Photochemical isomerization of trans- to cis-[RuCl2(dppb) (4,4′-X2-2,2′-bipy)] (X = -H, -NO2, -Me, -COOH, -SMe, -O=SMe, -Cl, -OMe) complexes

Article abstract of DOI:10.1016/j.poly.2003.07.007

A series of [RuCl₂(dppb)(4,4′-X₂-2,2′ -bipy)] complexes was synthesized from [RuCl₂(dppb)(PPh₃) and 4,4′-X₂-2,2′-bipy [dppb=1,4-bis(diphenylphosphino)butane and 4,4′-X₂-2,2′ -bipy=2,2′-bipyridine (bipy), 4,4′-dimethoxy-2,2′-bipyridine (MeO-bipy), 4,4′-dimethyl-2,2′-bipyridine (Me-bipy), 4,4′-dicarboxy-2,2′-bipyridine (COOH-bipy), 4,4′ -dithiomethyl-2,2′-bipyridine (MeS-bipy), 4,4′-dichloro-2,2′ -bipyridine (Cl-bipy), 4,4′-dinitro-2,2′-bipyridine (NO ₂-bipy), and 4,4′-dithiomethylsulphoxide-2, 2′-bipyridine (MeS=O-bipy)]. The complexes were characterized by elemental analysis, cyclic voltammetry and differential pulse voltammetry, and by UV-Vis and ³¹P{¹H}NMR spectroscopies. The pK _a of the ligands were determined. The trans-[RuCl ₂(dppb)(4,4′-X₂-2,2′-bipy) complexes were photoisomerized under white light and the isomerization rate constants from trans to cis isomers were consistent with first order reactions and were correlated with the pK_a of the ligands and with the E_1/2 of the complexes. The suggested mechanism of isomerization reactions is consistent with an intramolecular twist process.

Full text of DOI:10.1016/j.poly.2003.07.007

Polyhedron 22 (2003) 3205–3211
www.elsevier.com/locate/poly
Photochemical isomerization of trans- to cis-[RuCl2(dppb)
(4,40-X2-2,20-bipy)] (X ¼ –H, –NO2, –Me, –COOH, –SMe,
–O@SMe, –Cl, –OMe) complexes
a
Marcelo O. Santiago , Claudio L. Donicci Filho , Icaro de S. Moreira ,
b
c
a
Rose Maria Carlos , Salete L. Queiroz , Alzir A. Batista
d
a,
*
a
Departamento de Quꢀımica, Universidade Federal de Sa~o Carlos, CP 676, 13565-905 Sa~o Carlos, SP, Brazil
Departamento de Quꢀımica, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil
b
c
Departamento de Quꢀımica Orga^nica e Inorgaꢀnica, Universidade Federal do Cearaꢀ, Fortaleza, CE, Brazil
d
Instituto de Quꢀımica de Sa~o Carlos, Universidade de Sa~o Paulo, Sa~o Carlos, SP, Brazil
Received 17 March 2003; accepted 16 July 2003
Abstract
A series of [RuCl2(dppb)(4,40-X2-2,20-bipy)] complexes was synthesized from [RuCl2(dppb)(PPh3)] and 4,40-X2-2,20-bipy
[dppb ¼ 1,4-bis(diphenylphosphino)butane and 4,40-X2-2,20-bipy ¼ 2,20-bipyridine (bipy), 4,40-dimethoxy-2,20-bipyridine (MeO-
bipy), 4,40-dimethyl-2,20-bipyridine (Me-bipy), 4,40-dicarboxy-2,20-bipyridine (COOH-bipy), 4,40-dithiomethyl-2,20-bipyridine
(MeS-bipy), 4,40-dichloro-2,20-bipyridine (Cl-bipy), 4,40-dinitro-2,20-bipyridine (NO2-bipy), and 4,40-dithiomethylsulphoxide-2,
20-bipyridine (MeS@O-bipy)]. The complexes were characterized by elemental analysis, cyclic voltammetry and differential pulse
voltammetry, and by UV–Vis and 31P{1H}NMR spectroscopies. The pKa of the ligands were determined. The trans-
[RuCl2(dppb)(4,40-X2-2,20-bipy)] complexes were photoisomerized under white light and the isomerization rate constants from trans
to cis isomers were consistent with first order reactions and were correlated with the pKa of the ligands and with the E1=2 of the
complexes. The suggested mechanism of isomerization reactions is consistent with an intramolecular twist process.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium(II)-bipy derivative complexes; Photoisomerization; 1,4-bis(diphenylphosphino)butane
1. Introduction
charge transfer (MLCT) bands in the visible region,
which make them highly colored and, with the presence
of phosphines, they often become good catalysts for
homogeneous hydrogenation of unsaturated organics
[2–6]. This property depends on the dissociation of a
coordinated ligand in order to generate an electroni-
cally unsaturated species that undergoes reactions with
the substrate.
