Synthesis, spectral, thermal and kinetic studies on the adducts with pyridines of tungsten(V) binuclear thiocomplexes with di-isopropyl-dithiocarbamate

Article abstract of DOI:10.1016/j.poly.2006.05.029

We report the synthesis and the thermal and kinetic behaviour of di-μ-sulfido-bis-(sulfido N,N,di-i-propyldithiocarbamate)di-tungsten(V) adducts with pyridine or substituted pyridines, the formula of which is [W₂S₄(di-i-propyldtc)₂B₂], where dtc = dithiocarbamate and B = pyridine (Py), 3-methylpyridine (3-MP), 4-methylpyridine (4-MP), 3,5-dimethylpyridine (3,5-DMP), 3-aminopyridine (3-AP) and 4-aminopyridine (4-AP). The synthesized complexes have been identified by IR and electronic spectra, magnetic susceptibility measurements and analytical data. We have also inferred the thermal behaviour and kinetic parameters by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of the thermal decomposition of these adducts in the solid state. From the DSC curves, the activation energies and pre-exponential Arrhenius factors for the endothermic process corresponding to the loss of two moles of coordinated base were calculated. We have also deduced the reaction mechanism using a new non-isothermal kinetic method. Steric hindrance and inductive effects prompted by amino and methyl pyridine functional groups in the adducts formation are discussed. A relationship between the pyridines basicity, infrared and electronic spectral data and activation energies has also been explored.

Full text of DOI:10.1016/j.poly.2006.05.029

Polyhedron 25 (2006) 3127–3132
www.elsevier.com/locate/poly
Synthesis, spectral, thermal and kinetic studies on the adducts with
pyridines of tungsten(V) binuclear thiocomplexes with
di-isopropyl-dithiocarbamate
a,
b
b
R. Lozano *, F. de Jesu´s , M.C. Lozano
a
Departamento de Quı´mica Inorga´nica y Bioinorga´nica, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain
b
Facultad de Farmacia, Universidad Alfonso X el Sabio, 28691 Villanueva de la Can˜ada, Madrid, Spain
Received 22 March 2006; accepted 16 May 2006
Available online 7 June 2006
Abstract
We report the synthesis and the thermal and kinetic behaviour of di-l-sulfido-bis-(sulfido N,N,di-i-propyldithiocarbamate)di-tung-
sten(V) adducts with pyridine or substituted pyridines, the formula of which is [W2S4(di-i-propyldtc)2B2], where dtc = dithiocarbamate
and B = pyridine (Py), 3-methylpyridine (3-MP), 4-methylpyridine (4-MP), 3,5-dimethylpyridine (3,5-DMP), 3-aminopyridine (3-AP)
and 4-aminopyridine (4-AP). The synthesized complexes have been identified by IR and electronic spectra, magnetic susceptibility mea-
surements and analytical data. We have also inferred the thermal behaviour and kinetic parameters by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA) of the thermal decomposition of these adducts in the solid state. From the DSC curves, the
activation energies and pre-exponential Arrhenius factors for the endothermic process corresponding to the loss of two moles of coor-
dinated base were calculated. We have also deduced the reaction mechanism using a new non-isothermal kinetic method. Steric hin-
drance and inductive effects prompted by amino and methyl pyridine functional groups in the adducts formation are discussed. A
relationship between the pyridines basicity, infrared and electronic spectral data and activation energies has also been explored.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Tungsten(V) complexes; Calorimetry; Reaction mechanisms; Kinetics; Ligand effects; Adducts
1. Introduction
sten(V) have not been studied as extensively. However we
have reported the study and characterization of W(V)
dimeric complexes with dithiocarbamates and oxines as
ligands [11–18].
Papers about Mo– and W–S complexes and clusters
have increased during the last few years. There are no other
transition metals for which so many pure and discrete het-
erometal-S species are known. The relevance to the bioin-
organic chemistry of molybdenum and tungsten and also
to heterogeneous catalysis has been stressed several times
in the literature [1–10].
