Pyrazine-bridged Pt(II)-sulfoxide complexes: Multinuclear magnetic resonance spectroscopy and crystal structures of trans,trans-{Pt(R 2SO)Cl2}2(μ-pyrazine)

Article abstract of DOI:10.1016/j.ica.2003.11.048

A new type of pyrazine-bridged platinum(II) complexes, trans,trans- Pt(R₂SO)Cl₂(μ-pyrazine)Pt(R₂SO)Cl ₂ was synthesized and characterized mainly by IR and multinuclear magnetic resonance spectroscopies (¹H, ¹³C and ¹⁹⁵Pt) and by crystallographic methods. Compounds with dimethylsulfoxide, tetramethylenesulfoxide, di-n-propylsulfoxide, di-n-butylsulfoxide, dibenzylsulfoxide and diphenylsulfoxide were prepared. The compounds were synthezised in good yields from the aqueous reaction of K[Pt(R₂SO)Cl₃] with pyrazine. IR spectroscopy showed only one ν(Pt-Cl) band suggesting trans isomers. The ¹⁹⁵Pt NMR signals of the dimeric species were observed between -3041 and -3113 ppm. The resonance of the diphenylsulfoxide complex was found at higher fields than the other compounds. In ¹H NMR, the pyrazine protons are more deshielded (ave. 0.52 ppm) than in free pyrazine. The coupling constants with the pyrazine protons ³J(¹⁹⁵Pt-¹H) are between 28 and 35 Hz. The crystal structures of three pyrazine-bridged dimers, {trans-Pt(R ₂SO)Cl₂}₂(μ-pyrazine) were studied by X-ray diffraction methods. The results confirmed the trans,trans configuration of the compounds. All the molecules contain an inversion centre.

Full text of DOI:10.1016/j.ica.2003.11.048

Inorganica Chimica Acta 357 (2004) 2167–2175
www.elsevier.com/locate/ica
Pyrazine-bridged Pt(II)-sulfoxide complexes: multinuclear
magnetic resonance spectroscopy and crystal structures of
trans,trans-{Pt(R2SO)Cl2}2(l-pyrazine)
*
Julien R.L. Priqueler, Fernande D. Rochon
Dꢀepartement de Chimie, Universitꢀe du Quꢀebec ꢁa Montrꢀeal, C.P. 8888, Succ. Centre-ville, Montrꢀeal, Canada H3C 3P8
Received 23 July 2003; accepted 19 November 2003
Abstract
A new type of pyrazine-bridged platinum(II) complexes, trans,trans-Pt(R2SO)Cl2(l-pyrazine)Pt(R2SO)Cl2 was synthesized and
characterized mainly by IR and multinuclear magnetic resonance spectroscopies (1H, 13C and 195Pt) and by crystallographic
methods. Compounds with dimethylsulfoxide, tetramethylenesulfoxide, di-n-propylsulfoxide, di-n-butylsulfoxide, dibenzylsulfoxide
and diphenylsulfoxide were prepared. The compounds were synthezised in good yields from the aqueous reaction of K[Pt(R2SO)Cl3]
with pyrazine. IR spectroscopy showed only one m(Pt–Cl) band suggesting trans isomers. The 195Pt NMR signals of the dimeric
species were observed between )3041 and )3113 ppm. The resonance of the diphenylsulfoxide complex was found at higher fields
than the other compounds. In 1H NMR, the pyrazine protons are more deshielded (ave. 0.52 ppm) than in free pyrazine. The
coupling constants with the pyrazine protons 3J(195Pt–1H) are between 28 and 35 Hz. The crystal structures of three pyrazine-
bridged dimers, {trans-Pt(R2SO)Cl2}2(l-pyrazine) were studied by X-ray diffraction methods. The results confirmed the trans,trans
configuration of the compounds. All the molecules contain an inversion centre.
Ó 2004 Published by Elsevier B.V.
Keywords: Platinum complexes; Sulfoxide; Pyrazine; Crystal structure; NMR; IR
1. Introduction
the other active compounds, since it is ionic and there-
fore should be more soluble than the conventional an-
titumor compounds. They seem to possess an
appropriate Ptꢀ ꢀ ꢀPt distance and the flexibility to pro-
vide the cross-links with a minimal distortion of the
DNA molecule. A few other studies [2,3] seem also to
indicate that pyrazine-bridged complexes might have a
promising future in chemotherapy.
