Studies on the reactivity of alkoxo-niobium(V) compounds towards O,O- or S,S-enolate ligands. Synthesis and structural characterisation of (alkoxo)(β-diketonato)- and (alkoxo)[dialkyl(aryl)dithiocarbamato] complexes of niobium(V)

Article abstract of DOI:10.1016/S0022-328X(00)00168-6

Reaction of the alkoxo-containing niobium complexes [NbCl₃(OR)₂]₂ (R=Me, Et, ⁱPr) with β-diketones R′COCH₂COR′ [R′=Me (Hacac); R′=^tBu (Hdpm); R′=Ph (Hdbm)] in a 1:2 molar ratio, afforded the corresponding (alkoxo)(β-diketonato) complexes [NbCl₂(OR)₂(O,O′-R′COCHCOR′)] (R=Me, R′=Me 1; R=Et, R′=Me 2; R=ⁱPr, R′=Me 3; R=Me, R′=^tBu 4; R=Et, R′=^tBu 5; R=ⁱPr, R′=^tBu 6; R=Me, R′=Ph 7; R=Et, R′=Ph 8; R=ⁱPr, R′=Ph 9). The molecular structure of 8 was elucidated by an X-ray diffraction study. The kinetics of the displacement reaction of 5 and 8 with Hacac to give complex 2 and the corresponding β-diketone Hdpm or Hdbm have also been studied by ¹H-NMR spectroscopy. Finally, the complexes [NbCl₃(OR)₂]₂ reacted with reagents of the type Na(S₂CNR′₂), in a 1:4 molar ratio, to give the corresponding (alkoxo)[dialkyl(aryl)dithiocarbamato] complexes [NbCl(OR)₂(S,S′-S₂CNR′)₂] (R=Me, R′=Me 10; R=Me, R′=Et 11; R=Me, R′=Bz 12; R=Et, R′=Me 13; R=Et, R′=Et 14; R=Et, R′=Bz 15; R=ⁱPr, R′=Me 16; R=ⁱPr, R′=Et 17; R=ⁱPr, R′=Bz 18). All complexes were characterised spectroscopically.

Full text of DOI:10.1016/S0022-328X(00)00168-6

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 603 (2000) 194–202
Studies on the reactivity of alkoxo-niobium(V) compounds
towards O,O- or S,S-enolate ligands. Synthesis and structural
characterisation of (alkoxo)(b-diketonato)- and
(alkoxo)[dialkyl(aryl)dithiocarbamato] complexes of niobium(V)
A. Antin˜olo, F. Carrillo-Hermosilla, J. Ferna´ndez-Baeza, S. Garc´ıa-Yuste, A. Otero *,
E. Palomares, A.M. Rodriguez, L.F. Sa´nchez-Barba
Departamento de Qu´ımica Inorga´nica, Orga´nica y Bioqu´ımica, Facultad de Ciencias Qu´ımicas, Uni6ersidad de Castilla-La-Mancha,
Campus Uni6ersitario, E-13071 Ciudad Real, Spain
Received 27 January 2000; received in revised form 6 March 2000; accepted 6 March 2000
Abstract
i
Reaction of the alkoxo-containing niobium complexes [NbCl3(OR)2]2 (R=Me, Et, Pr) with b-diketones R%COCH2COR%
[R%=Me (Hacac); R%=tBu (Hdpm); R%=Ph (Hdbm)] in a 1:2 molar ratio, afforded the corresponding (alkoxo)(b-diketonato)
complexes [NbCl2(OR)2(O,O%-R%COCHCOR%)] (R=Me, R%=Me 1; R=Et, R%=Me 2; R=iPr, R%=Me 3; R=Me, R%=tBu 4;
R=Et, R%=tBu 5; R=iPr, R%=tBu 6; R=Me, R%=Ph 7; R=Et, R%=Ph 8; R=iPr, R%=Ph 9). The molecular structure of
8 was elucidated by an X-ray diffraction study. The kinetics of the displacement reaction of 5 and 8 with Hacac to give complex
1
2 and the corresponding b-diketone Hdpm or Hdbm have also been studied by H-NMR spectroscopy. Finally, the complexes
[NbCl3(OR)2]2 reacted with reagents of the type Na(S2CNR%2), in
a 1:4 molar ratio, to give the corresponding
(alkoxo)[dialkyl(aryl)dithiocarbamato] complexes [NbCl(OR)2(S,S%-S2CNR%)2] (R=Me, R%=Me 10; R=Me, R%=Et 11; R=
Me, R%=Bz 12; R=Et, R%=Me 13; R=Et, R%=Et 14; R=Et, R%=Bz 15; R=iPr, R%=Me 16; R=iPr, R%=Et 17; R=iPr,
R%=Bz 18). All complexes were characterised spectroscopically. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Alkoxy; Niobium; Enolate ligands; Dithiocarbamate
ity of these complexes towards O,O%- or S,S%-chelate
ligands, namely b-diketonate or dialkyl(aryl)dithio-
carbamato anions, in order to prepare new families of
1. Introduction
Alkoxide compounds of a wide variety of elements
alkoxo-containing niobium(V) derivatives. Several b-
diketonato-containing niobium complexes have been
previously described [5], and in some cases one alkoxide
is also present as an ancillary ligand in the coordination
sphere of the niobium centre. Examples of such com-
plexes include [Nb(OR)5−x(acac)x] [6] and [Nb(OR)5−x
(dbm)x] [7], although few X-ray crystal structures have
have been extensively investigated [1] often because
they can be used as precursor compounds for solving
technical materials-related problems [2]. With respect to
the Group 5 metals, this class of complex especially
those of the elements Nb and Ta, have been known for
many years [1,3]. Ten years ago, some of us reported [4]
a general method for the preparation of alkoxoniobi-
um(V) derivatives of the type NbCl5−x(OR)x by reac-
tion of NbCl5 with the appropriate Me3SiOR reagents.
At present we are interested in the study of the reactiv-
been
determined.
Furthermore,
alkyl(aryl)-
dithiocarbamato-containing metal complexes are well
known [8]. For Group 5 metals, the most important
family of this class of complex corresponds to the
compounds of stoichiometry ML(S2CNR2)3 (M=Nb
or Ta; L=O, S, NR, S2, N2R2 or RCꢀCR%) [9]. The
aim of this work was to investigate the synthesis and
* Corresponding author. Tel.: +34-26-295326; fax: +34-26-
295318.
