex Mo(Nmes)Cl2(PMe3)3 (1) (mes=2,4,6-Me3C6H2, 2,4,6-trimethylphenyl) with two equivalents of the
potassium salt of methyl xanthate, KS2COMe, or the potassium 1-pyrrole-carbodithioate, KS2C(NC4H4), gave Mo(Nmes)(S-
S)2(PMe3) (S-S=S2COMe 2; S2C(NC4H4) 3). When the reaction of 1 with KS2COMe is carried out in a 1:1 stoichiometry, the
complex Mo(Nmes)(S2COMe)Cl(PMe3)2 (4) is obtained. The chlorine atom in 4 occupies the trans position with respect to the
organoimido ligand. MO calculations of the EH type rationalize the preferential formation of 4 as the thermodynamically
preferred isomer in this metathesis reaction. The reaction of 4 with CO produces Mo(Nmes)(S2COMe)Cl(CO)(PMe3) (5). Reaction
of 1 with carbon disulfide yields the complex Mo(Nmes)Cl2(S2CPMe3) (6), containing a trihapto-(S,S%,C) phosphoniumdithiocar-
boxylate ligand. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Imido; Molybdenum; Dithio-ligand; EHMO calculations
material. A molecular orbital (MO) rationalization of
the preferential formation of only one isomer of the
complex Mo(Nmes)(S2COMe)Cl(PMe3)2 (4) is also
presented.
1. Introduction
Interest in the chemistry of compounds containing
organoimido ligands has increased enormously in the
last two decades. In particular, the use of this ligand in
stabilizing transition metals in high oxidation states [1]
and the role they play in the development of well-
defined catalysts are well documented [2].
2. Experimental
Previous work from our laboratories has resulted in
the preparation of a variety of oxo- and imido-deriva-
tives of molybdenum containing dithioacid and related
ligands [3,4]. As an extension of this work [5], we now
wish to report the synthesis and characterization of
some new d2-imido complexes of molybdenum contain-
ing xanthate, dithiocarbamate and phosphoniumdithio-
carboxylate ligands. As discussed below, they have been
prepared using the complex Mo(Nmes)Cl2(PMe3)3 (1)
(mes=2,4,6-trimethylphenyl, Me3C6H2) as the starting
Microanalyses were carried out by the Microanalyti-
cal Service of the University of Sevilla. IR spectra were
recorded on a Perkin–Elmer model 883 spectrophoto-
meter. 1H-, 13C- and 31P-NMR spectra were run on
Bruker AMX-300 and AMX-500 spectrometers. 31P
shifts were measured with respect to external 85%
H3PO4. 13C-NMR spectra were referenced using the 13
C
resonance of the solvent as an internal standard but are
reported with respect to SiMe4. All preparations and
other operations were carried out under oxygen-free
nitrogen, following conventional Schlenk techniques.
Solvents were dried and degassed before use. The light
petroleum ether used had b.p. 40–60°C. The compound
* Corresponding author. Fax: +34-954557156.
E-mail address: galindo@cica.es (A. Galindo)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00314-9

128
F. Montilla et al. / Journal of Organometallic Chemistry 587 (1999) 127–131
Mo(Nmes)Cl2(PMe3)3 was prepared according to the
literature procedure [5].
(s). 1H-NMR (500 MHz, C6D6): l 6.49 (s, 2H, CH, Ph),
3.71 (s, 3H, OCH3), 2.44 (s, 6H, o-CH3), 1.94 (s, 3H,
p-CH3), 1.37 (d, JHP=7.7 Hz, 18H, PMe3). 13C{1H}-
NMR (125 MHz, C6D6): l 235.4 (s, S2C), 150.8 (s,
ipso-C), 135.7, 134.7 (s, p-C and o-C), 128.8 (s, m-C),
57.9 (s, OCH3), 19.7 (s, p-CH3), 19.1 (m, PMe3), 18.3
(s, o-CH3). Anal. Calc. for C17H32NClMoOP2S2: C,
39.0; H, 6.1; N, 2.7. Found: C, 39.0; H, 6.3; N, 2.7%.