Polynuclear Pt(II) complexes bridged by N-donors
are becoming very popular as potent anticancer agents,
some of them are in terminal clinical phases [4–6]. In
1993, Caluccia et al. [7] showed that trans complexes
with amine ligands could exhibit antitumor properties
for very specific organs. Furthermore, neutral ligands
with non-N-donors, such as S ligands, produced several
active compounds [8]. Pt(II) complexes containing sulf-
oxide and amine ligand such as [Pt(R2SO)(di-
amine)Cl]NO3 and cis and trans-Pt(R2SO)(quinoline)
Cl2 have shown some biological activity, which seems to
depend on the sulfoxide chain [9,10].
The antitumor activity of the Pt(II) complexes of the
type cis-Pt(amine)2Cl2 are now well known. Nowadays,
cisplatin (cis-Pt(NH3)2Cl2) still remains the most com-
monly used anticancer agent in chemotherapy, even if it
exhibits numerous and important side effects. Never-
theless, the search for more specific and less toxic drugs
is still continuing, especially in view of the significant
cross-resistance of cisplatin.
Recently, the pyrazine-bridged dinuclear Pt(II) com-
plexes, [{cis-Pt(NH3)2Cl}2(l-pz)]Cl2, and its derivatives
were reported to possess good anticancer properties,
especially since they seem to overcome the cross-resis-
tance of cisplatin [1]. The complex is quite different from
* Corresponding author. Tel.: +1-514-9873000x4896; fax: +1-514-
987-4054.
E-mail address: rochon.fernande@uqam.ca (F.D. Rochon).
0020-1693/$ - see front matter Ó 2004 Published by Elsevier B.V.
doi:10.1016/j.ica.2003.11.048

2168
J.R.L. Priqueler, F.D. Rochon / Inorganica Chimica Acta 357 (2004) 2167–2175
Our research group has been involved in Pt–sulfoxide
bought from Anachemia Chemicals Ltd. and dipropyl-
sulfoxide from Phillips Petroleum Co. The latter was
purified by distillation before use.
complexes for many years. Sulfoxides have interesting
behaviours since they can accept p electron density from
the metal center. For these types of ligands, the cis com-
plexes are usually more stable than the trans compounds
(contrary to the amine system). We have recently under-
taken a study on a new type of mixed-ligands platinum(II)
complexes containing pyrimidine (pm) and sulfoxide
ligands. Pyrimidine also contains empty p antibonding
orbitals, which could accept electron density from Pt. The
reaction of the K[Pt(R2SO)Cl3] with pm produces first the
trans isomer, which then isomerizes to the cis compound
[11]. When the Pt:pm ratio was 2:1, pyrimidine-bridged
dimers were isolated [12].
The decomposition points were measured on a
Fisher–Johns instrument and were not corrected. The
IR spectra were recorded in the solid state (KBr pellets)
on a Perkin–Elmer 783 spectrometer between 4000 and
280 cmꢂ1. All the NMR spectra were measured in
CDCl3 or DMF-d7 on a Varian Gemini 300BB spec-
trometer operating at 300.069, 75.460 and 64.326 MHz
for 1H, 13C and 195Pt, respectively. The chloroform
peaks were used as an internal standard for the 1H (7.24
ppm) and 13C (77.00 ppm) NMR spectra. For 195Pt, the
external reference was K[Pt(DMSO)Cl3] in D2O, ad-
justed at )2998 ppm from K2[PtCl6] (dðPtÞ ¼ 0 ppm).
The 195Pt NMR spectra were measured between )2500
and )4000 ppm.
It appeared interesting to determine the influence of
the pm ligand in these reactions. We have therefore
studied a new type of mixed-ligands complexes with
sulfoxides and pyrazine (pz). Contrary to pm, pz is a
symmetric molecule, but it has also empty pꢁ orbitals,
which could form p bonds with Pt. There are no re-
ported studies on platinum compounds with sulfoxides
and pyrazine and only four papers were found in the
literature on mixed-ligands platinum compounds with
non-substituted pyrazine. The compounds trans-
Pt(phosphine)(pz)Cl2 and trans-{Pt(phosphine)Cl2}2
(l-pz) were synthesized and characterized [13], while
Hall et al. [14] reported trans-{Pt(CO)Cl2}2(l-pz), which
The crystallographic measurements were done on a
Siemens P4 diffractometer using graphite-monochroma-
ꢂ
tized Mo Ka (k ¼ 0:71073 A) radiation. All the crystals
were selected after examination under a polarizing mi-
croscope for homogeneity. The compounds containing
DPrSO (II) and DBuSO (III) were crystallized in a
chloroform–acetone–methanol (3:1:1) mixture, whereas
the TMSO crystal (I) was obtained in a dichlorome-
thane–acetone (1:1) solution. The cell dimensions were
determined at room temperature from a least-squares
refinement of the angles 2h, x and v obtained for well-
centered reflections. The data collections were made by
the 2h=x scan technique using the XSCANS program [16].