The mechanism of stereochemical rearrangements of
metal chelate compounds has been an important prob-
lem in coordination chemistry since the days of Alfred
Werner [7]. For intramolecular rearrangements of oc-
tahedral chelates, two classes of mechanisms have been
considered: (1) metal–ligand bond rupture to give a five-
coordinate species and (2) twisting mechanisms that
produce rearrangement without rupture of any metal–
ligands bonds [8].
The 2,20-bipyridine species has been used in a variety
of studies dealing with structural coordination chemis-
try. Since its discovery at the end of the 19th century,
bipyridine has been extensively used in the complexa-
tion of metal ions as a metal chelating ligand, due to its
robust redox stability and ease of functionalization [1].
Ruthenium(II) is one of the most currently used ions
for complexation with bipyridine ligands and com-
plexes of this type have been known for many years.
They are characterized by strong metal-to-ligand
* Corresponding author. Tel.: +55-16-2608285; fax:
2608350.
E-mail address: daab@power.ufscar.br (A.A. Batista).
+ 55-16-
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2003.07.007

3206
M.O. Santiago et al. / Polyhedron 22 (2003) 3205–3211
In the present work we have studied the photo-
bipy)] complexes were performed at room temperature
and in the dark to avoid the isomerization processes.
The microanalytical data for all trans-[RuCl2
(dppb)(4,40-X2-2,20-bipy)] complexes synthesized are
consistent with the formulation shown below [experi-
mental(calculated)] (Table 1).
All complexes, including the products of the pho-
tochemical isomerizations, were characterized by
cyclic voltammetric (CV) or differential pulse vol-
tammetric (DPV) measurements and 31P{1H}NMR
spectroscopy.
chemical isomerization of trans-[RuCl2(dppb)(4,40-X2-
2,20-bipy)] to cis-isomers. Quantitative data show that
the isomerization rate constants increase with the in-
creasing pKa of the 4,40-X2-2,20-bipy ligand. This study
also leads us to propose that the isomerization process
in the series progresses through a twisting mechanism.
2. Experimental
2.1. Materials
ReineckeÕs salt (NH4)[Cr(NH3)2(SCN)4] (Eastman)
used in the photochemical experiments was converted to
the potassium salt by recrystallization from KNO3 so-
lution, prior to being used for actinometry at 519 nm
[13]. Potassium(tris-oxalate)ferrate(III), used in acti-
nometry at 304 and 370 nm, was prepared according to
Calvert and Pitts [14].
The chemicals employed were of reagent grade
quality (Aldrich). Ligands with general formula 4,40-
X2-2,20-bipy (X ¼ –H, –NO2, –Me, –COOH, –SMe,
–O@SMe, –Cl, –OMe) and the [RuCl2(dppb)(PPh3)]
and the [RuCl2(PPh3)3] complexes, were synthesized as
described in the literature [9–11]. Tetrabutylammonium
perchlorate (TBAP) (Fluka purum) was recrystallized
from ethanol/water and dried overnight, under vacuum,
at 100 °C. Reagent grade solvents (Merck) were ap-
2.3. Apparatus and methods
Monochromatic irradiation at 300, 370 and 500 nm is
generated using a 200-W Xenon lamp in an Oriel mod.
68805 Universal Arc Lamp source selected with an ap-
propriate interference filter (Oriel). The progress of the
photoreaction was monitored by UV–Vis spectra on a
HP 8452A spectrophotometer, or electrochemically by
differential pulse voltammetry (DPV) on a EG&PARC
electrochemical system consisting of a 273A potentio-
stat. This instrument was also used for characterization
of the complexes. The working and auxiliary electrodes
were a stationary platinum foil and wire, respectively.
The reference electrode was Ag j AgCl in a Luggin cap-
illary. The measurements were carried out at room
temperature in freshly distilled CH2Cl2 containing 0.1
ꢁ
propriately distilled, dried and stored over Linde 4 A
molecular sieves. Purified argon was used for the re-
moval of dissolved oxygen.