E-mail address: aotero@qino-cr.uclm.es (A. Otero)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00168-

A. Antin˜olo et al. / Journal of Organometallic Chemistry 603 (2000) 194–202
195
structural characterisation of new b-diketonato
[NbCl2(OR)2(O,O%-R%COCHCOR%)] and dialkyl-
(aryl)dithiocarbamato [NbCl(OR)2(S,S%-S2CNR%) ] nio-
2 2
bium complexes containing alkoxides as ancillary lig-
ands.
2. Results and discussion
2.1. Preparation of the i-diketonato-containing
niobium complexes
Firstly, we considered the reactivity of alkoxo-con-
taining niobium complexes [NbCl3(OR)2]2 towards
some b-diketones, namely MeCOCH2COMe (Hacac),
tBuCOCH2COtBu (Hdpm), PhCOCH2COPh (Hdbm).
The different complexes react at room temperature (r.t.)
with the appropriate b-diketones in CH2Cl2 in a 1:2
molar ratio to give, after the appropriate work-up, the
complexes [NbCl2(OR)2(O,O%-R%COCHCOR%)] (Eq.
(1)). The different complexes were isolated in high
yields as yellow moisture-sensitive solids (Section 3).
Fig. 2. ORTEP drawing of complex 8.
1
ture of the complexes. H- and 13C{1H}-NMR spectra
at r.t. show only one set of highly resolved resonances
for the alkoxide groups, indicating that the units are
equivalent. In addition, one set of resonances is also
present for the alkyl (aryl) groups of the b-diketonate
ligand (see Section 3). The spectroscopic results are in
accordance with two symmetrical octahedral structures,
as shown in Fig. 1, in which two chloride or, alterna-
tively, two alkoxide ligands occupy two axial positions
(Fig. 1(a) and (b), respectively).
1/2[NbCl3(OR)2]2+R%COCH2COR%
−HCl
ꢁꢁꢁꢁꢂ [NbCl2(OR)2(O,O%-R%COCHCOR%)]
(1)
It is noteworthy that at r.t. a static structure is
present in complexes 1–9 with highly resolved reso-
nances for the alkoxide groups. This is in contrast with
the situation for the starting complexes [NbCl3(OR)2]2
where fluxionality was observed, as indicated by the
presence of broad and weak resonances, which renders
the alkoxide groups equivalent [4]. The X-ray crystal
structure determination for 8, which is described in
more detail below, indicates that Fig. 1(a), with the
chloride ligands occupying axial positions, is most
probably present in our complexes in the solid state. An
ORTEP drawing of complex 8 is presented in Fig. 2 and
selected bond lengths and angles are listed in Table 1.
Complex 8 crystallises in the monoclinic space group
P21/n, with one molecule in the asymmetric unit cell,
and can be described as a pseudooctahedral Nb(V)
complex containing two trans-chloride ligands in the
axial positions; in the equatorial plane are placed two
cis-ethoxide ligands and a b-diketonate ligand.
1 R=Me, R%=Me; 2 R=Et, R%=Me;
3
R=iPr, R%=Me;
4 R=Me, R%=tBu; 5 R=Et, R%=tBu;
R=iPr, R%=tBu;
7 R=Me, R%=Ph; 8 R=Et, R%=Ph;
R=iPr, R%=Ph.
The complexes are soluble in polar solvents such as
6
9
dichloromethane, chloroform and acetone, but insolu-
ble in alkanes.
Complexes 1–9 were characterised by standard spec-
troscopic techniques. Their IR spectra exhibit charac-
teristic bands at ca. 1575 and 1520 cm−1
,
corresponding to the w(C···C) and w(C···O) of a b-dike-
tonate unit coordinated in a bidentate fashion. In addi-
tion, the bands corresponding to w(NbꢁCl) appear at
ca. 340 cm−1 (see Section 3). However, the NMR
spectroscopy data are valuable in establishing the struc-
The terminal alkoxides form a very strong bond to
,
the metal [NbꢁO distances are 1.804(5) and 1.809(5) A]
due to considerable p-interactions, evidence of which is
provided by NbꢁOꢁC angles [143.0(5) and 157.2(10)°].
The b-diketonate group is coordinated to the Nb centre
through both oxygen atoms, forming a six-membered
metallacycle. The metallacycle ring is reasonably planar
[distances of the atoms from the plane defined by
Nb(1), O(1), C(1), C(2), C(3) and O(2) are Nb(1),
−0.0016(7); O(1), 0.051(4); C(1), 0.008(5); C(2), −
,
0.082(6); C(3), −0.027(6); O(2), 0.077(5) A] and the
parameters within this ring suggest significant delocal-
Fig. 1. Proposed structural dispositions for complexes 1–9.

196
A. Antin˜olo et al. / Journal of Organometallic Chemistry 603 (2000) 194–202
ization. As a result, both CꢁO distances [1.282(6) and
2.2. Kinetic studies on the displacement reactions of 5
and 8 with Hacac
,
1.298(7) A] [10] are intermediate between the single-
,
and double-bonded extremes (1.43 and 1.23 A, respec-
tively [10]). Similarly, the C(1)ꢁC(2) and C(2)ꢁC(3)
The reactivity of complexes 5 and 8, which bear the
dpm and dbm b-diketonates respectively, towards the
b-diketone Hacac in the corresponding exchange reac-
tions has been studied. The reactions were monitored
,
distances [1.386(8) and 1.378(8) A] are intermediate
between single and double bond distances involving sp2
,
carbons (1.44 and 1.34 A). Rings C(10)ꢁC(15) and
1
C(20)ꢁC(25) are displaced slightly on opposite sides of
the metallacycle plane, thereby minimising contacts be-
tween each other.
by H-NMR spectroscopy. In fact, complexes 5 and 8
react with Hacac in CDCl3 to afford the exchange
product 2, along with the free diketones Hdpm and
Hdbm, respectively (Eq. (2)).
(2)
Kinetic studies on these exchange processes have
been carried out by following the reactions as a func-
tion of time and temperature by 1H-NMR spec-
troscopy. Several values of rate constant (Table 2), were
obtained for different [Hacac]/[5] molar ratios at 20°C
(see Section 3, experiment 1). A plot of these values
versus the [Hacac]/[5] values is shown in Fig. 3.