The coordinates of the Pt atoms were determined from
Patterson map calculations. All the other non-hydrogen
atoms were found by the usual Fourier methods. The
refinement of the structures was done on F 2 by full-ma-
trix least-squares analysis. The hydrogen atom positions
were fixed in their calculated position with Ueq ¼ 1:2Ueq
(or 1.5 for methyl groups) of the carbon to which they are
bonded. Corrections were made for absorption (semi-
empirical except for crystal II (integration)), Lorentz and
polarization effects. The residual peaks were located in
the close environment of the platinum atoms. The cal-
culations were done using the Siemens SHELXTL system
[16]. The pertinent crystal data and the experimental
details are summarized in Table 1.
1
was characterized by H NMR, IR and UV spectros-
copies. The compounds trans-Pt(C2H4)(pz)Cl2 and its
derivatives were prepared and used as catalyst into hy-
drosilation reactions [15]. The fourth paper is on the
ionic cis diammine compound mentioned above [1]. The
nature of the platinum–pyrazine bond has not been in-
vestigated yet.
In the present project, we have synthesized and
characterized pyrazine-bridged dinuclear complexes
with sulfoxide ligands. Six sulfoxides with different ste-
ric hindrance were used: dimethylsulfoxide (DMSO),
tetramethylenesulfoxide (TMSO), di-n-propylsulfoxide
(DPrSO), di-n-butylsulfoxide (DBuSO), dibenzylsulfox-
ide (DBzSO) and diphenylsulfoxide (DPhSO). The
complexes (R2SO)Cl2Pt(l-pz)Pt(R2SO)Cl2 which were
found to have the trans,trans configuration were char-
acterized by infrared and multinuclear magnetic reso-
nance (1H, 13C and 195Pt) spectroscopies. The crystal
structures of three compounds with different sulfoxides
(TMSO, DPrSO and DBuSO) were determined.
2.1. {trans-Pt(R2SO)Cl2}2(l-pyrazine)
The K[Pt(R2SO)Cl3] complexes were synthesized ac-
cording to the method described by Kukushkin et al.
[17]. The DBuSO, DBzSO and DPhSO complexes were
obtained in small yields due to the great aqueous
insolubility of the ligands and the very favorable
formation of the insoluble disubstituted compound
Pt(R2SO)2Cl2.
2. Experimental
K2[PtCl4] was obtained from Johnson Matthey Inc.
and was purified by recrystallization in water before use.
CDCl3 and DMF-d7 were purchased from CDN Iso-
topes. Pyrazine and the sulfoxide ligands were obtained
from Aldrich except dimethylsulfoxide, which was
The K[Pt(R2SO)Cl3] compound was dissolved in a
minimum amount of water. Pyrazine, dissolved in water,

J.R.L. Priqueler, F.D. Rochon / Inorganica Chimica Acta 357 (2004) 2167–2175
2169
Table 1
Crystallographic data for the compounds {Pt(R2SO)Cl2}2(l-pz), crystals I–III
Crystal
I
II
III
R2SO
Chemical formula
TMSO
C
DPrSO
C16H32Cl4N2O2Pt2S2
880.54
DBuSO
C20H40Cl4N2O2Pt2S2
936.64
12H20Cl4N2O2Pt2S2
Mw
Space group
820.40
P21=n
ꢃ
P21=n
R3
ꢂ
a (A)
7.537(2)
10.823(4)
12.639(3)
100.160(10)
1014.8(5)
2
7.736(2)
17.759(2)
10.1180(10)
105.00(2)
1342.7(4)
2
26.638(4)
26.638(4)
11.437(3)
ꢂ
b (A)
ꢂ
c (A)
b (°)
Volume (A )
3
ꢂ
7028(2)
9
1.992
Z
qcalc (g cmꢂ3
)
2.685
2.178
10.976
l(Mo Ka) (mmꢂ1
F ð000Þ
)
14.511
756
8508
9.442
4014
828
12 090
Measured reflections
Independent reflections (Rint
10 469
3405 (0.0711)
2349
)
2335 (0.1028)
1578
3079 (0.0795)
2166
0.0555
Observed reflections (I > 2rðIÞ)
R1ðF > 2rðIÞÞ
0.0547
0.1191
1.076
0.0444
0.0890
1.079
wR2 (all data)
0.1211
1.055
S
P
P
P
2
2
1=2
R1 ¼ ðjFo ꢂ FcjÞ=jFoj, wR2 ¼ ½ ðwðFo2 ꢂ Fc2Þ Þ= ðwðFo2Þ Þꢃ
.