Although the chemistry of Mo(V) is dominated by binu-
clear complexes with monoxo, dioxo, dithio and oxo-thio
bridges, curiously, similar binuclear complexes of tung-
In this paper we describe the synthesis by reactions in
different solvents, characterization and thermal decomposi-
tion of [di-l-sulfido-bis-(sulfido N,N,di-i-propyldithiocar-
bamate)-di-tungsten(V)] adducts with pyridine or
substituted pyridines (3-methyl, 4-methyl, 3,5-dimethyl,
3-amino and 4-aminopyridine), by direct reaction between
the complex and the pyridines and conversion of five to six-
coordinate metal centres. The study of these complexes by
IR and electronic spectroscopy, magnetic susceptibilities
and analytical data shows that the stoichiometry of the
complex is 1:1 (ligand:metal) and 2:1 (base:complex) for
these adducts.
*
Corresponding author. Tel.: +34913941717; fax: +34 913941705.
E-mail address: rlozano@farm.ucm.es (R. Lozano).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.05.029

3128
R. Lozano et al. / Polyhedron 25 (2006) 3127–3132
We have determined the kinetic parameters and the
3. Instrumentation
reaction mechanism of the process to be:
Elemental analyses were performed in a Perkin–Elmer
240 B elemental analyzer. Tungsten was determined by
Atomic Absorption with a Perkin–Elmer 430 Atomic
Absorption spectrophotometer, after decomposing the
complexes with a 1:1 mixture of concentrated nitric
and sulfuric acid. Magnetic susceptibility determinations
were performed at room temperature on a Mettler
H-51 AR balance and a type C Newport electromagnet.
Reported magnetic moments are corrected for ligand dia-
magnetism. Infrared spectra were measured on a Perkin–
Elmer 283 spectrophotometer. Visible–near UV spectra
of the compounds were measured on a Beckman DU7
spectrophotometer, using complex solutions in DMSO.
Thermogravimetric analysis and differential scanning cal-
orimetry, were performed between 25 and 600 ꢁC on a
Mettler TA 3000 system fitted with a Mettler TG-50
thermobalance and a Mettler DSC-20 differential scan-
ning calorimeter. The scanning rate used was 2 ꢁC minꢁ1
and samples of about 5 mg, as a very fine powder, were
used, so as to render insignificant the temperature non-
uniformity within the sample. An aluminium pan was
used under a dynamic nitrogen atmosphere. The instru-
ment calibration was checked periodically with standard
samples of indium (99.99 % purity). Several runs were
made.
½W2S4ðdi-i-propyldtcÞ2B2ꢀ ðsolidÞ
! ½W2S4ððdi-i-propyldtcÞÞ2ꢀ ðsolidÞ þ 2B ðgasÞ
and we have determined the activation energy and the pre-
exponential Arrhenius factor for all the adducts synthe-
sized, using a new non-isothermal kinetic method, reported
previously by us.
The determination of kinetic parameters by non-isother-
mal methods offers interesting advantages over conven-
tional isothermal studies [19–26]. Only a single sample and
fewer data are required and the kinetics can be calculated
over an entire temperature range in a continuous manner.
A disadvantage of the non-isothermal methods, when
are compared with the isothermal techniques, is that the
reaction mechanism cannot usually be determined, and
hence the meaning of the kinetics parameters is uncertain.
However, with the new proposed method, the reaction
mechanism can be determined [27–29].
A comparison between activation energies (Ea), steric or
inductive factors, infrared and electronic spectral data and
pKb values of the coordinated bases has also been explored.
2. Experimental
2.1. Synthesis
3.1. Kinetic studies
[W2S4(di-i-propyldtc)2]: Prepared as previously reported
[15].
The rate of a thermal decomposition reaction of a solid
can be expressed by the Arrhenius equation:
[W2S4(di-i-propyldtc)2B2]: 0.01 mol of [W2S4(di-i-propyl-
dtc)2] was dissolved in 0.02 mol of the organic base (pyri-
dine, 3-methyl, 4-methyl and 3,5-dimethylpyridine) or in
an ethanolic solution of 0.02 mol of 3-amino and 4-amino-
pyridine and heated in a water bath for about 15 min. After
cooling for 2 days, the adducts, obtained as brownish pow-
ders, were separated by filtration in vacuo and dried over
P4O10 in a nitrogen atmosphere.