2.2. Synthesis
A typical procedure for preparation of the trans-
[RuCl2(dppb)(4,40-X2-2,20-bipy)] complexes was: (0.100
g; 0.110 mmols) of [RuCl2(dppb)(PPh3)] was dissolved
in benzene (2 ml) and 0.120 mmols of the ligand was
added. The solution was stirred under Ar for 30 min and
hexane was subsequently added to precipitate the re-
sulting powder, which was filtered, well washed with
ether and dried under vacuum. The yield of these com-
plexes was about 90%.
In a previous study was detected that heating the
trans-isomers to reflux in CH2Cl2 or C6H6 results in
isomerization to the cis-forms [12]. Thus, in this work
the synthesis of the trans-[RuCl2(dppb)(4,40-X2-2,20-
ꢀ
mol lꢀ1 of Bu4NþClO4 in the three-electrode system
with resistance compensation, a medium in which fer-
rocene is oxidized at 0.43 V versus Fcþ/Fc; all potentials
are referred to this electrode.
31P{1H} NMR spectra (31P 161 MHz) were obtained
in a CH2Cl2 or CDCl3 solution at room temperature
Table 1
Micro analytical data for the trans-[RuCl2(dppb)(4,40-X2-2,20-bipy)] complexes
RuCl2(dppb)2,(X-bipy), X ¼
MeO–
Me–
MeS–a
H–
%H(calc)
%C(calc)
%N(calc)
4.92(4.95)
5.15(5.15)
4.90(4.76)
4.90(4.85)
4.22(4.31)
4.93(4.81)
4.66(4.59)
4.04(4.06)
4.95(4.66)
58.35(58.97)
61.11(61.38)
56.67(56.73)
60.48(60.50)
56.68(57.01)
56.99(57.35)
53.99(54.67)
54.04(54.67)
61.56(61.70)
3.46(3.44)
3.66(3.58)
3.47(3.31)
3.81(3.75)
3.03(3.32)
3.20(3.18)
3.20(3.19)
6.41(6.36)
3.75(3.60)
–COOH
Cl–
MeS@O–b
NO2–
RuCl2(dppb)(phen)
a S ¼ 7.60(7.78)%.
b S ¼ 7.81(7.57)%.

M.O. Santiago et al. / Polyhedron 22 (2003) 3205–3211
3207
with a Bruker 400 MHz spectrometer and chemical shifts
4. Results and discussion
are reported relative to H3PO4 85%. Microanalytical
data (C, H and N) were obtained with the use of a Fisons
model EA 1108 elemental analyser.
The 31P{1H} chemical shifts of the [RuCl2(dppb)
(4,40-X2-2,20-bipy)] complexes, synthesized in this work,
close to 30.0–33.0 ppm, are consistent with a geometry
where the nitrogen are trans to the phosphorus atoms
[12,15].
3. Procedures
In CH2Cl2 solutions, in the UV–Vis region, the
complexes show intense bands below 330 nm which are
also present in the free ligands and are assigned to
p ! pꢁ transitions of the aromatic rings. The two
moderately intense bands observed in the complexes
between 350 and 550 nm can be assigned to charge
transfer transitions (CT) (Ru(II) dp ! pꢁðbipyÞÞ [16]. The
energy of these transitions depend on the substituent
group on the bipyridine rings, analogous to those pre-
viously reported for other ruthenium(II) complexes
[16–19]. Their energies depend on the electron-acceptor
properties of the bipyridine ligands, and follow a good
linear correlation with the pKas of the ligands (Fig. 1).
The transitions of the metal ion (d–d transitions) can
be obscured by charge transfer transitions at lower en-
ergies in these complexes [20]. The UV–Vis spectra of
our complexes with bipyridine derivative ligands (see
Table 3) are very similar to those described in the lit-
erature where two charge transfer transitions were
found [21]. As a general tendency, bands for complexes
with 1,4-bis(diphenylphosphino)butane are shifted to
the infrared region when compared to those for triphe-
nylphosphine complexes with the same bipyridine de-
rivative ligands [15].