was slowly added to the previous solution in a 2.1:1
proportion at room temperature. A pale yellow precip-
itate appeared rapidly, but the stirring of the mixture
was pursued from several minutes to several hours for
the most hindered sulfoxides. After filtration, the pre-
cipitate was dried, washed with ether and dried in vac-
uum. For the DBuSO and DBzSO complexes, the
reactions were done in a 2:1 MeOH/H2O mixture, while
for DPhSO, the reaction was performed in a 2:1 EtOH/
H2O solution.
trans-{Pt(DBuSO)Cl2}2(l-pz). Yield 94%; dec. 168–
195 °C; IR (cmꢂ1): pz: 3118 w (m13(B1u)), 1462 w
(m19a(B1u)), 1416 s (m19b(B1u)), 1380 w (m19b(B3u)), 1139 s
(m18a(B1u)), 1104 s (m3(B2g)), 1089 s (m14(B3u)), 817 s
(m11(B2u)); m(S–O) 1162 m, m(Pt–N) 511 vw, m(Pt–S) 458
w, m(Pt–Cl) 354 m; 1H NMR (ppm): Hpz 9.104 s, Ha 3.67
ddd (J ¼ 12:9, 11.4 and 5.2), 3.25 ddd (J ¼ 12:9, 10.8
and 4.8), Hb 2.18 m, 2.08 m, Hc 1.59 tq (J ¼ 6:8, 7.2),
Hd 1.02 t (J ¼ 7:2);13C NMR (ppm): Cpz 148.21, Ca
55.11, Cb 24.67, Cc 21.54, Cd 13.66.
trans-{Pt(DMSO)Cl2}2(l-pz). Yield 93%; dec. 228–
247 °C; IR (cmꢂ1): pz (vibration number [18]): 3118 m
(m13(B1u)), 1415 s (m19b(B1u)), 1339 m (m19b(B3u)), 1106 m
(m3(B2g)), 1028 s (m1(Ag)), 810 s (m11(B2u)); m(S–O) 1147 s,
m(Pt–N) 518 m, m(Pt–S) 443 w, m(Pt–Cl) 349 w; 1H NMR
(ppm): Hpz 9.100 s, Ha 3.50 s, 3.47 s; 13C NMR (DMF-d7)
(ppm): Cpz 148.03, Ca 44.31.
trans-{Pt(TMSO)Cl2}2(l-pz). Yield 92%; dec. 173–
206 °C; IR (cmꢂ1): pz: 3118 m (m13(B1u)), 1419 s
(m19b(B1u)), 1383 w (m19b(B3u)), 1140 s (m18a(B1u)), 1108 s
(m3(B2g)), 1079 s (m14(B3u)), 1030 w (m1(Ag)), 803 w
(m11(B2u)); m(S–O) 1149 s, m(Pt–N) 508 m, m(Pt–S) 453 w,
m(Pt–Cl) 347 w; 1H NMR (ppm): Hpz 9.105 s, Ha 4.03 m,
3.67 m, Hb 2.41 m, 2.23 m; 13C NMR (ppm): Cpz 148.25,
Ca 57.36, Cb 24.65.
trans-{Pt(DBzSO)Cl2}2(l-pz). Yield 88%. dec. 122–
203 °C; IR (cmꢂ1): pz: 3120 w (m13(B1u)), 1448 m
(m19a(B1u)), 1418 s (m19b(B1u)), 1376 w (m19b(B3u)), 1123 m
(m18a(B1u)), 1110 m (m3(B2g)), 1068 w (m14(B3u)), 1024 w
(m1(Ag)), 801 w (m11(B2u)); m(S–O) 1160 m, m(Pt–N) 516
vw, m(Pt–S) 478 s, m(Pt–Cl) 349 w.
trans-{Pt(DPhSO)Cl2}2(l-pz). Yield 78%; dec. 167–
214 °C; IR (cmꢂ1): pz: 3106 w (m13(B1u)), 1442 s
(m19a(B1u)), 1418 s (m19b(B1u)), 1380 w (m19b(B3u)), 1140 s
(m18a(B1u)), 1110 m (m3(B2g)), 1068 m (m14(B3u)), 1019 w
(m1(Ag)), 812 w (m11(B2u)); m(S–O) 1160 m, m(Pt–N) 508
1
w, m(Pt–S) 448 w, m(Pt–Cl) 351 m; H NMR (ppm): Hpz
9.134 s, Hortho 7.91 m, Hmeta;para 7.55 m; 13C NMR
(ppm): Cpz 148.41, Cphenyl 138.80, 127.42, 128.86, 133.17.