[W2S4(C7H14NS2)2(C5H5N)2]. Anal. Calc.: W, 36.58;
C, 28.62; N, 5.56; H, 3.77; S, 25.44. Found: W, 36.92; C,
29.01; N, 5.66; H, 3.81; S, 25.39%. l = 0.03 B.M.
[W2S4(C7H14NS2)2(C5H4N,3-CH3)2]. Anal. Calc.: W,
35.58; C, 30.17; N, 5.41; H, 4.06; S, 24.75. Found: W,
35.61; C, 30.00; N, 5.92; H, 3.85; S, 24.64%. l = 0.00 B.M.
[W2S4(C7H14NS2)2(C5H4N,4-CH3)2]. Anal. Calc.: W,
35.58; C, 30.17; N, 5.41; H, 4.06; S, 24.75. Found: W,
35.44; C, 29.87; N, 5.32; H, 4.12; S, 24.52%. l = 0.07 B.M.
[W2S4(C7H14NS2)2[C5H3N(CH3)2]2]. Anal. Calc.: W,
34.65; C, 31.63; N, 5.27; H, 4.33; S, 24.10. Found: W,
34.81; C, 31.27; N, 5.18; H, 4.51; S, 24.00%. l = 0.10 B.M.
[W2S4(C7H14NS2)2(C5H4N,3-NH2)2]. Anal. Calc.: W,
35.52; C, 27.79; N, 8.10; H, 3.86; S, 24.71. Found: W,
35.77; C, 27.80; N, 8.74; H, 3.91; S, 24.53%. l = 0.05 B.M.
[W2S4(C7H14NS2)2(C5H4N,4-NH2)2]. Anal. Calc.: W,
35.52; C, 27.79; N, 8.10; H, 3.86; S, 24.71. Found: W,
35.66; C, 27.50; N, 7.87; H, 3.74; S, 24.78%. l = 0.03 B.M.
da=dt ¼ AeꢁE =RT f ðaÞ
ð1Þ
a
In Eq. (1), a is the fraction of material which has reacted at
time t, Ea is the activation energy, f(a) is a function which
depends on the actual reaction mechanism and A is the pre-
exponential Arrhenius factor.
We have collected in Table 1 the mathematical expres-
sions of the functions f(a) corresponding to some of the
thermal decomposition mechanisms found in the literature
[19].
When the temperature of the sample is increased at a
constant rate, we can write Eq. (2), where b is the heating
rate, dT/dt:
da=dT ¼ a0 ¼ ðA=bÞeꢁE =RT f ðaÞ
ð2Þ
a
By differentiating the logarithmic form of Eq. (2) with re-
spect to dln(1 ꢁ a), we obtain the following equation:
dlna0=dlnð1 ꢁ aÞ ¼ ꢁðEa=RÞ½dð1=TÞ=dlnð1 ꢁ aÞꢀ
þ ½dlnf ðaÞ=dlnð1 ꢁ aÞꢀ or
Dlna0=Dlnð1 ꢁ aÞ ¼ ꢁðEa=RÞ½Dð1=TÞ=Dlnð1 ꢁ aÞꢀ
þ ½Dlnf ðaÞ=Dlnð1 ꢁ aÞꢀ and
½Dlna0 ꢁ Dlnf ðaÞꢀ=Dlnð1 ꢁ aÞ ¼ ꢁðEa=RÞ½Dð1=TÞ=Dlnð1 ꢁ aÞꢀ
ð3Þ

R. Lozano et al. / Polyhedron 25 (2006) 3127–3132
3129
Table 1
Kinetic equations
4.2. Infrared spectra
Mechanism (rate-controlling process)
f(a)
The IR spectra show, in all cases, a band in the range 552–
507 cmꢁ1, which we attribute to the vibration of the terminal
W@S bond. If we compare the complex [W2S4(di-i-propyl-
dtc)2] with the adducts [W2S4(di-i-propyldtc)2B2], one
observes that the adducts show the W@S band displaced
to lower frequencies (539–507 cmꢁ1) than the complex
(552–545 cmꢁ1). This displacement can be attributed to the
different basicity of the pyridine or substituted pyridine
employed [33,34]. The pKb value reflects the basicity of the
ligand towards the proton. It may be assumed that the pKb
of the ligand provides a measure of the ligand ability to form
r-bonds to metal ions. In general (see Table 2), the greater
the basicity the lower the frequency of the W@S vibration.