2.1. Preparation of Mo(Nmes)(S2COMe)2(PMe3) (2)
A mixture of Mo(Nmes)Cl2(PMe3)3 (0.07 g, 0.15
mmol) and KS2COMe (0.04 g, 0.30 mmol) was dis-
solved in THF (25 ml). The suspension was refluxed
overnight, the solvent was removed by pumping under
vacuum and the resulting residue extracted with light
petroleum. The solution was concentrated and cooled
to −20°C. Compound 2 was isolated as an orange–
brownish solid in 30% yield. 31P{1H}-NMR (C6D6): l
2.4. Interaction of 4 with CO: synthesis of
Mo(Nmes)(S2COMe)Cl(CO)(PMe3) (5)
A solution of Mo(Nmes)(S2COMe)Cl(PMe3)3 (0.06 g,
0.11 mmol) in Et2O (20 ml) was pressurized with CO (2
atm) in a pressure bottle. The mixture was stirred
overnight and then the solvent was removed under
vacuum. Crystallization of the residue always gave a
mixture of the starting material 4 and compound 5. The
separation by fractional crystallization was not possi-
ble, but 5 was fully characterized by solution NMR.
31P{1H}-NMR (C6D6): l −5.65 (s). 1H-NMR (500
MHz, C6D6): l 6.38 (s, 2H, CH, Ph), 3.54 (s, 3H,
OCH3), 2.31 (s, 6H, o-CH3), 1.90 (s, 3H, p-CH3), 1.22
(d, JHP=8.9 Hz, 9H, PMe3). 13C{1H}-NMR (125
MHz, C6D6): l 238.6 (d, JCP=7.6 Hz, CO), 235.8 (s,
S2C), 150.1 (s, ipso-C), 136.9, 136.7 (s, p-C and o-C),
129.0 (s, m-C), 58.2 (s, OCH3), 20.8 (s, p-CH3), 19.1 (s,
o-CH3), 16.5 (d, JCP=27.5 Hz, PMe3).
1
−4.1 (s). H-NMR (C6D6): l 6.44 (s, 2H, CH, Ph),
3.47, 3.43 (s, 3H, OCH3), 2.33 (s, 6H, o-CH3), 1.91 (s,
3H, p-CH3), 1.17 (d, JHP=8.6 Hz, 9H, PMe3).
13C{1H}-NMR (125 MHz, C6D6): l 218.4 (s, S2C),
172.2 (s, S2C), 153.7 (s, ipso-C), 135.4, 135.3 (s, p-C
and o-C), 128.7 (s, m-C), 57.4, 57.0 (s, OCH3), 20.8 (s,
p-CH3), 19.7 (s, o-CH3), 15.1 (d, JCP=26.7 Hz, PMe3).
Crystals of complex 2 were contaminated by small
amounts of 4. This makes it difficult to obtain reliable
microanalytical
data.
Anal.
Calc.
for
C16H26NMoO2PS4: C, 37.0; H, 5.0; N, 2.7. Found: C,
38.5; H, 5.3; N, 2.7%.