trans-{Pt(DPrSO)Cl2}2(l-pz). Yield 83%; dec. 188–
203 °C; IR (cmꢂ1): pz: 3095 m (m13(B1u)), 1451 w
(m19a(B1u)), 1417 s (m19b(B1u)), 1380 w (m19b(B3u)), 1130 s
(m18a(B1u)), 1106 s (m3(B2g)), 1076 m (m14(B3u)), 1057 w
(m1(Ag)), 806 w (m11(B2u)); m(S–O) 1164 s, m(Pt–N) 519 m,
m(Pt–S) 450 w, m(Pt–Cl) 351 m;1H NMR (ppm): Hpz
9.098 s, Ha 3.65 ddd (J ¼ 12:9, 11.1 and 5.7), 3.25 ddd
(J ¼ 12:9, 10.8 and 5.4), Hb 2.24 m, 2.16 m, Hc 1.21 t
(J ¼ 7:2);13C NMR (ppm): Cpz 148.20, Ca 56.97, Cb
16.66, Cc 12.87.
3. Results and discussion
3.1. Synthesis of the complexes
The aqueous reaction of K[Pt(R2SO)Cl3] with pyra-
zine in a 2.1:1 proportion produced the dimeric com-
plexes {trans-Pt(R2SO)Cl2}2(l-pz). Compounds with
R2SO ¼ DMSO, TMSO, DPrSO, DBuSO, DBzSO and
DPhSO were prepared. The pyrazine solution must be

2170
J.R.L. Priqueler, F.D. Rochon / Inorganica Chimica Acta 357 (2004) 2167–2175
added to the K[Pt(R2SO)Cl3] solution slowly, in order
to have constantly an excess of K[Pt(R2SO)Cl3]. For the
DBuSO and DBzSO complexes, the reactions were
performed in a 1:2 water–methanol mixture, while a 1:2
water–ethanol solution was preferred for the DPhSO
compound
vibration between 347 and 354 cmꢂ1, suggesting a trans
geometry, since two bands are usually observed for cis
complexes. One absorption band observed around 450
cmꢂ1 was assigned to a m(Pt–S) vibration as suggested in
the literature [19–23], while a band between 508 and 519
cmꢂ1 was assigned to the m(Pt–N) vibration, based also
on some published results [24,25].
2K½PtðR2SOÞCl3ꢃ þ pz ! trans;
trans-ðR2SOÞCl2Ptðl-pzÞPtðR2SOÞCl2 þ 2KCl
3.3. 195Pt NMR spectroscopy
A pale yellow to yellow-orange precipitate was
formed. The time of reaction varied with the bulkiness
of the sulfoxide, from 2 min for DMSO to several hours
for the more hindered sulfoxides, DBuSO, DBzSO and
DPhSO. The yields varied between 78% and 94%. These
dimeric species are very insoluble and it was a major
problem in this project. The most insoluble compound
{trans-Pt(DBzSO)Cl2}2(l-pz) could not be analyzed by
NMR spectroscopy, being too insoluble in all the sol-
vents tested (H2O, MeOH, EtOH, CHCl3, CH2Cl2,
CH3COCH3 and DMF). The 195Pt and 13C NMR
spectra of the DMSO complex were measured in DMF-
d7 after heating for a few hours.
Since sulfoxide ligands exhibit a much larger trans
effect than chlorides, the trans isomers are expected to be
first formed. Isomerization to the cis complexes were
expected, especially for the less bulky sulfoxides as ob-
served with the pyrimidine analogues [12]. Nevertheless,
isomerization was not detected. Numerous attempts to
obtain the cis complexes in different organic solvents
were all unsuccessful. It seems that the pyrazine-bridged
compounds exhibit different reactivities than their py-
rimidine analogues {Pt(R2SO)Cl2}2(l-pm) [12].
The 195Pt NMR spectra of the pyrazine-bridged di-
nuclear complexes were measured in CDCl3 (except the
DMSO complex which was obtained after a very long
accumulation time in DMF-d7) and the chemical shifts
are shown in Table 2. The dibenzylsulfoxide compound
was found to be insoluble in all the classical organic
solvants. All the synthesized complexes are pure as
shown by multinuclear (195Pt, 1H and 13C) magnetic
1
resonance spectroscopy. The H NMR spectrum of the
DMSO compound (in DMF-d7) was measured before
and after the 195Pt and 13C spectra were recorded. No
change was noted. The d(195Pt) indicated clearly that
DMSO is still bonded to Pt(II) through its S atom. No
signal was observed in the region where a complex
containing bonded DMF would be observed.
The 195Pt NMR chemical shifts vary between )3041
and )3051 ppm for the aliphatic sulfoxides. They do not
seem to be very dependent on the substituents on the
sulfoxide ligands. The DPhSO complex which contains
two aromatic groups was observed at higher field, )3113
ppm. These results are similar to those observed for
K[Pt(R2SO)Cl3] [26,27], Pt(R2SO)(pm)Cl2 [11] and the
pyrimidine-bridged dimers {Pt(R2SO)Cl2}2(l-pm) [12].