The IR spectra of the adducts exhibit a band over 348–
333 cmꢁ1, which is assigned to the stretching vibration of
the W–N(base) bond [35–37]. The frequencies of this band
listed in decreasing order are:
D1 (one-dimensional diffusion)
D2 (two-dimensional diffusion)
D3 (three-dimensional diffusion:
Jander equation)
D4 (three-dimensional diffusion:
Ginstling–Brounshtein equation)
F1 (random nucleation)
R2 (phase-boundary reaction:
cylindrical symmetry)
R3 (phase-boundary reaction:
spherical symmetry)
(1/2a)
[ꢁln(1 ꢁ a)]ꢁ1
(3/2)(1 ꢁ a)2/3[1 ꢁ (1 ꢁ a)1/3 ꢁ1
(3/2)[(1 ꢁ a)ꢁ1/3 ꢁ 1]ꢁ1
]
(1 ꢁ a)
2(1 ꢁ a)1/2
3(1 ꢁ a)2/3
Thus, the plots of [Dlna0 ꢁ Dlnf(a)]/Dln(1 ꢁ a) versus
[D(1/T)/Dln(1 ꢁ a)] should be a straight line with a slope
ꢁEa/R, irrespective of the f(a) employed. However, we
can select the f(a) that best fits the actual reaction mecha-
nism studied, by means of the intercept value, which in
an ideal agreement with Eq. (3) should be zero.
In order to test the validity of the above conclusions we
have substituted the seven forms of f(a) into the Arrhenius
equation in the logarithmic form (2), giving the following
equation:
½W2S4ðdi-i-propyldtcÞ2ð4-APÞ2ꢀ
> ½W2S4ðdi-i-propyldtcÞ2ð3-APÞ2ꢀ
> ½W2S4ðdi-i-propyldtcÞ2ð3; 5-DMPÞ2ꢀ
> ½W2S4ðdi-i-propyldtcÞ2ð3-MPÞ2ꢀ
> ½W2S4ðdi-i-propyldtcÞ2ð4-MPÞ2ꢀ
> ½W2S4ðdi-i-propyldtcÞ2Py2ꢀ:
These differences can be attributed to the electronic
donation of the pyridine or substituted pyridine to the
tungsten (N ! W) which increases the electron density on
the metal d-orbitals and consequently the pp ! dp dona-
tion from the sulfur atom to tungsten is expected to be
reduced to an extent which depends of the base donor abil-
ity. As a result there will be a lowering of the W@S bond
[hence m(W@S)] and a greater frequency of the W–N bond.
We assign the bands corresponding to W–S–W bridge,
which appear over 460 and 365 cmꢁ1, to the antisymmetric
and symmetric stretching mode, respectively.
ln a0 ¼ lnðA=bÞ ꢁ Ea=RT þ ln f ðaÞ or
ln a0 ꢁ ln f ðaÞ ¼ lnðA=bÞ ꢁ Ea=RT
ð4Þ
The plot of lna0 ꢁ lnf(a) versus 1/T should be a straight
line with a slope ꢁEa/R and intercept ln(A/b).
If the suggested mechanism is correct, the activation
energy value should be the same as the value previously
obtained.
4. Discussion
4.1. Magnetic susceptibility
All the complexes studied present very low values for
the magnetic moments (0.10–0.00 B.M.), as occurs in
similar W(V) complexes [12–18], which can be attributed
to an interaction through the bridge atoms, or to the
possible formation of a direct metal–metal bond, as has
been proved for similar molybdenum(V) oxo-complexes
[30–32].