Photolysis experiments were carried out at 298 K in a
10 cm path length quartz cells but some of them were
initiated directly in a 3 ml electrochemical cell. The so-
lutions (10ꢀ3 M of the initial complex concentrations)
were deaerated by bubbling argon prior to photolysis
and were stirred during irradiation. Solutions protected
from light were completely stable on a timescale longer
than that of the photochemical experiments. Experi-
ments with different monochromatic lights in different
solvents, had, within experimental uncertainties, shown
no wavelength dependence and an identical quantum
yield was found, as can be seen in Table 2. This behavior
indicates that the photochemical reactions are processed
through ligand field paths even when the irradiation is in
a charge transfer transition region (TC). This procedure
was performed for the ruthenium complexes with the
2,20-bipyridine and the 4,40-dimethoxy-2,20-bipyridine
ligands and assumed to be the same for the other li-
gands. Since the quantum yields of the isomerization
processes are the same, independent of the irradiation
wavelength (Table 2), all the photochemical experiments
were performed using white light irradiation. The re-
ported yields are the average of, at least, two indepen-
dent experiments. Isomerization rate constants for the
photoreaction were evaluated from cyclic voltammetry
changes (electrochemical RuIII ! RuII processes) by
using the initial and final differential pulse voltammo-
grams as reactant and as product voltammogram, re-
spectively. As the half-wave potential values for the
trans- and cis-RuIII/RuII redox processes are easily dis-
tinguishable the isomerization from trans- to cis-
[RuCl2(dppb)(4,40-X2-2,20-bipy)], complexes were mon-
itored by observing the growth of a new electrochemical
wave present in the cis isomers. Successive cyclic vol-
tammograms were collected at time intervals from 20 to
180 min.
As can be seen in Fig. 2, in the visible spectrum of the
trans-[RuCl2(dppb)(2,20-bipy)] obtained during the
isomerization process, there are three isosbestic points
indicating that the final product is only the cis form of
this complex, which absorbs at 360 nm (shoulder) and
452 nm. Thus, the only photoproducts upon irradiation
Table 2
Photoreaction quantum yield for trans-[RuCl2(dppb)(bipy)] isomeri-
zation for different irradiations and different solvents at 25 °C
kirr (nm)
Solvent
/ (mol/einstein)a
300
370
500
370
370
CH2Cl2
CH2Cl2
0.53 ꢂ 0.03
0.54 ꢂ 0.03
0.52 ꢂ 0.03
0.55 ꢂ 0.03
0.56 ꢂ 0.03
CH2Cl2
CH3CN
CH3SOCH3
Fig. 1. Correlations between pKas of the (4,40-X2-2,20-bipy)] ligands
and energies of the CT transitions of the [RuCl2(dppb)(4,40-X2-2,20-
bipy)] complexes (j, CT1*; d, CT2).
a Determined by UV–Vis experiments.

3208
M.O. Santiago et al. / Polyhedron 22 (2003) 3205–3211
Table 3
Charge transfer transitions (TC) for the trans-[RuCl2(dppb)(4,40-X2-
2,20-bipy)] complexes
[RuCl2(dppb)(X-bipy)] X ¼
MeO–
CT1a
CT2a
348 (3.56)
350 (3.49)
372 (3.67)
360 (3.52)
390 (3.84)
386 (3.52)
386 (3.51)
438 (3.16)
472 (3.25)
478 (3.32)
500 (3.46)
500 (3.37)
496 (3.53)
514 (3.39)
510 (3.38)
538 (3.64)
Me–
MeS–
H–b
COOH–
Cl–
MeSO–
NO2–
a Electronic spectral data, in CH2Cl2 solution. kmax, nm (log emax
,
M
ꢀ1 cmꢀ1).
b Ref. [12].
Fig. 3. Changes (isomerization process) in the differential pulse vol-
tammogram resulting from white light photolysis of
[RuCl2(dppb)(2,20-bipy)] ꢃ10ꢀ3 M in CH2Cl2 (0.1 M PTBA, Pt aux-
of the trans isomers for all bipyridine derivative ligands
are the respective cis isomers. The photochemical reac-
tion products were also quantitatively detected by
31P{1H} measurements showing two doublets at about
32.0 ppm (phosphorus trans nitrogen) and at about
43.0 ppm (phosphorus trans chlorine) what represents a
iliary and working electrodes and Ag j AgCl as reference).
The differential pulse voltammogram of an exhaus-
tively photolyzed solution of [RuCl2(dppb)(2,20-bipy)]
withwhite light ꢃ10ꢀ3 M, 3 ml, at 298 K is shown in Fig. 3.
In cyclic or differential pulse voltammetry in CH2Cl2,
all complexes (trans and cis isomers) reveal a one-elec-
tron (RuIII ! RuII) process, corresponding to a revers-
ible wave (iopx/irped values are close to unity at scan rate
v ¼ 10–200 mV sꢀ1).