In all these compounds, the DPhSO complexes were
observed at higher fields that the other sulfoxides.
The d(Pt) on the trans pyrazine-bridged dimers are
quite close to those observed for the trans pyrimidine
analogues (ave. )3070 ppm and )3155 ppm for the
DPhSO compound) [12]. They are also in agreement
with the published work on sulfoxide–nitrile compounds
Pt(R2SO)(R-CN)Cl2 [26,27]. They were observed at
lower fields than the phosphine-pyrazine compounds
trans-Pt(PR3)(pz)Cl2 (ave. )3589 ppm) and {Pt(PR3)
Cl2}2(l-pz) (ave. )3587 ppm) [13]. Phosphine Pt(II)
All the dinuclear pyrazine species were characterized
by IR and (except for the DBzSO complex) by multi-
nuclear magnetic resonance spectroscopies. The syn-
thesized complexes were pure, since only one resonance
was observed in 195Pt NMR spectroscopy (confirmed
1
also by H and 13C NMR). The TMSO, DPrSO and
DBuSO complexes were also studied by X-ray diffrac-
tion methods.
3.2. IR spectroscopy
The vibrational spectra of the complexes were mea-
sured in the solid state between 4000 and about 280
cmꢂ1. The IR spectrum of pyrazine has been reported
[18] and our assignments on the pyrazine ligand (ex-
perimental section) are based on this study. The vibra-
tions of coordinated pyrazine were observed at higher
or identical energies than those of free pyrazine. The
Table 2
195Pt, 1H and 13C NMR chemical shifts and Dd (dcomplex ꢂ dpyrazine
)
(ppm) of the complexes {Pt(R2SO)Cl2}2(l-pz) (in CDCl3)
R2SO
d(195Pt)
d(1H) (Dd)
3J(195Pt–H)
d(13C) (Dd)
DMSO
TMSO
DPrSO
)3049a
)3041
)3048
9.100 (0.515) n.o.
9.105 (0.520) 34
9.098 (0.513) 28
9.104 (0.519) 29
9.134 (0.549) 35
148.03a (2.96)
148.25 (3.18)
148.20 (3.13)
148.21 (3.14)
148.41 (3.34)
m(S–O) vibrations absorb between 1147 and 1164 cmꢂ1
,
at higher energy than in the free sulfoxides (ave. 130
cmꢂ1), since the ligands are bonded to Pt by the S atom.
The S–O dp–pp bond is strengthened upon coordination
to the metal. The IR spectra showed only one m(Pt–Cl)
DBuSO )3051
DPhSO )3113
a DMF-d7.

J.R.L. Priqueler, F.D. Rochon / Inorganica Chimica Acta 357 (2004) 2167–2175
2171
H3
H2
complexes are observed at higher fields than sulfoxide
analogues.
2
R
S
Cl
Pt
Cl
Cl
Pt
Cl
R
S
3
1
O
N
N
O
The DPhSO pyrazine-bridged dimer showed a signal
at higher field (about 65 ppm) than the other complexes.
This ligand is the most sterically hindered but it is also
different from the other ligands, since it holds aromatic
groups directly attached to the sulfur binding atom.
These groups are probably responsible for the different
electronic effects observed in this compound. The d(Pt)
values show that the electronic density on the Pt atom is
increased in the DPhSO complex compared to the other
ligands. The shielded resonance is probably not caused
by bulkiness on the bonding atom, since this usually
causes a deshielded effect. For Pt–amine complexes, the
d(Pt) were found at lower fields for amines containing
bulky substituents on the binding atom [28,29]. For
K[Pt(R2SO)Cl3] [30] and the sulfoxide-pyrimidine com-
plexes [11,12], we have suggested that the difference
between the DPhSO complex and the others was due to
inverse polarization of the the p-electrons of the S@O
bond. Again this concept might be a judicious inter-
pretation of our results. This phenomenon has been
suggested in the literature to explain some 13C NMR
results on Pt–CBO [31] and Pt–carboxylato complexes
[32]. This effect would be present in all Pt–R2SO com-
plexes, but would be more important for a ligand con-
taining electron attracting aromatic groups directly on
the binding atom. This effect would increase the electron
density on the Pt atom resulting in a higher field signal.
The pyrazine-bridged dimers are observed at slightly
lower fields than the pyrimidine-bridged dimers (around
25 ppm) [12]. It seems therefore than the r-bond between
Pt and pyrazine is slightly weaker than the one between
Pt and pyrimidine. Since both these ligands contain
empty pꢁ orbitals, the difference might also be caused by
a stronger p-bonding in the pyrazine compound.