The IR spectra of these complexes show the C–N and
C–S vibrations over 1515 and 1090 cmꢁ1. The displacement
to greater frequencies for the C–N band and to lower fre-
quencies for the C–S band, in comparison with the sodium
dithiocarbamate, are indicative of bidentate ligands.
Table 2
Infrared absorption maxima, in cmꢁ1, of tungsten(V) thiocomplexes
Compound
mW@S
maW–Sb
msW–Sb
mC–N
mC–S
mW–N
[W2S4(di-i-propyldtc)2]
552–545
539
532
535
521
461
458
462
457
460
462
459
363
361
360
364
365
367
368
1518
1515
1513
1517
1514
1513
1512
1090
1093
1094
1087
1089
1091
1083
[W2S4(di-i-propyldtc)2(Py)2]
[W2S4(di-i-propyldtc)2(3-MP)2]
[W2S4(di-i-propyldtc)2(4-MP)2]
[W2S4(di-i-propyldtc)2(3,5DMP)2]
[W2S4(di-i-propyldtc)2(3-AP)2]
[W2S4(di-i-propyldtc)2(4-AP)2]
333
338
334
343
343
348
513
507

3130
R. Lozano et al. / Polyhedron 25 (2006) 3127–3132
4.3. Visible and ultraviolet spectra
The electronic spectra for all the complexes present a
band in the 18500–16900 cmꢁ1 region, which can be
2
2
The electronic spectra of the compounds obtained is in
accordance with the Ballhausen–Gray and Gray–Hare
schemes [38,39] with a square pyramidal structure for the
complex and a distorted octahedral structure for the
adducts.
assigned to a B2 ! B1 transition. This band shows the
greater or lesser bonding force from the tungsten to the
ligand. Therefore the band is displaced to higher frequen-
cies in the adducts, because the introduction of the base
in the six-coordination position and the ‘‘trans’’ effect
strengthen the ligand–tungsten r-bond, as we have
observed in the IR spectra (see Table 3).
In the visible zone we can detect three other bands over
20000, 22000 and 26500 cmꢁ1 that we attribute to charge
transfer transitions. The band which appears over
22000 cmꢁ1 can be attributed to a S2ꢁ ! W(V) charge
transfer, as has been observed in others transition metal
complexes with a sulfur bridge [40,41].
In the ultraviolet zone, three transitions are detected
over 30000, 32000 and 35000 cmꢁ1, attributable to intral-
igand transitions like n ! n*, p ! p* and n ! r*,
respectively.
Table 3
Electronic absorption spectra of tungsten(V) thiocomplexes
Compound
m
e
Transition
(cmꢁ1) (Mꢁ1.cmꢁ1
)
2
[W2S4(di-i-propyldtc)2]
16993
20422
21775
26467
28990 13048
33658 20404
36088 31412
213
2840
4828
8853
2B2 ! B1
charge transfer
charge transfer
charge transfer
n ! n*
p ! p*
n ! r*
2
[W2S4(di-i-propyldtc)2(Py)2]
[W2S4(di-i-propyldtc)2(3-MP)2]
[W2S4(di-i-propyldtc)2(4-MP)2]
17547
20658
22315
26587
30457 21125
31989 14995
34785 34114
117
2654
3672
8143
2B2 ! B1
4.4. DSC, TGA and kinetic studies
charge transfer
charge transfer
charge transfer
n ! n*
On the DSC curves of all the compounds, a first endo-
thermic peak can be appreciated. The mass loss accompa-
nying this transition corresponds on the TG curves to the
loss of two moles of coordinated base to tungsten atoms,
the residue being [W2S4(di-i-propyldtc)2] in all cases.