We can observe in the Fig. 3 that, for molar ratios
higher than 4:1, [Hacac]/[Nb complex], respectively, a
negligible dependence on the rate constant can be con-
sidered. A possible mechanism for these processes, for
these conditions of high [Hacac]/[Nb complex] molar
ratios, is shown in Scheme 1 and this corresponds to a
consecutive reaction with reversible step. The ‘satura-
tion’ behaviour observed for the rate constant is typical
of a pseudo-first-order dissociative process [11] and, as
such, the formation of a pentacoordinated and elec-
tronically unsaturated species can be proposed as the
rate-limiting step in these experimental conditions for
the commented processes.
Table 1
,
Selected bonds lengths (A) and angles (°) for 8
Nb(1)ꢁO(4)
Nb(1)ꢁO(3)
Nb(1)ꢁO(2)
Nb(1)ꢁO(1)
Nb(1)ꢁCl(3)
Nb(1)ꢁCl(2)
O(1)ꢁC(1)
1.804(5)
1.809(5)
2.045(4)
2.085(4)
2.392(2)
2.403(2)
1.282(6)
O(2)ꢁC(3)
C(1)ꢁC(2)
C(2)ꢁC(3)
O(3)ꢁC(4)
C(4)ꢁC(5)
O(4)ꢁC(6)
C(6)ꢁC(7)
1.298(7)
1.386(8)
1.378(8)
1.465(10)
1.335(14)
1.462(18)
1.34(3)
O(4)ꢁNb(1)ꢁO(3) 100.7(2)
O(4)ꢁNb(1)ꢁO(2) 92.9(2)
O(3)ꢁNb(1)ꢁO(2) 165.67(19)
O(4)ꢁNb(1)ꢁO(1) 172.6(2)
O(3)ꢁNb(1)ꢁO(1)
O(2)ꢁNb(1)ꢁO(1)
O(4)ꢁNb(1)ꢁCl(3)
O(3)ꢁNb(1)ꢁCl(3)
O(2)ꢁNb(1)ꢁCl(3)
O(1)ꢁNb(1)ꢁCl(3)
O(4)ꢁNb(1)ꢁCl(2)
O(3)ꢁNb(1)ꢁCl(2)
O(2)ꢁNb(1)ꢁCl(2)
O(1)ꢁNb(1)ꢁCl(2)
Cl(3)ꢁNb(1)ꢁCl(2) 170.23(8)
C(1)ꢁO(1)ꢁNb(1)
C(3)ꢁO(2)ꢁNb(1)
O(1)ꢁC(1)ꢁC(2)
C(3)ꢁC(2)ꢁC(1)
O(2)ꢁC(3)ꢁC(2)
C(15)ꢁC(10)ꢁC(11) 118.2(6)
C(10)ꢁC(15)ꢁC(14) 120.8(6)
C(4)ꢁO(3)ꢁNb(1)
C(5)ꢁC(4)ꢁO(3)
C(6)ꢁO(4)ꢁNb(1)
C(71)ꢁC(6)ꢁC(7)
C(71)ꢁC(6)ꢁO(4)
C(7)ꢁC(6)ꢁO(4)
134.8(4)
135.7(4)
122.5(5)
124.3(5)
122.1(5)
86.66(18)
79.72(16)
92.20(18)
96.39(17)
87.39(15)
87.09(14)
92.80(18)
90.92(16)
83.98(15)
86.88(14)
143.0(5)
115.2(9)
157.2(10)
46(3)
118(3)
109(2)
This observation agrees with the intermediate pro-
posed in the isotopic interchange processes for tris-b
Table 2
Values of rate constant at 313 K, for different molar ratios [Hacac]/
[5], in experiment 1
[Complex 5]
[Hacac] (M)
[Hacac]/
k
app×103
(M)
[complex 5]
0.09
0.09
0.09
0.09
0.09
0.09
0.18
0.36
0.45
0.63
1:1
2:1
4:1
5:1
7:1
0.790.2
2.090.2
3.190.2
3.190.2
3.190.2
Fig. 3. Plot of kapp (s−1) vs. [Hacac]/[5], in experiment 1.

A. Antin˜olo et al. / Journal of Organometallic Chemistry 603 (2000) 194–202
197
diketonato derivatives of Group 13 elements [12]. Sev-
eral irreversible steps, which correspond to the coordi-
nation of the Hacac, probably throughout the enol
tautomer, which enhances its acidic character, and the
de-coordination of the appropriate b-diketone (Hdpm
or Hdbm) on the metal centre, can be considered in the
proposed mechanism.
Activation parameters have been estimated from
these studies. In order to optimise and simplify the
experiments, they were carried out under the com-
mented pseudo-first-order conditions with respect to the
species 5 and 8, using a high initial concentration of
Hacac (see Section 3, experiments 1 and 2). Values of
the apparent pseudo-first-order rate constants, kapp, at
five temperature values were obtained for the two stud-
ied processes (Table 3).
The kapp values give a satisfactory fit to the Arrhenius
plot, kapp=A exp(−Ea/RT), and the Ea values for the
different processes were calculated (Table 3). The
Eyring plot, ln(kapp/T) against 1/T,
kapp=(kT/
h) exp(−DHc/RT) exp(DSc/R), gives the activation
parameters DHc and DSc for the two processes (Table
3). The calculated Ea and DHc values are comparable
to those previously described in the isotopic exchange
processes for tris-b-diketonato derivatives of Group 13
elements [12]. The difference observed in the activation
energy calculated in the two processes could be ex-
plained in terms of a more stable bonding between the
dbm ligand and the niobium atom in complex 8,
throughout a delocalized p system implying the phenyl
rings of the ligand, than in complex 5 with the dpm
ligand.
Scheme 1. A possible mechanism on the displacement reactions of 5 and 8 with Hacac.
Table 3
Kinetic and thermodynamic parameters on the displacement reactions of 5 and 8 with Hacac
Experiment
1
k
app×103 (s−1
)
T (K)
Ea (kJ mol−1
)
DH" (kJ mol−1
)
DS" (J mol−1 K−1
)
0.390.2
0.690.2
1.290.2
2.190.2
3.190.2
293
298
303
308
313
90.390.1
87.890.1
−1194
2
0.190.2
0.290.2
0.590.2
0.990.2
1.590.2
293
298
303
308
313
105.590.1
102.990.1
3194

198
A. Antin˜olo et al. / Journal of Organometallic Chemistry 603 (2000) 194–202
bipyramidal structure in the solid state, as shown in
Fig. 4, where two dithiocarbamate ligands occupy four
equatorial coordination sites and the alkoxide ligands
two axial sites.