4
6
R
5
R
H5
H4
Scheme 1.
reduced due to the formation of two r-bonds. With the
exception of the DPhSO compound, the chemical shifts
are constant and do not vary with the sulfoxide (ave.
9.102 ppm), similarly to the 195Pt NMR spectra. These
values can be compared with those observed for the
dinuclear complexes cis-[{Pt(NH3)2Cl}2(l-pz)](NO3)2,
9.04 ppm [1], and trans-{Pt(CO)Cl2}2(l-pz), 9.24 ppm
[14].
The DPhSO complex was observed at slightly lower
field than the others, which is in agreement with the
195Pt NMR results, where the DPhSO compound was
observed at higher field. When the electronic density is
reduced on the ligand, it should increase on the platinum
atom. As discussed above, the difference would be a
consequence of the specific electronic density induced by
the phenyl rings attached directly to the S-bonded atom.
Satellites arising from the coupling of the protons
with the platinum-195 isotope were observed. The
3J(195Pt–1H) coupling constants are listed in Table 2.
However, this coupling was not noted for
{Pt(DMSO)Cl2}2(l-pz) due to low solubility of the
compound causing an important background noise. The
coupling constants 3J(195Pt–1H) vary between 28 and 35
Hz and they agree well with the only 3J(195Pt–1H) value
(25 Hz) published for a pyrazine-bridged dimeric com-
pound, trans-{Pt(PMePh2)Cl2}2(l-pz) [13]. The cou-
pling constant between pyridine derivatives or amine
protons and 195Pt are known to be geometry-dependent.
The coupling constants 3J(195Pt–1H) are usually around
24–32 Hz for trans complexes and about 10 Hz higher
for their cis analogues [11,12,33–36]. Even if it cannot be
regarded as a definitive proof, the coupling constants
reported in this work suggest a trans geometry. These
results reflect a general trans influence on the coupling
constants for Pt(II) complexes [37].
In 13C NMR spectroscopy, similar results were noted.
All pyrazine carbon atoms are again equivalent in free or
in pyrazine-bridged dimers. The signals are deshielded
upon coordination (Table 2). The DMSO compound was
measured in DMF. For the TMSO, DPrSO and DBuSO,
the chemical shifts are almost constant (Dd ¼ 3:15 ppm).
Again, the DPhSO is slightly more deshielded (Dd ¼ 3:34
ppm). Coordination of pyrazine to two Pt atoms should
considerably reduce the electronic density on the C atoms.
The Dd values are not as large as expected for N donors
like amine ligands, probably caused by inverse polariza-
tion on the S–O p-bond of the sulfoxide, which would
increase the electron density on the Pt atom and will also
affect the trans ligand.
1
3.4. H and 13C NMR spectroscopy
The 1H and 13C NMR spectra of the complexes were
measured in CDCl3, except the 13C NMR spectrum of
the DMSO complex which was measured in DMF-d7.
The chemical shifts are shown in the experimental sec-
tion. The pyrazine signals of the complexes will first be
1
discussed. Free pyrazine has a single signal in H NMR
at 8.585 ppm and at 145.07 ppm in 13C NMR spec-
troscopy. In the pyrazine-bridged dimers, pyrazine is
again symmetric contrary to monomeric species
(Scheme 1). Therefore, only one signal is also observed
and the dðHÞ are shown in Table 2, along with the
Dd(dcomplex ꢂ dpyrazine) values. The H spectra confirmed
the purity of the samples.
Coordination of pyrazine to two platinum atoms
causes a deshielding of the pyrazine protons of about
0.52 and 0.549 ppm for the DPhSO compound. The
electronic density on the bridged ligand is considerably

2172
J.R.L. Priqueler, F.D. Rochon / Inorganica Chimica Acta 357 (2004) 2167–2175
The chemical shifts of the sulfoxide ligands are pre-
The signals of the C atoms (Table 4) in a position
were observed at lower field than in the free molecule
(Dd ꢅ 3 ppm), except for the DPhSO complex, which
was observed at very high field (Dd ¼ ꢂ6:61 ppm).
Similar results were already noted for the Pt(R2SO)
(pm)Cl2 and pyrimidine-bridged complexes [11,12].
All the sulfoxides C signals are usually less deshielded
upon coordination than other types of ligands. The
presence of the p(Pt ! S) bond increases the elec-
tronic density on the sulfoxide ligand. Furthermore,
we have suggested in the past the presence of inverse
polarization of the p(S@O) bond which would ex-
plain the results observed in Pt–sulfoxide complexes.