p ! p*
n ! r*
2
17555
20508
21825
26602
30132 14419
32256 22623
35598 47989
198
3204
4654
7601
2B2 ! B1
charge transfer
charge transfer
charge transfer
n ! n*
Table 4
DH, a, T and a0 obtained from the DSC curve for the complex [W2S4(di-i-
p ! p*
propyldtc)2Py2]
n ! r*
DH (mJ)
a
T (ꢁC)
a0 (Kꢁ1
)
2
17325
20232
21878
26119
30358 16985
31983 19657
36021 39568
131
3125
4125
6319
2B2 ! B1
144.91
205.28
257.84
319.89
399.19
494.20
604.32
737.04
887.00
1062.37
1252.43
1453.74
1628.18
0.08045
0.11396
0.14314
0.17759
0.22161
0.27436
0.33549
0.40917
0.49242
0.58978
0.69529
0.80705
0.90389
121.0
123.0
125.0
127.0
129.0
131.0
133.0
135.0
137.0
139.0
141.0
143.0
145.0
0.01111
0.01398
0.01758
0.02260
0.02641
0.03194
0.03733
0.04344
0.04918
0.05349
0.05206
0.04056
0.02174
charge transfer
charge transfer
charge transfer
n ! n*
p ! p*
n ! r*
2
[W2S4(di-i-propyldtc)2(3,5DMP)2] 17702
216
2154
3125
7893
2B2 ! B1
21989
21358
26596
39930 16489
32301 22531
34995 35106
charge transfer
charge transfer
charge transfer
n ! n*
p ! p*
n ! r*
2
[W2S4(di-i-propyldtc)2(3-AP)2]
17959
20186
21987
26668
30129 14206
31657 19293
35369 31136
229
2435
4023
7003
2B2 ! B1
charge transfer
charge transfer
charge transfer
n ! n*
Table 5
Results obtained by using the seven mechanisms for the plot of
[Dlna0 ꢁ Dlnf(a)]/Dln(1 ꢁ a)] vs. [D(1/T)/Dln(1 ꢁ a)] for the complex
[W2S4(di-i-propyldtc)2Py2]
p ! p*
Ea (kJ molꢁ1
)
n ! r*
Mechanism
r
m
i
2
D1
D2
D3
D4
F1
ꢁ0.99761
ꢁ0.99762
ꢁ0.99762
ꢁ0.99762
ꢁ0.99984
ꢁ0.99999
ꢁ0.99987
ꢁ36837.84
ꢁ37014.92
ꢁ36924.59
ꢁ36924.63
ꢁ17323.45
ꢁ17323.45
ꢁ17323.45
0.64519
0.34992
294
295
295
295
137
137
137
3
4
4
4
3
2
3
[W2S4(di-i-propyldtc)2(4-AP)2]
18458
19961
22153
26354
30096 13483
32627 21969
35250 33364
376
2023
3951
6302
2B2 ! B1
charge transfer
charge transfer
charge transfer
n ! n*
ꢁ0.16012
0.18125
ꢁ0.44752
0.00592
ꢁ0.16253
p ! p*
R2
R3
n ! r*

R. Lozano et al. / Polyhedron 25 (2006) 3127–3132
3131
Table 6
Thermal and kinetic parameters for the complexes W2S4(di-i-propyldtc)2B2
Compound
Mechanism
DH (kJ molꢁ1
)
Ea (kJ molꢁ1
)
A (sꢁ1
)
pKb
[W2S4(di-i-propyldtc)2(Py)2]
R2
R2
R2
R2
R2
R2
21.7 0.3
26.5 0.4
25.0 0.3
34.6 0.7
40.1 0.3
31.3 0.9
137
162
154
187
203
257
3
2
1
2
1
3
(3.3 0.2) · 1013
(3.4 0.1) · 1020
(6.8 0.3) · 109
(1.2 0.1) · 1027
(7.5 0.3) · 1032
(2.5 0.2) · 1027
8.87
8.32
7.98
7.85
7.49
4.88
[W2S4(di-i-propyldtc)2(3-MP)2]
[W2S4(di-i-propyldtc)2(4-MP)2]
[W2S4(di-i-propyldtc)2(3,5DMP)2]
[W2S4(di-i-propyldtc)2(3-AP)2]
[W2S4(di-i-propyldtc)2(4-AP)2]
of the base. The outstanding exception is the 4-MP and
4-AP adducts, which can be attributed to the existence of
resonance structures and a pronounced effect on the stabil-
ity of the complex [29,33,34]. It can be seen that the steric
impediment of 3,5-DMP, does not modify the kinetic
parameters.