This bipyramidal disposition was previously found in
the dialkyldithiocarbamato complexes [Nb(RCꢀCR%)-
Fig. 4. Proposed structural disposition for complexes 10–18.
(S2CNR¦) ] [15].
2 3
In conclusion, this study has revealed that the
alkoxo-containing complexes [NbCl3OR)2]2 react read-
ily with b-diketones and sodium dialkyl(aryl)dithio-
carbamato salts, giving rise to the b-diketonato and
dialkyl(aryl)dithiocarbamato-containing niobium spe-
cies, namely [NbCl2(OR)2(O,O%-R%COCHCOR%)] and
In the course of these studies, we have observed that,
for long reaction times, higher than 24 h, equilibrium
was reached between reagents and products. Equi-
librium constants at 20°C were calculated from the
spectra (see Section 3). The obtained values, near 0.45
(Complex 5: Keq=0.44; complex 8: Keq=0.46), were
similar for both niobium complexes, 5 and 8, indicating
that they exhibit similar stability with regard to com-
plex 2.
[NbCl(OR)2(S,S%-S2CNR%) ]. In addition, kinetic stud-
2 2
ies on the exchange processes with b-diketonato com-
plexes have been considered. We believe that the results
reported offer an advance in the chemistry of alkoxo-
containing niobium complexes and further work in this
field is ongoing.
2.3. Preparation of dialkyl(aryl)dithiocarbamato-
containing niobium complexes
The reactivity of our starting complexes
[NbCl3(OR)2]2 towards sodium dialkyl(aryl)dithio-
3. Experimental
carbamato salts, Na(S2CNR%), was considered. The
2
3.1. General
study involved the addition of Na(S2CNR%) to a THF
2
solution of the complex in a 1:4 molar ratio to give,
after stirring for 20 h at 60°C, a solution from which
the corresponding products were isolated, after appro-
priate work-up, as white crystalline air-stable solids
All reactions were performed using standard Schlenk-
tube techniques under an atmosphere of dry nitrogen.
Solvents were distilled from appropriate drying agents
and degassed before use. Microanalyses were carried
out with a Perkin–Elmer 2400 CHN analyser. IR spec-
tra were obtained in the 4000–200 cm−1 region, using
[NbCl(OR)2(S,S%-S2CNR%) ] (10–18) (Eq. (3)). Com-
2 2
plexes 10, 11 and 12 have been described previously [13]
and were obtained by reaction of NbCl5 with the
a
Perkin–Elmer 883 spectrophotometer. 1H- and
appropriate Na(S2CNR%) salt in methanol. The differ-
13C{1H}-NMR spectra were recorded on a Varian
Unity FT-300 spectrometer and referenced to the resid-
ual deuterated solvent. The NMR probe temperatures
were varied using an Oxford Instruments VTC 4 unit,
measured by a thermocouple and calibrated with
CH3OH.
2
ent complexes were isolated in high yields and are
soluble in polar solvents, but insoluble in alkanes.
1/2[NbCl3(OR)2]2+2Na(S2CNR%)
2
“[NbCl(OR)2(S2CNR%) ]+2NaCl
2 2
(3)
10 R=Me, R%=Me; 11 R=Me, R%=Et;
i
The complexes [NbCl3(OR)2]2 (R=Me, Et, Pr) were
prepared as reported previously [4]. The b-diketones
and the sodium dialkyldithiocarbamates were pur-
chased from Fluka.
12
13 R=Et, R%=Me; 14 R=Et, R%=Et;
15 R=Et, R%=Bz;
16 R=iPr, R%=Me; 17 R=iPr, R%=Et;
18
R=iPr, R%=Bz.
Complexes 10–18 were characterised spectroscopi-
R=Me, R%=Bz;
3.2. Preparation of [NbCl2(OMe)2(acac)] (1)
To a CH2Cl2 (50 cm3) solution of [NbCl3(OMe)2]2
(0.670 g, 1.28 mmol) was added Hacac (0.26 cm3, 2.56
mmol). The suspension was stirred for 6 h at r.t. The
solvent was removed under reduced pressure and a
yellow solid was obtained. Yield 81%. Found: C, 25.87;
H, 4.03. Anal. Calc. for NbCl2O4C7H13: C, 25.94; H,
4.00%. 1H-NMR (CDCl3, 295 K), l 5.97 (s, 1H,
MeCOCHCOMe), 2.16 (s, 6H, MeCOCHCOMe), 4.60
(s, 6H, OMe). 13C{1H} (CDCl3), l 191.4 (MeCOCH-
COMe), 108.0 (MeCOCHCOMe), 26.6 (MeCOCH-
COMe), 67.6 (OMe). IR (Nujol, cm−1) 1590 [w(C···C)],
1542 [w(C···O)], 334 [w(NbꢁCl)].
cally. Their IR spectra show a characteristic band at ca.
990 cm−1, corresponding to the w(C···S) of a dithiocar-
bamato unit coordinated in a bidentate fashion [14],
and another band at ca. 360 cm−1 for w(NbꢁCl) (see
Section 3). The complexes have also been characterised
1
by NMR spectroscopy. H- and 13C{1H}-NMR spectra
at r.t. show only one set of resonances for the alkoxide
groups as well as two sets of resonances for the alkyl-
(aryl) groups on the dithiocarbamate ligands (see Sec-
tion 3). These data are in accordance with a pentagonal

A. Antin˜olo et al. / Journal of Organometallic Chemistry 603 (2000) 194–202
199
3.3. Preparation of [NbCl2(OEt)2(acac)] (2)
(t, 6H, 3JHH=7.2, OCH2CH3). 13C{1H} (CDCl3), l
201.1 (Me3CCOCHCOCMe3), 98.5 (Me3CCOCH-
The synthetic procedure was the same as for com-
plex 1, using [NbCl3(OEt)2]2 (0.226 g, 0.39 mmol) and
Hacac (0.08 cm3, 0.78 mmol), to give complex 2 as a
yellow solid. Yield 86%. Found: C, 30.62; H, 4.85.
Anal. Calc. for NbCl2O4C9H17: C, 29.95; H, 4.44%.
1H-NMR (CDCl3, 295 K), l 5.93 (s, 1H, MeCOCH-
COMe), 2.13 (s, 6H, MeCOCHCOMe), 4.87 (q, 4H,
3JHH=7.0, OCH2CH3), 1.48 (t, 6H, 3JHH=7.0,
OCH2CH3). 13C{1H} (CDCl3), l 191.1 (MeCOCH-
COMe), 107.7 (MeCOCHCOMe), 26.6 (MeCOCH-
COMe), 76.6 (OCH2CH3), 16.1 (OCH2CH3). IR
(Nujol, cm−1) 1583 [w(C···C)], 1533 [w(C···O)], 340
[w(NbꢁCl)].