2.2. Preparation of Mo(Nmes)[S2C(NC4H4)]2(PMe3) (3)
To a solution of Mo(Nmes)Cl2(PMe3)3 (0.12 g, 0.23
mmol) in THF (15 ml) was added KS2C(NC4H4) (0.09
g, 0.46 mmol) in THF (15 ml). The mixture was stirred
overnight at room temperature (r.t.). The solvent was
then removed and the residue extracted with ether/light
petroleum. Concentration of the solution and cooling
at −20°C gave orange crystals of compound 3 in 50%
yield. 31P{1H}-NMR (C6D6): l −8.33 (s). 1H-NMR
2.5. Preparation of Mo(Nmes)Cl2(S2CPMe3) (6)
A solution of Mo(Nmes)Cl2(PMe3)3 (0.20 g, 0.38
mmol) in THF (20 ml) was treated with one equivalent
of CS2 (0.38 ml of a solution 1 M in THF). The mixture
was stirred overnight at r.t. and an orange solid was
formed. The resulting suspension was filtered, the solu-
tion evaporated to dryness and the residue extracted
with THF. Concentration and cooling at −30°C af-
forded orange crystals of 6. 31P{1H}-NMR (acetone-
3
(300 MHz, C6D6): l 7.63, 7.52 (t, JHH=2.3 Hz, 2H,
3
NC4H4), 6.33 (s, 2H, CH, Ph), 6.01, 5.96 (t, JHH=2.3
Hz, 2H, NC4H4), 1.96 (s, 6H, o-CH3), 1.84 (s, 3H,
p-CH3), 1.02 (d, JHP=9.2 Hz, 9H, PMe3). 13C{1H}-
NMR (75 MHz, C6D6): l 206.7 (d, JCP=4.3 Hz,
eq-S2C), 187.3 (s, ax-S2C), 153.3 (s, ipso-C), 136.4,
135.4 (s, p-C and o-C), 128.4 (s, m-C), 121.2, 118.5,
114.3, 109.2 (s, NC4H4), 20.7 (s, p-CH3), 19.3 (s, o-
CH3), 14.7 (d, JCP=26.2 Hz, PMe3). Anal. Calc. for
C22H28N3MoPS4: C, 44.8; H, 4.4; N, 7.1. Found: C,
44.7; H, 4.4; N, 7.0%.
1
d6): l 42.5 (s). H-NMR (500 MHz, acetone-d6): l 6.80
(s, 2H, CH, Ph), 2.36 (s, 6H, o-CH3), 2.20 (s, 3H,
p-CH3), 1.94 (d, JHP=13.8 Hz, 9H, PMe3). 13C{1H}-
NMR (125 MHz, acetone-d6): l 153.6 (s, ipso-C), 139.5,
136.6 (s, p-C and o-C), 129.7 (s, m-C), 97.5 (d, JCP
78.2 Hz, CS2), 21.3 (s, p-CH3), 18.6 (s, o-CH3), 11.2 (d,
=
J
CP=57.2
Hz,
PMe3).
Anal.
Calc.
for
C13H20NCl2MoPS2·14THF: C, 35.7; H, 4.7; N, 3.0.
Found: C, 35.3; H, 5.0; N, 2.9%.
2.3. Preparation of Mo(Nmes)(S2COMe)Cl(PMe3)2 (4)
2.6. Extended Hu¨ckel molecular orbital study
A mixture of Mo(Nmes)Cl2(PMe3)3 (0.26 g, 0.49
mmol) and KS2COMe (0.07 g, 0.5 mmol) was dissolved
in THF (25 ml). The solution was stirred for 24 h at r.t.
The solvent was removed and the dark red solid was
recrystallized from ether/light petroleum to give 4 as
red crystals in 55% yield. 31P{1H}-NMR (C6D6): l 0.3
The relative stability of the three isomers (I–III) of
complex Mo(Nmes)(S2COMe)Cl(PMe3)2 (4) was stud-
ied
employing
the
model
compounds
Mo(NH)(S2COH)Cl(PH3)2 at the extended Hu¨ckel level
[6] by using a weighted modified Wolfsberg–Helmholz

F. Montilla et al. / Journal of Organometallic Chemistry 587 (1999) 127–131
129
formula [7]. The calculations were performed by means
of the CACAO package [8]. The STO parameters used
are the standard ones tabulated in the program. The
geometry of the model compounds was based on the
ideal octahedron and bond distances and angles were
adapted from the results on a structural search of the
Cambridge Crystallographic Database [9].
KS2COMe in a 1:1 ratio, at r.t. If the substitution of
one chloride and one PMe3 groups by the xanthate,
occurs without ligand redistribution, three possible iso-
mers (I–III) can be envisaged.