The plot of pKb versus activation energy presented in
Fig. 1, shows that, except for the 4-AP adduct, a very sat-
isfactory correlation was obtained. It is obvious from this
figure that the activation energy is linearly related to the
basicity of the ligands.
Fig. 1. Activation energy for decomposition of [W2S4(di-i-propyldtc)2B2]
complexes vs. pKb. Points 1–6 correspond to complexes with Py, 3-MP, 4-
MP, 3,5-DMP, 3-AP and 4-AP, respectively.
References
[1] E.I. Stiefel, Prog. Inorg. Chem. 1 (1977) 22.
Table 4 shows the data obtained from the DSC curve of
the adduct [W2S4(di-i-propyldtc)2Py2], a being the reacted
fraction at time t and a0 = da/dT.
[2] B.K. Burgess, D.B. Jacobs, E.I. Stiefel, Biochim. Biophys. Acta
Enzymol. 614 (1980) 196.
[3] R. Lozano, A.L. Doadrio, A. Doadrio, Polyhedron 1 (1982) 163.
´
´
[4] R. Lozano, J. Roman, E. Alarcon, A.L. Doadrio, A. Doadrio, Rev.
We have plotted [Dlna0 ꢁ Dlnf(a)]/Dln(1 ꢁ a)] versus
[D(1/T)/Dln(1 ꢁ a)]. From these representations, we can
calculate the activation energy of the process. Table 5
shows the results for r (correlation coefficient), m (slope),
i (intercept value) and Ea (activation energy) obtained for
the seven mechanisms.
Chim. Miner. 20 (1983) 173.
´
[5] R. Lozano, J. Roman, D. Armada, A. Doadrio, Polyhedron 4 (1985)
1563.
´
´
´
[6] R. Lozano, J. Roman, F. De Jesus, E. Alarcon, Transit. Met. Chem.
15 (1990) 141.
[7] C. Sharp, A.G. Sykes, Inorg. Chem. 27 (1988) 501.
[8] M.R. Maurya, N. Bharti, Transit. Met. Chem. 24 (1999) 389.
[9] K. Umakoshi, E. Nishimoto, M. Sokolov, H. Kawano, Y. Sasaki, M.
Onishi, J. Organomet. Chem. 611 (2000) 370.
[10] B. Keshavan, K. Gowda, Proc. Indian Acad. Sci. 3 (2001) 165.
[11] D. Miguel, V. Riera, M. Wang, Inorg. Chim. Acta 307 (2000) 82.
We can observe that in all cases the correlation coeffi-
cients (r) are very near to unity, consistent with the results
of Criado [42]. Nevertheless, only when the analysis was
performed with the mechanism R2 does the straight line
present an intercept value (i) close to zero.
´
´
[12] R. Lozano, A.L. Doadrio, E. Alarcon, J. Roman, A. Doadrio, Rev.
´
Chim. Miner. 20 (1983) 109.
Therefore, according with our proposed method, we
deduce that the thermal decomposition of the complex
[W2S4(di-i-propyldtc)2Py2] and the loss of pyridine, is carry
out by a R2 mechanism (Phase boundary reaction: cylin-
drical symmetry) with a function f(a) = 2(1 ꢁ a)1/2, an acti-
vation energy Ea = 137 3 kJ molꢁ1 and a pre-exponential
[13] R. Lozano, E. Alarco´n, A.L. Doadrio, A. Doadrio, Polyhedron 2
(1983) 435.
´
[14] R. Lozano, E. Alarcon, A.L. Doadrio, A. Doadrio, Polyhedron 3
(1984) 25.
´
´
´
[15] R. Lozano, E. Alarcon, J. Roman, A. Doadrio, Rev. Chim. Miner. 21
(1984) 177.