2
standard AX patterns with JAX approximately 32.0 Hz
[22,23].
Irradiation of trans-isomers in CH2Cl2 solutions with
white light leads to the decrease of their electrochemical
currents (Fig. 3) and the appearance of new electro-
chemical processes at higher potentials, showing the
isomerization processes from trans to cis forms. The
isosbestic points at ca. 400 and 480 nm were retained
during the conversion, indicating that the photoreaction
is not complicated by side or subsequent reactions. The
spectral changes observed during the photolysis were
essentially independent of the excitation energy, of the
complex or of the light intensity. The photoproducts of
the reactions (cis-isomers) were also identified by CV or
DPV measurements (Table 4).
The E1=2 values for the complexes depend on the type
of isomer present in solution and on the donor proper-
ties of the ligands. The cis-isomers always show
oxidation potentials higher than the corresponding
trans-isomers; this increase in potential on going from
the trans- to the cis-isomer is well established in Ru(II)
chemistry [21,24,25]. The oxidation potential of the
complexes reflects the donor property of the substituent
group in the X-bipy ligands: electron-withdrawing
groups in the 4,40-positions make the oxidation of the
complex more difficult, as expected, while electron-do-
nor groups in these positions make the oxidation easier.
This electrochemical behavior was the subject of a very
elegant work by LeverÕs group for an extended series of
transition metal complexes [26,27]. The electrochemical
redox potentials (E1=2) of the trans-[RuCl2(dppb)(4,40-
X2-2,20-bipy)] series can be correlated with the pKas of
the ligands (Fig. 4). This correlation has been also de-
tected for the cis-[RuCl2(PPh3)2(X2-2,20-bipy)] series
where the PPh3 are in trans position [15].
Fig. 5 shows the correlation between energies of
charge transitions (CT1* and CT2*) of the complexes
and their respective electrochemical redox potentials
(E1=2) where can be observed that complexes with lower
energies of transition present lower redox potentials.
The isomerization constant rates for the photoreac-
tion in the [RuCl2(dppb)(X-bipy)] series are given in
Table 4 and as can be seen in Fig. 6, there is a good
Fig. 2. Photoisomerization process under white light of
[RuCl2(dppb)(2,20-bipy)] in CH2Cl2 solution (2.1 ꢄ 10ꢀ4 mol lꢀ1), at
298 K.

M.O. Santiago et al. / Polyhedron 22 (2003) 3205–3211
3209
Table 4
pKa and cyclic voltammetric (v ¼ 100 mV sꢀ1) and isomerization rate constant data for the [RuCl2(dppb)(4,40-X2-2,20-bipy)] complexes
[RuCl2(dppb)(X-bipy)] X ¼
LigandÕs pKa
E1=2 (V) Trans
E1=2 (V) Cis
Isomerisation rate
constant 105ꢄ (sꢀ1
)
MeO–
Me–
MeS–
5.74
4.92
4.74
4.44
0.37
0.43
0.45
0.46
0.57
0.60
0.62
0.65
86.22
85.00
74.50
63.10
35.3
H– (298 K)
H– (293 K)
H– (288 K)
H– (283 K)
COOH–
Cl–
22.8
13.2
4.26a
2.60
2.30
1.81
0.50
0.60
0.63
0.70
0.46
0.60
0.76
0.77
0.86
0.66
52.10
35.70
13.50
1.98
MeS@O–
NO2–
[RuCl2(dppb)(fen)]
84.00
a Ref. [21].
Fig. 4. Correlation of measured values of E1=2 for the [RuCl2
(dppb)(4,40-X2-2,20-bipy)] complexes with pKa of free 4,40-X2-2,20-bipy
ligands.
Fig. 5. Correlation between energies of the charge transfer transitions
(CT) and potential redox (E1=2 V vs. Ag/AgCl) of the [RuCl2(dppb)
(4,40-X2-2,20-bipy)] complexes.
correlation between these data and the pKa of the li-
gands. The results collected in Table 4 clearly show that
the isomerization rate constants depend on the donor
properties of the ligands which is a consequence of the
electron densities of the metal centers. This in the end is
reflected in the Ru–Cl bond character, which is mainly
responsible for the isomerization process of the com-
plexes. Thus, as the metal center becomes softer due to
its coordination to ligands with high pKa, for instance,
lower is the attraction of the metal cation for the
chlorine ion. Therefore, the isomerization process is
faster.