3. Results and discussion
The imido ligand exerts a strong trans influence
[1b,10]. By assuming that the trans effect follows the
same trend of the trans influence, one would expect that
the metathesis occurs with the initial substitution of the
chloride ligand that occupies the trans position with
respect to the imido group. This substitution will pro-
duce the isomers II or III. However, the isolated com-
plex 4 is characterized by a singlet in the 31P{1H}-NMR
spectrum and by the presence of a doublet for the PMe3
ligands in the 1H-NMR spectrum, with a coupling
constant JHP of 7.7 Hz. Consequently, the spectroscopic
data of 4 are in agreement with structure I, in which the
two PMe3 ligands are equivalent and mutually cis. We
have no evidence for the formation of complexes with
either structure II or III.
We have recently reported [5] that treatment of com-
pound Mo(Nmes)Cl3(dme) with two equivalents of
PMe3 and subsequent NaꢀHg reduction, in the presence
of one additional equivalent of PMe3, gives Mo(N-
mes)Cl2(PMe3)3 (1). This synthetic methodology pro-
vides easy access to the chemistry of the d2-Mo(Nmes)
moiety. We also showed that interaction of 1 with two
equivalents
of
KS2COiPr
gave
Mo(Nmes)-
(S2COiPr)2(PMe3), in contrast with the behavior previ-
ously observed for the oxo-analogue of 1, which under
similar conditions gave MoO[S2C(PMe3)OiPr](S2COiPr)
[3b]. With the aim of ascertaining if the attainment of
the classical six-coordinate structure is a general obser-
vation, we have enlarged the number of d2-imido
derivatives of molybdenum by preparing the complexes
Mo(Nmes)(S2COMe)2(PMe3) (2) and Mo(Nmes)-
[S2C(NC4H4)]2(PMe3) (3). They can be synthesized by
reacting 1 with the potassium salts of methyl xanthate
and 1-pyrrole-carbodithioate, respectively (Eq. (1)), and
are obtained as orange–brownish and orange crys-
talline solids, respectively.
In order to rationalize the preferential formation of
only isomer I in this reaction, we have performed an
EHMO
study
with
the
model
compound
Mo(NH)(S2COH)Cl(PH3)2 in the three geometries, I–
III. The main electronic features are not affected by the
substitution of the terminal 2,4,6-Me3C6H2 group by a
single H atom. The absolute energy minimum is calcu-
lated to correspond to the experimentally observed
isomer I, which is ca. 0.2 eV more stable than II and
III.
(1)
In view of the limitations of the EHMO method for
the prediction of the correct energies, we decided to
analyze the fragment molecular orbital (FMO) interac-
tions of the three model compounds. In all cases, the
common dithio-ligand, S2COH, was considered to in-
teract with a metallic fragment of the type d2-ML4 and
the geometry defined in IV–VI.
Both compounds are moderately stable upon exposure
to air in the solid state. Their NMR spectra are consis-
tent with the schematic structure shown in Eq. (1). The
PMe3 ligand of compounds 2 and 3 gives a 31P{1H}-
NMR singlet, with a chemical shift characteristic of
coordinated PMe3 ligands. The two dithio-ligands give
rise to two different sets of signals. For instance, two
resonances are observed for the S2C carbon atoms of 3
in the 13C{1H}-NMR spectra (206.7 (d, JCP=4.3 Hz,
S2C eq) and 187.3 (s, S2C ax) ppm). These resonances
are in the range expected for bidentate dithio-ligands.
The formation of 2 requires heating for completion.
When the reaction is carried out at r.t., the analysis of
the reaction mixture by 31P{1H}-NMR reveals three
species: the starting material 1, complex 2 and a singlet
resonance corresponding to a new compound. This
complex was identified as the monosubstituted xanthate
derivative Mo(Nmes)(S2COMe)Cl(PMe3)2, (4), which is
best prepared by carrying out the reaction of 1 and
Despite the fact that the presence of the triple bonded
imido functionality makes these d2-ML4 fragments un-
usual, their FMOs can be inferred from the classical
C26-ML4 fragment [11]. Fig. 1 illustrates the metal
FMOs of IV. For comparison, the energies of the
analogous FMOs of V and VI have been also included.