COCMe3),
40.9
(Me3CCOCHCOCMe3),
27.5
(Me3CCOCHCOCMe3), 75.9 (OCH2CH3), 16.4
(OCH2CH3). IR (Nujol, cm−1) 1540 [w(C···C)], 1507
[w(C···O)], 340 [w(NbꢁCl)].
3.7. Preparation of [NbCl2(OiPr)2(dpm)] (6)
The synthetic procedure was the same as for com-
plex 1, using [NbCl3(OiPr)2]2 (0.727 g, 1.14 mmol) and
Hdpm (0.48 cm3, 2.28 mmol), to give complex 6 as a
yellow solid. Yield 90%. Found: C, 41.55; H, 6.79.
Anal. Calc. for NbCl2O4C17H33: C, 41.45; H, 6.71%.
1H-NMR (CDCl3, 295 K),
l
6.16 (s, 1H,
Me3CCOCHCOCMe3), 1.22 (s, 18H, Me3CCOCH-
COCMe3), 5.25 (m, 2H, OCH(CH3)2), 1.48 (d, 12H,
3JHH=6.3, OCH(CH3)2). 13C{1H} (CDCl3), l 200.7
(Me3CCOCHCOCMe3), 98.1 (Me3CCOCHCOCMe3),
40.9 (Me3CCOCHCOCMe3), 27.6 (Me3CCOCH-
COCMe3), 84.5 [OCH(CH3)2], 23.3 [OCH(CH3)2]. IR
(Nujol, cm−1) 1553 [w(C···C)], 1507 [w(C···O)], 342
[w(NbꢁCl)].
3.4. Preparation of [NbCl2(OiPr)2(acac)] (3)
The synthetic procedure was the same as for com-
plex 1, using [NbCl3(OiPr)2]2 (0.529 g, 0.83 mmol) and
Hacac (0.17 cm3, 1.66 mmol), to give complex 3 as a
yellow solid. Yield 84%. Found: C, 34.64; H, 5.51.
Anal. Calc. for NbCl2O4C11H21: C, 34.74; H, 5.47%.
1H-NMR (CDCl3, 295 K), l 5.90 (s, 1H, MeCOCH-
COMe), 2.10 (s, 6H, MeCOCHCOMe), 5.19 (m, 2H,
3.8. Preparation of [NbCl2(OMe)2(dbm)] (7)
3
OCH(CH3)2), 1.46 (d, 12H, JHH=6.4, OCH(CH3)2).
13C{1H} (CDCl3), l 190.7 (MeCOCHCOMe), 107.3
(MeCOCHCOMe), 26.4 (MeCOCHCOMe), 84.3
The synthetic procedure was the same as for com-
plex 1, using [NbCl3(OMe)2]2 (1.10 g, 2.11 mmol) and
Hdbm (0.94 g, 4.22 mmol), to give complex 7 as an
orange solid. Yield 98%. Found: C, 45.34; H, 3.93.
Anal. Calc. for NbCl2O4C17H17: C, 45.59; H, 3.79%.
1H-NMR (CDCl3, 295 K), l 7.28 (s, 1H, PhCOCH-
COPh), 8.08–7.63 (m, 10H, PhCOCHCOPh), 4.76 (s,
6H, OMe). 13C{1H} (CDCl3), l 183.8 (PhCOCH-
COPh), 100.7 (PhCOCHCOPh), 135.4, 133.7, 128.3,
128.9 (PhCOCHCOPh), 67.6 (OMe). IR (Nujol,
cm−1) 1595 [w(C···C)], 1531 [w(C···O)], 334 [w(NbꢁCl)].
[OCH(CH3)2], 23.03 [OCH(CH3)2]. IR (Nujol, cm−1
1585 [w(C···C)], 1532 [w(C···O)], 341 [w(NbꢁCl)].
)
3.5. Preparation of [NbCl2(OMe)2(dpm)] (4)
The synthetic procedure was the same as for com-
plex 1, using [NbCl3(OMe)2]2 (0.672 g, 1.28 mmol) and
Hdpm (0.54 cm3, 2.57 mmol), to give complex 4 as a
yellow solid. Yield 98%. Found: C, 37.31; H, 6.03.
Anal. Calc. for NbCl2O4C13H25: C, 38.23; H, 6.18%.
1H-NMR (CDCl3, 295 K),
l
6.23 (s, 1H,
(Me3CCOCHCOCMe3), 1.24 (s, 18H, Me3CCOCH-
COCMe3), 4.61 (s, 6H, OMe). 13C{1H} (CDCl3), l
201.0 (Me3CCOCHCOCMe3), 98.9 (Me3CCOCH-
3.9. Preparation of [NbCl2(OEt)2(dbm)] (8)
The synthetic procedure was the same as for com-
plex 1, using [NbCl3(OEt)2]2 (0.612 g, 1.05 mmol) and
Hdbm (0.474 g, 2.10 mmol), to give complex 8 as an
orange solid. Yield 98%. Crystals for X-ray diffraction
were grown from a solution of CH2Cl2 by slow evapo-
ration of the solvent. Found: C, 47.30; H, 4.20. Anal.
Calc. for NbCl2O4C19H21: C, 47.81; H, 4.40%. 1H-
NMR (CDCl3, 295 K), l 7.25 (s, 1H, PhCOCH-
COPh), 8.08–7.63 (m, 10H, PhCOCHCOPh), 5.05 (q,
4H, 3JHH=7.2, OCH2CH3), 1.59 (t, 6H, 3JHH=7.2,
OCH2CH3). 13C{1H} (CDCl3), l 183.7 (PhCOCH-
COPh), 100.6 (PhCOCHCOPh), 135.7, 133.5, 128.8,
128.2 (PhCOCHCOPh), 76.6 (OCH2CH3), 16.5
(OCH2CH3). IR (Nujol, cm−1) 1602 [w(C···C)], 1534
[w(C···O)], 342 [w(NbꢁCl)].
COCMe3),
40.9
(Me3CCOCHCOCMe3),
27.5
(Me3CCOCHCOCMe3), 66.98 (s, OMe). IR (Nujol,
cm−1) 1550 [w(C···C)], 1500 [w(C···O)], 332 [w(NbꢁCl)].