Fig. 7 shows the correlation between isomerization
rate constants and electrochemical redox potentials
(E1=2) for the series of complexes. To explain the in-
crease of the isomerization rate constant of the com-
plexes with the increase of the pKa of the coordinate
ligand, it is possible to suggest a model where the at-
traction between the metal center and the chlorine is
affected by the donor property of the nitrogen donor
atoms of the bipyridine derivative ligands. Thus, as the
Fig. 6. Correlation between isomerization rate constants of
[RuCl2(dppb)(4,40-X2-2,20-bipy)] complexes and pKa of 4,40-X2-2,20-
bipy ligands.
metal center, in the case coordinated to ligands with
high pKa, becomes more electron rich, the faster is the
isomerization process.

3210
M.O. Santiago et al. / Polyhedron 22 (2003) 3205–3211
isomers the electron densities are more centered in the
metal centers making easier their oxidation processes.
It is interesting to mention that the kinetics of the
isomerization process of the compounds can be conve-
niently followed also by 31P{1H} NMR spectroscopy
(changing from a singlet to two doublets) and that t1=2
for the conversion of trans to cis of the [RuCl2(dppb)(H-
bipy)] complex at 298 K is ꢃ18 min, under white light.
5. Conclusions
The photoreaction quantum yields for the trans-
[RuCl2(dppb)(4,40-X2-2,20-bipy)] complexes are inde-
pendent of both, the wavelength of the irradiation and
the solvent used.
Fig. 7. Correlation between isomerization rate constants of the
[RuCl2(dppb)(4,40-X2-2,20-bipy)] and E1=2(V vs. Ag/AgCl) of the
complexes.
The present study demonstrates that the trans-
[RuCl2(dppb)(4,40-X2-2,20-bipy)] complexes are photo-
isomerized under white light; that the isomerization
constant rates are dependent on the pKa of the ligands
and that they follow a twist type mechanism. The elec-
trochemical oxidation potentials of the complexes are
also dependent on the pKa of the ligands, decreasing
with the increasing of the donor properties of the X-bipy
molecules.
The isomerization rate constants of the complexes
increase with the donor properties of the ligands show-
ing that the electron enrichment of the metal center of
electrons facilitates the isomerization process. This
suggests that in this case, the Ru–Cl bonds become
weaker or more relaxed allowing the isomerization
process to occur faster. This suggestion finds support in
the fact that in presence of coordinating solvent for long
periods of time the chlorine ligand dissociates.
The large negative activation entropy of the isomer-
ization is consistent with an intramolecular twist process
for the isomerization reaction.
As has been shown, the trans complexes can be easily
converted into the cis isomers in solution, by photolysis
at room temperature, under white light. The formation
of the cis-[RuCl2(dppb)(2,20-bipy)] complex was nearly
quantitative as determined from the relative spectral
changes at 450 and 520 nm, after 3 h of reaction. For
practical purposes these processes are remarkable be-
cause they constitute quantitative synthetic procedures
for preparation of the cis isomers, which are consider-
ably easier than the original syntheses [25].
The lack of substantial dependence of the quantum
yields in the isomerization reactions for the trans-
[RuCl2(dppb)(2,20-bipy)] complex (Table 2) on the sol-
vents (coordinating or non-coordinating) suggests a
twisting mechanism where no ligand is totally dissoci-
ated from the metal center. Since in the trans-
[RuCl2(dppb)(2,20-bipy)] series only the chlorine ligand
is not quelated probably the mechanism involved in the
isomerization process of these complexes is more influ-
enced by the flexibility of the Ru–Cl bond than of the
Ru–P or Ru–N bonds. This suggestion finds support in
the fact that the chlorine ligand is easily and totally
dissociated from the complexes when they are in the
presence of coordinating solvent for long period of time.
Variable-temperature data afforded the activation
parameters for the isomerization process of the
[RuCl2(dppb)(2,20-bipy)] complex (DHz ¼ 69:6 kJ molꢀ1
and DSz ¼ ꢀ73:1 J Kꢀ1 molꢀ1). The large, negative
entropy factor for this conversion is in agreement with
the occurrence of a twist pathway of the isomerization
process as suggested above [28–30].
Acknowledgements
We thank CNPq, CAPES, FINEP and FAPESP for
financial support. We are grateful to Professor Edward
R. Dockal for discussion and suggestions.
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