130
F. Montilla et al. / Journal of Organometallic Chemistry 587 (1999) 127–131
They consist of a high-lying s-hybrid, 3a%, a pair of
hybridized dxy and dyz orbitals (2a%% and 1a%%) and the 2a%
FMO (dxz orbital). The FMOs 1a%%, 2a% and 2a%% show
the antibonding combination with the p-contribution of
the imido and the chloride ligands. The HOMO 1a% of
with respect to the imido ligand is electronically more
favorable than the structure containing the chloride
ligand occupying a cis-coordination site. In conclusion,
our calculations indicate that isomer I is the most stable
configuration
for
the
model
compound
Mo(NH)(S2COH)Cl(PH3)2 and this fact is in agreement
with the preferential formation of this isomer, observed
experimentally.
Interaction of solutions of 4 with CO takes place
with substitution of one PMe3 ligand and formation of
complex Mo(Nmes)(S2COMe)Cl(CO)(PMe3) (5) (Eq.
(2)). The presence of the CO group was spectroscopi-
cally confirmed by IR and was further indicated by the
appearance of a doublet at 238.6 ppm in the 13C{1H}-
NMR spectrum (2JCP=7.6 Hz).
IV is essentially the dx
orbital. The presence of the
2
2
−y
formally p-donor chloride ligand in the cis position
with respect to the imido functionality produces a
destabilization (four electron repulsion) of the HOMO
in the fragments V and VI. Accordingly, the minimum
energy corresponds to the metal fragment IV, which is
found to be ca. 0.6 eV more stable than the other two
structures.
The FMOs of some dithio-ligands have been outlined
previously [12], and include two filled in-plane combi-
nations of sulfur lone pairs (in phase, sip, and out-of-
phase, sop) and the p-S2C system. The interaction
diagrams for the three isomers (not represented) reveal
that the p combinations of the dithio-ligand basically
do not participate in the bonding scheme and only the
two filled s-FMOs (sop and sip) contribute to the
construction of the MOs.
(2)
Other NMR data are collected in Section 2 and need no
further comment. As outlined before, the MO analysis
of 4 shows a HOMO composed essentially of the filled
dx
metal orbital. Accordingly, the substitution of
2
2
one−PyMe3 ligand by CO stabilizes the HOMO by the
p-acceptor capability of the latter ligand [1b,5,13].
Addition of carbon disulfide to transition metal–
phosphine derivatives is a common entry to the chem-
istry of S2CPR3 complexes [14]. The easy dissociation of
PMe3 from complex 1, makes it a useful candidate for
the investigation of this reaction. Treatment of solu-
tions of 1 with one equivalent of carbon disulfide
affords compound Mo(Nmes)Cl2(S2CPMe3) (6), as an
orange microcrystalline solid (Eq. (3)). The S2CPMe3
ligand gives characteristic 1H (doublet at 1.94 ppm,
As stated above, the energy minimum corresponds to
isomer I. Moreover, the best overlap populations be-
tween the metallic fragments and the S2COH ligand
occurs also for this isomer. In particular, the FMO 1a%%
shows an appreciable overlap population with the sop
contribution. This interaction has no match for the
isomers II or III. Finally, the decrease in the energy
difference between I and the other isomers (from 0.6 eV
in the metal fragments to 0.2 eV) has its origin in a four
electron repulsion between the 1a% fragment (HOMO
for IV) and the sip FMO.
For a d2 metal configuration, the presence of the
formally p-donor chloride ligand in the trans position
J
HP=13.8 Hz) and 31P{1H} (singlet at 42.5 ppm) reso-
nances, attributable to a phosphonium group.
(3)
In addition, a 13C doublet at 97.5 ppm (JCP=78.2 Hz)
can be assigned to the central carbon atom of the
S2CPMe3 ligand. The chemical shift of this signal is
indicative of a trihapto-(S,S%,C) coordination [14,15].