´
´
[16] R. Lozano, E. Alarcon, J. Roman, A. Doadrio, Polyhedron 3 (1984)
Arrhenius factor A = (3.3 0.2) · 1013 sꢁ1
.
1021.
´
[17] R. Lozano, J. Roman, M.D. Armada, E. Parrondo, Eur. J. Solid
The results obtained for the compounds studied, are
listed in Table 6.
State Inorg. Chem. 25 (1988) 191.
´
´
[18] R. Lozano, J. Roman, F. de Jesus, Polyhedron 7 (1989) 947.
[19] W.W. Wendlant, Thermal Methods of Analysis, Wiley, New York,
1974.
We can assume that the thermal decomposition and loss
of coordinated base of the adducts with pyridine, 3-methyl-
pyridine, 4-methylpyridine, 3,5-dimethylpyridine, 3-amino-
pyridine and 4-aminopyridine follows a R2 mechanism.
If we compare the results, in general, as can be seen from
the data in Table 6, the greater the basicity of the base
employed, the higher the activation energy and the pre-
exponential Arrhenius factor for the process of the loss
[20] A.E. Nerwkirk, Anal. Chem. 32 (1960) 1558.
[21] E.S. Freeman, B. Carroll, J. Phys. Chem. 62 (1958) 394.
[22] H.H. Horowitz, G. Metzger, Anal. Chem. 35 (1963) 1464.
[23] A.W. Coats, J.P. Redfern, Nature 201 (1964) 68.
[24] C.D. Doyle, J. Appl. Polym. Sci. 5 (1961) 285.
[25] T.R. Ingraham, P. Marrier, Can. J. Chem. Eng. (1984) 161.
[26] J.M. Thomas, T.A. Clarke, J. Chem. Soc. (A) (1968) 457.

3132
R. Lozano et al. / Polyhedron 25 (2006) 3127–3132
´
´
´
´
´
´
[27] R. Lozano, J. Roman, F. de Jesus, A. Jerez, M. Gaitan, E. Ramos,
[34] R. Lozano, J. Roman, J. Martınez, A. Martınez, A. Doadrio,
Thermochim. Acta 141 (1989) 261.
Polyhedron 5 (1986) 1341.
´
´
´
[28] R. Lozano, J. Roman, F. de Jesus, A. Jerez, M. Gaitan, E. Ramos,
[35] V.C. Gibson, T.P. Kee, A. Shaw, Polyhedron 7 (1988) 579.
[36] M.A. Ansari, J. Chandrasekaran, S. Sarkar, Polyhedron 7 (1988)
471.
Thermochim. Acta 143 (1989) 93.
[29] R. Lozano, J. Martınez, A. Martınez, J. Roman, F. de Jesus,
´
´
´
´
Polyhedron 8 (1989) 2611.
[30] F.A. Cotton, S.M. Morehouse, J.S. Wood, Inorg. Chem. 3 (1964)
1063.
[31] L. Ricard, C. Martin, R. Wiest, R. Weiss, Inorg. Chem. 14 (1975)
2300.
[32] L. Ricard, J. Stienne, J. Karagiannidis, P. Toledano, J. Fisher, A.
Mitschler, J. Coord. Chem. 3 (1974) 277.
[37] M.H. Chisholm, J.C. Huffman, J.W. Pasterczyk, Polyhedron 6 (1987)
551.
[38] C.J. Ballhausen, H.B. Gray, Molecular Orbital Theory, Benjamin,
New York, 1965.
[39] H.B. Gray, C.R. Hare, Inorg. Chem. 1 (1962) 363.
[40] I.G. Dance, A.E. Landers, Inorg. Chem. 18 (1979) 3487.
[41] A. Kay, P.C.H. Mitchell, J. Chem. Soc. (A) (1970) 2421.
[42] J.M. Criado, D. Dollimore, G.R. Heal, Thermochim. Acta 54 (1982)
159.
´
´
[33] R. Lozano, J. Martınez, A. Martınez, A. Doadrio, Polyhedron 2
(1983) 997.