3.6. Preparation of [NbCl2(OEt)2(dpm)] (5)
The synthetic procedure was the same as for com-
plex 1, using [NbCl3(OEt)2]2 (0.808 g, 1.36 mmol) and
Hdpm (0.582 cm3, 2.72 mmol), to give complex 5 as a
yellow solid. Yield 98%. Found: C, 41.03; H, 6.84.
Anal. Calc. for NbCl2O4C15H29: C, 41.28; H, 6.70%.
1H-NMR (CDCl3, 295 K),
l
6.20 (s, 1H,
Me3CCOCHCOCMe3), 1.23 (s, 18H, Me3CCOCH-
COCMe3), 4.90 (q, 4H, 3JHH=7.2, OCH2CH3), 1.48

200
A. Antin˜olo et al. / Journal of Organometallic Chemistry 603 (2000) 194–202
3.10. Preparation of [NbCl2(OiPr)2(dbm)] (9)
K), l 7.37–7.33 (m, 20H, S2CN(CH2Ph)2), 4.95 (s,
8H, S2CN(CH2Ph)2), 4.09 (s, 6H, OMe). 13C{1H}
The synthetic procedure was the same as for com-
plex 1, using [NbCl3(OiPr)2]2 (0.752 g, 1.18 mmol) and
Hdpm (0.531 g, 2.36 mmol), to give complex 9 as a
yellow solid. Yield 90%. Found: C, 47.33; H, 4.42.
Anal. Calc. for NbCl2O4C21H25: C, 47.21; H, 4.75%.
1H-NMR (CDCl3, 295 K), l 7.20 (s, 1H, PhCOCH-
COPh), 8.08–7.63 (m, 10H, PhCOCHCOPh), 5.38 (m,
2H, OCH(CH3)2), 1.59 (d, 12H, 3JHH=6.3,
OCH(CH3)2). 13C{1H} (CDCl3), l 183.7 (PhCOCH-
COPh), 100.3 (PhCOCHCOPh), 136.1, 133.2, 128.7,
128.1 (PhCOCHCOPh), 84.3 [OCH(CH3)2], 23.4
[OCH(CH3)2]. IR (Nujol, cm−1) 1585 [w(C···C)], 1532
[w(C···O)], 344 [w(NbꢁCl)].
(CDCl3),
[S2CN(CH2Ph)2], 133.8–127.9 [S2CN(CH2Ph)2], 63.2
(OMe). IR (Nujol, cm−1
1485 [w(C···N)], 1273
l
203.5
(S2CNBz2),
51.0,
50.0
)
[w(C···S)], 1077 [w(OꢁC)], 275 [w(NbꢁCl)].
3.14. Preparation of [NbCl(OEt)2(S2CNMe2)2] (13)
The synthetic procedure was the same as for com-
plex 10, using [NbCl3(OEt)2]2 (0.663 g, 1.15 mmol) and
Na(S2CNMe2) (0.656 g, 4.58 mmol), to give complex
13 as a white solid. Yield 66%. Found: C, 25.77; H,
4.85; N, 5.81. Anal. Calc. for NbClO2S4N2C10H22: C,
1
26.17; H, 4.83; N, 6.10%. H-NMR (CDCl3, 295 K), l
3
3.40, 3.39 (all s, 12H, S2CNMe2), 4.17 (q, 4H, JHH
=
3
3.11. Preparation of [NbCl(OMe)2(S2CNMe2)2] (10)
7.0, OCH2CH3), 1.09 (t, 6H, JHH=7.0, OCH2CH3).
13C{1H} (CDCl3), l 202.8 (S2CNMe2), 39.5, 39.4
(S2CNMe2), 71.0 (OCH2CH3), 17.23 (OCH2CH3). IR
(Nujol, cm−1) 1520 [w(C···N)], 1247 [w(C···S)], 1066
[w(OꢁC)], 275 [w(NbꢁCl)].
To a THF (50 cm3) solution of [NbCl3(OMe)2]2
(0.470 g, 0.90 mmol) was added Na(S2CNMe2) (0.515
g, 3.60 mmol). The suspension was stirred for 20 h at
60°C. The solvent was removed under reduced pres-
sure and, after extraction with CH2Cl2 (30 cm3), a
white solid was obtained. Yield 78%. Found: C, 22.35;
H, 4.27; N, 6.08. Anal. Calc. for NbClO2S4N2C8H18:
C, 22.30; H, 4.21; N, 6.50%. 1H-NMR (CDCl3, 295
K), l 3.42, 3.40 (all s, 12H, S2CNMe2), 3.94 (s, 6H,
OMe). 13C{1H} (CDCl3), l 202.1 (S2CNMe2), 38.3,
38.2 (S2CNMe2), 68.1 (OMe). IR (Nujol, cm−1) 1528
[w(C···N)], 1258 [w(C···S)], 1094 [[w(OꢁC)], 274
[w(NbꢁCl)].
3.15. Preparation of [NbCl(OEt)2(S2CNEt2)2] (14)
The synthetic procedure was the same as for com-
plex 10, using [NbCl3(OEt)2]2 (0.371 g, 0.64 mmol) and
Na(S2CNEt2) (0.439 g, 2.56 mmol), to give complex 14
as a white solid. Yield 77%. Found: C, 32.58; H, 5.62;
N, 5.37. Anal. Calc. for NbClO2S4N2C14H30: C, 32.65;
1
H, 5.87; N, 5.42%. H-NMR (CDCl3, 295 K), l 3.84
(q, 8H, 3JHH=7.4, S2CN(CH2CH3)2), 1.31 (t, 12H,
3
3JHH=7.4, S2CN(CH2CH3)2), 4.20 (q, 4H, JHH=7.0,
3.12. Preparation of [NbCl(OMe)2(S2CNEt2)2] (11)
OCH2CH3), 1.12 (t, 6H, 3JHH=7.0, OCH2CH3).
13C{1H} (CDCl3), l 201.3 (S2CNEt2), 44.8, 44.9
[S2CN(CH2CH3)2], 12.9, 13.1 [S2CN(CH2CH3)2], 71.0
(OCH2CH3), 17.1 (OCH2CH3). IR (Nujol, cm−1) 1502
[w(C···N)], 1260 [w(C···S)], 1062 [w(OꢁC)], 283
[w(NbꢁCl)].