The structure displayed in Eq. (3) for complex 6 was
proposed on the basis of previous MO studies [16]
carried out on related d2-systems that contain a multi-
ple-bonded ligand and the trihapto-(S,S%,C) S2CPR3
group.
Acknowledgements
Financial support from DGES and Junta de Andalu-
c´ıa are gratefully acknowledged. F.M. thanks the Junta
de Andaluc´ıa for a research studentship and A.P.
Fig. 1. CACAO drawings for the metallic FMOs of IV and compara-
tive energies for the same metallic fragments V and VI.

F. Montilla et al. / Journal of Organometallic Chemistry 587 (1999) 127–131
131
[5] F. Montilla, A. Galindo, E. Carmona, E. Gutie´rrez-Puebla, A.
Monge, J. Chem. Soc. Dalton Trans. (1998) 1299.
[6] (a) R. Hoffmann, J. Chem. Phys. 39 (1963) 1397. (b) R. Hoffmann,
W.N. Lipscomb, J. Chem. Phys. 37 (1962) 3489. (c) R. Hoffmann,
W.N. Lipscomb, J. Chem. Phys. 36 (1962) 2179.
thanks the Spanish Ministry of Education for a post-
doctoral contract.
[7] J.H. Ammeter, H.-B. Bu¨rgi, J.C. Thibeault, R. Hoffmann, J. Am.
Chem. Soc. 100 (1978) 3686.
References
[8] C. Mealli, D. Proserpio, J. Chem. Educ. 67 (1990) 399.
[9] Cambridge Structural Database System, Version 5.15, Cambridge
Crystallographic Data Centre, UK.
[10] P.D. Lyne, D.M.P. Mingos, J. Organomet. Chem. 478 (1994) 141.
[11] T.A. Albright, J.K. Burdett, M.-H. Whangbo, Orbital Interactions
in Chemistry, Wiley, New York, 1985.
[12] A. Galindo, C. Mealli, J. Cuya´s, D. Miguel, V. Riera, J.A.
Pe´rez-Mart´ınez, C. Bois, Y. Jeannin, Organometallics 15 (1996)
2735.
[1] (a) D.E. Wigley, Prog. Inorg. Chem. 42 (1994) 239. (b) W.A.
Nugent, J.M. Mayer, Metal–Ligand Multiple Bonds, Wiley, New
York, 1988. (c) W.A. Nugent, B.L. Haymore, Coord. Chem. Rev.
31 (1980) 123.
[2] See for example: Y. Imamoglu (Ed.), Metathesis Polymerization
of Olefins and Polymerization of Alkynes, NATO ASI Series C
506, Kluwer, Dordrecht, 1998.
[3] (a) E. Carmona, A. Galindo, C. Guille-Photin, R. Lai, A. Monge,
C. Ruiz, L. Sa´nchez, Inorg. Chem. 27 (1988) 488. (b) E. Carmona,
A. Galindo, E. Gutie´rrez-Puebla, A. Monge, C. Puerta, Inorg.
Chem. 25 (1986) 3804. (c) E. Carmona, A. Galindo, L. Sa´nchez,
A.J. Nielson, G. Wilkinson, Polyhedron 3 (1984) 347.
[4] A. Galindo, F. Montilla, A. Pastor, E. Carmona, E. Gutie´rrez-
Puebla, A. Monge, C. Ruiz, Inorg. Chem. 36 (1997) 2379.
[13] See for example Ref. [11] of Ref. [4].
[14] A. Galindo, D. Miguel, J. Pe´rez, Coord. Chem. Rev. (1999) in press.
[15] A. Galindo, E. Gutie´rrez-Puebla, A. Monge, A. Pastor, A. Pizzano,
C. Ruiz, L. Sa´nchez, E. Carmona, Inorg. Chem. 32 (1993) 5569.
[16] L. Zhang, M.P. Gamasa, J. Gimeno, A. Galindo, C. Mealli, M.
Lanfranchi, A. Tiripicchio, Organometallics 16 (1997) 4099.
.
.