The synthetic procedure was the same as for com-
plex 10, using [NbCl3(OMe)2]2 (0.632 g, 1.21 mmol)
and Na(S2CNEt2) (0.828 g, 4.84 mmol), to give com-
plex 11 as a white solid. Yield 82%. Found: C, 29.72;
H, 5.21; N, 5.75. Anal. Calc. for NbClO2S4N2C12H26:
C, 29.60; H, 5.38; N, 5.75%. 1H-NMR (CDCl3, 295
3
K), l 3.86 (q, 8H, JHH=7.4, S2CN(CH2CH3)2), 1.34
3.16. Preparation of [NbCl(OEt)2(S2CNBz2)2] (15)
(t, 12H, 3JHH=7.4, S2CN(CH2CH3)2), 3.96 (s, 6H,
OMe). 13C{1H} (CDCl3), l 202.9 (S2CNEt2), 44.4,
43.5 [S2CN(CH2CH3)2], 12.5, 12.3 [S2CN(CH2CH3)2],
62.9 (OMe). IR (Nujol, cm−1) 1510 [w(C···N)], 1260
[w(C···S)], 1090 [w(OꢁC)], 273 [w(NbꢁCl)].
The synthetic procedure was the same as for com-
plex 10, using [NbCl3(OEt)2]2 (0.515 g, 0.89 mmol) and
Na(S2CNBz2) (1.051 g, 3.56 mmol), to give complex
15 as a white solid. Yield 68%. Found: C, 51.97; H,
4.61; N, 3.78. Anal. Calc. for NbClO2S4N2C34H38: C,
1
3.13. Preparation of [NbCl(OMe)2(S2CNBz2)2] (12)
52.28; H, 4.66; N, 3.81%. H-NMR (CDCl3, 295 K), l
7.38–7.29 (m, 20H, S2CN(CH2Ph)2), 4.96 (s, 8H,
3
The synthetic procedure was the same as for com-
plex 10, using [NbCl3(OMe)2]2 (0.495 g, 0.95 mmol)
and Na(S2CNBz2) (1.119 g, 3.79 mmol), to give com-
plex 12 as a white solid. Yield 68%. Found: C, 51.97;
H, 4.61; N, 3.78. Anal. Calc. for NbClO2S4N2C32H34:
C, 52.28; H, 4.66; N, 3.81%. 1H-NMR (CDCl3, 295
S2CN(CH2Ph)2), 4.36 (q, 4H, JHH=7.0, OCH2CH3),
1.12 (t, 6H, JHH=7.0, OCH2CH3). 13C{1H} (CDCl3),
3
l 204.4 (S2CNBz2), 51.4, 50.4 [S2CN(CH2Ph)2], 134.5–
128.7 [S2CN(CH2Ph)2], 71.3 (OCH2CH3), 17.3
(OCH2CH3). IR (Nujol, cm−1) 1480 [w(C···N)], 1254
[w(C···S)], 1098 [w(OꢁC)], 276 [w(NbꢁCl)].

A. Antin˜olo et al. / Journal of Organometallic Chemistry 603 (2000) 194–202
201
3.17. Preparation of [NbCl(OiPr)2(S2CNMe2)2] (16)
systematically varied (see Table 2). The tube was vig-
orously shaken for about 5 s and then placed into the
spectrometer. Each spectrum recorded at 313 K was
the result of the coaddition of 16 transients, which
required nearly 1 min to acquire. Spectra were auto-
matically recorded every 5 min using the kinetic analy-
sis software from Varian. Variations of the intensity in
The synthetic procedure was the same as for com-
plex 10, using [NbCl3(OiPr)2]2 (0.535 g, 0.84 mmol)
and Na(S2CNMe2) (0.483 g, 3.37 mmol), to give com-
plex 16 as a white solid. Yield 37%. Found: C, 29.48;
H, 5.27; N, 5.80. Anal. Calc. for NbClO2S4N2C12H26:
C, 29.60; H, 5.38; N, 5.75%. 1H-NMR (CDCl3, 295
K), l 3.40, 3.39 (all s, 12H, S2CNMe2), 4.55 (m, 2H,
t
the singlet signal corresponding to the Bu of dpm in
5, were measured for three t1/2. A computer analysis
method for intensities gives a straight-line segment in
a plot of ln(intensity) against time. The error is given
by standard deviation in the regression analysis. For
the [Hacac]/[5] molar ratio of 7:1, similar experiments
were carried out at 293, 298, 303 and 308 K.
3
OCH(CH3)2), 1.11 (d, 12H, JHH=6.2, OCH(CH3)2).
13C{1H} (CDCl3), l 202.9 (S2CNMe2), 39.4, 38.6
(S2CNMe2), 78.0 [OCH(CH3)2], 23.9 [OCH(CH3)2]. IR
(Nujol, cm−1) 1529 [w(C···N)], 1258 [w(C···S)], 1121
[w(OꢁC)], 260 [w(NbꢁCl)].
3.18. Preparation of [NbCl(OiPr)2(S2CNEt2)2] (17)
3.21. Kinetics of the reaction of 8 with Hacac —
experiment 2
The synthetic procedure was the same as for com-
plex 10, using [NbCl3(OiPr)2]2 (0.327 g, 0.52 mmol)
and Na(S2CNEt2) (0.353 g, 2.06 mmol), to give com-
plex 17 as a white solid. Yield 35%. Found: C, 33.38;
H, 5.73; N, 4.91. Anal. Calc. for NbClO2S4N2C16H34:
C, 33.65; H, 6.00; N, 4.91%. 1H-NMR (CDCl3, 295
Complex 8 (40 mg) was dissolved in CDCl3 in an
NMR tube (5 mm diameter) and Hacac (0.060 ml)
was added. The final volume was 1 ml. The concentra-
tion of 8 was 0.083 M, and the [Hacac]/[8] molar ratio
was 7:1. The tube was vigorously shaken for about 5 s
and then placed into the spectrometer. Each spectrum
recorded at 293 K was the result of the coaddition of
16 transients, which required nearly 1 min to acquire.
Spectra were automatically recorded every 5 min using
the kinetic analysis software from Varian. Variations
of the intensity in the singlet signal corresponding to
the CH of dbm in 8 were measured for three t1/2. A
computer analysis method for intensities gives a
straight-line segment in a plot of ln(intensity) against
time. The error is given by standard deviation in the
regression analysis. Similar experiments were carried
out at 298, 303, 308 and 313 K.
3
K), l 3.85 (q, 8H, JHH=7.4, S2CN(CH2CH3)2), 1.33
(t, 12H, 3JHH=7.4, S2CN(CH2CH3)2), 4.72 (m, 2H,
3
OCH(CH3)2), 1.12 (d, 12H, JHH=6.2, OCH(CH3)2).
13C{1H} (CDCl3), l 202.6 (S2CNEt2), 44.8, 44.7
[S2CN(CH2CH3)2], 24.6, 24.5 [S2CN(CH2CH3)2], 77.4
[OCH(CH3)2], 13.1 [OCH(CH3)2]. IR (Nujol, cm−1
)
1510 [w(C···N)], 1262 [w(C···S)], 1100 [w(OꢁC)], 270
[w(NbꢁCl)].
3.19. Preparation of [NbCl(OiPr)2(S2CNBz2)2] (18)
The synthetic procedure was the same as for com-
plex 10, using [NbCl3(OiPr)2]2 (0.228 g, 0.36 mmol)
and Na(S2CNBz2) (0.424 g, 1.44 mmol), to give com-
plex 18 as a white solid. Yield 20%. Found: C, 54.32;
H, 5.26; N, 3.65. Anal. Calc. for NbClO2S4N2C36H42:
C, 54.64; H, 5.35; N, 3.54%. 1H-NMR (CDCl3, 295
K), l 7.38–7.30 (m, 20H, S2CN(CH2Ph)2), 4.98 (s,
8H, S2CN(CH2Ph)2), 4.80 (m, 2H, OCH(CH3)2), 1.15
3.22. Measurements of exchange equilibrium constants
Complex 5 (40 mg) was dissolved in CDCl3 in an
NMR tube (5 mm diameter) and Hacac (0.006 ml)
was added. The final volume was 1 ml and the
[Hacac]/[5] molar ratio was 7:1. The mixture was al-
lowed to react for 24 h, at 293 K, in order to reach
the equilibrium. Normalized integrals of characteristic
peaks of the present species were measured from the
1H-NMR spectra, in order to obtain the equilibrium
constant, Keq=[NbCl2(acac)(OEt)2][Hdpm]/[NbCl2(O,-
O-tBuCOCHCOtBu)(OEt)2][Hacac]. An average value
(see above) was calculated from five similar experi-
ments.
3
(d, 12H, JHH=6.2, OCH(CH3)2). 13C{1H} (CDCl3), l
203.5 (S2CNBz2), 51.4, 50.4 [S2CN(CH2Ph)2], 134.6–
128.9 [S2CN(CH2Ph)2], 77.5 [OCH(CH3)2], 13.1
[OCH(CH3)2]. IR (Nujol, cm−1) 1480 [w(C···N)], 1265
[w(C···S)], 1099 [w(OꢁC)], 272 [w(NbꢁCl)].
3.20. Kinetics of the reaction of 5 with Hacac —
experiment 1
A similar process was performed for complex 8, in
order to obtain the average equilibrium constant (see
above), Keq=[NbCl2(acac)(OEt)2][Hdbm]/[NbCl2(O,O-
PhCOCHCOPh)(OEt)2][Hacac].
Complex 5 (40 mg) was dissolved in CDCl3 in an
NMR tube (5 mm diameter) and Hacac was added.
The final volume was 1 ml. The concentration of 5
was 0.09 M and the concentrations of Hacac were

202
A. Antin˜olo et al. / Journal of Organometallic Chemistry 603 (2000) 194–202
3.23. Crystal structure determination of 8
Yellow crystal (0.30×0.20×0.20
C19H21Cl2NbO4, which is monoclinic with space group
P21/n and lattice constants a=10.224(2), b=
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of
,
15.4315(8), c=14.248(5) A, i=108.83(2)°, V=
−3
2127.6(9)
M=477.17. Reflections were collected at 25°C on a
Nonius-MACH3 diffractometer equipped with
graphite-monochromated radiation source (u=0.7107
A
,
Z=4,
D
calc=1.490
g
cm−3
,
,
a
,
A), 5122 reflections collected (2BqB28), 2209 reflec-
tions with I\2|I. Data were corrected in the usual
fashion for Lorentz and polarisation effects, and empir-
ical absorption correction was not necessary (v=8.36
cm−1). Data/restraints/parameters 5122/2/237. The
structure was solved by direct methods [16] and refine-
ment on F2 was carried out by full-matrix least-squares
analysis [17]. The C7 is disordered over two positions
with different occupation factors (0.722 for C7 and
0.278 for C71). Anisotropic temperature parameters
were considered for all non-hydrogen atoms, while
hydrogen atoms were included in calculated positions
but not refined. Final disagreement indices are R1=
[7] R.N. Kapoor, S. Prakash, P.N. Kapoor, Bull. Chem. Soc. Jpn.
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0.0587, wR2=0.1578, Goodness-of-fit=0.853, largest
difference peak and hole 0.602 and −0.731 e A
−3
,
.
4. Supplementary material
[10] International Tables for X-ray Crystallography, vol. III, Kynoch
Press, Birmingham, UK, 1968, Tables 4.2.2 and 4.2.5.
[11] (a) R.B. Jordan, Reaction Mechanisms of Inorganic and
Organometallic Systems, Oxford University Press, London,
1991, pp. 31–32. (b) J.H. Espenson, Chemical Kinetics and
Reaction Mechanisms, 1st ed., McGraw-Hill, New York, 1981,
p. 76. (c) R.G. Wilkins, Kinetics and Mechanism of Reactions of
Transition Metal Complexes, 2nd ed., VCH, Weinheim, 1991,
p.11.
Crystallographic data for the structural analysis has
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 139411 for compound 8.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1E2, UK (fax: +44-1223-336033, or
e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
[12] K. Saito, Pure Appl. Chem. 38 (1974) 325.
[13] D.C. Pantaleo, R.C. Johnson, Inorg. Chem. 9 (1970) 1248.
[14] F. Boneti, R. Ugo, J. Organomet. Chem. 10 (1967) 257.
[15] J. Ferna´ndez-Baeza, F.A. Jalo´n, A. Otero, M.E. Rodrigo-
Blanco, M. Etienne, J. Chem. Soc. Dalton Trans. (1998) 769.
[16] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi,
M.C. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr.
(1994) 435.
[17] G.M. Sheldrick, Program for the Refinement of Crystal Struc-
tures from Diffraction data, University of Go¨ttingen, Germany,
1997.
Acknowledgements
We gratefully acknowledge financial support from
the Direccio´n General de Ensen˜anza Superior e Inves-
tigacio´n Cient´ıfica, Spain (Grant no. PB95-0023-C01-
02).
.