Bonding to Pt would reduce the p electron density on
the O atom (dꢂS Odþ) and increase it on the S
atom and its neighbouring atoms, including the Pt
atom as mentioned above. The chemical shifts in 13C
NMR spectroscopy are particularly sensitive to such
mesomeric effects. The inverse polarization of the p
electrons in the S@O bond would be more important
in the DPhSO complex, since the two electron at-
tracting groups are bonded directly on the S atom.
sented in the experimental section and the values
Dd(dcomplex ꢂ dsulfoxide) are listed in Tables 3 and 4. The
chemical shifts of the bonded sulfoxide ligands are more
deshielded than those of the free molecules. TMSO is a
cyclic molecule and the protons on Ca and Cb are not
equivalent. Therefore, two signals are observed for the
protons on Ca and two for the protons on Cb. The
protons closer to the O atom are the most deshielded
upon coordination. For the other bonded sulfoxides,
two series of signals were also observed due to limited
rotation of the S–C bonds. The separation between the
signals of the geminal protons decreases as the distance
from the coordination site increases, where the chemical
shifts become superimposable. Similar observations
were already reported for the pyrimidine-bridged di-
meric complexes {Pt(R2SO)Cl2}2(l-pm) [12].
As expected, the a protons are the most affected by
the coordination to the Pt atom and the Dd values be-
come negligible as the distance from the binding atom
increases as shown in Table 3. The Ha Dd value seems to
increase for less bulky sulfoxide ligands. The angle C–S–
C in TMSO is small (ꢄ95°), which makes this ligand the
least sterically hindered and its Ha Dd values are larger.
DPhSO is the most sterically demanding ligand, but it
cannot be compared with the other sulfoxide ligands.
For the DPrSO and DBuSO complexes, the signals of
the a protons are doublets of doublets of doublets, with
2Jave: ¼ 12:9, 3Jave: ¼ 11:0 and 3Jave: ¼ 5:6 Hz for DPrSO
and 2Jave: ¼ 12:9, 3Jave: ¼ 11:1 and 3Jave: ¼ 5:0 Hz for the
DBuSO compound. No coupling of the sulfoxide protons
with 195Pt was observed. The sulfoxide proton signals are
often multiplets of low intensity and consequently, the
195Pt satellites are very weak and overlap with the other
signals. Furthermore, the low solubility of the complexes
resulted in important background noise.
3.5. Crystal structures
The crystal structures of three complexes trans,trans-
Pt(R2SO)Cl2(l-pz)Pt(R2SO)Cl2, where R2SO ¼ TMSO
(I), DPrSO (II) and DBuSO (III) were studied by X-ray
diffraction methods. The crystal structures confirmed
the trans configuration suggested by IR and by the 195Pt
coupling constants in NMR spectroscopy. Figs. 1–3
show the labeled diagrams of the three complexes. The
bond distances and angles are listed in Table 5. All the
molecules contain an inversion centre. For crystal III,
the butyl chains are slightly disordered resulting in high
thermal factors for some of the C atoms.
Table 3
1H NMR Dd (dcomplex ꢂ dsulfoxide) (ppm) of the sulfoxide ligands in the complexes {Pt(R2SO)Cl2}2(l-pz)
Complex
Ha
Hb
Hc
Hd
{Pt(DMSO)Cl2}2(l-pz)
{Pt(TMSO)Cl2}2(l-pz)
{Pt(DPrSO)Cl2}2(l-pz)
{Pt(DBuSO)Cl2}2(l-pz)
{Pt(DPhSO)Cl2}2(l-pz)
1.02, 0.99
1.20, 0.84
1.09, 0.69
0.86, 0.58
0.01, 0.25
0.50, 0.42
0.43, 0.33
0.28 (ortho)
0.20
0.12
0.07
0.11 (para)
0.11 (meta)
Table 4
13C NMR Dd (dcomplex ꢂ dsulfoxide) (ppm) of the sulfoxide ligands in the complexes {Pt(R2SO)Cl2}2(l-pz)
Complex
Ca
Cb
Cc
Cd
{Pt(DMSO)Cl2}2(l-pz)
{Pt(TMSO)Cl2}2(l-pz)
{Pt(DPrSO)Cl2}2(l-pz)
{Pt(DBuSO)Cl2}2(l-pz)
{Pt(DPhSO)Cl2}2(l-pz)
3.61(DMF-d7)
2.97
2.77
–1.11
0.52
–0.40
–0.50
–0.24 (meta)
3.49
–6.61
0.03
2.85 (ortho)
0.00
2.34 (para)

J.R.L. Priqueler, F.D. Rochon / Inorganica Chimica Acta 357 (2004) 2167–2175
2173
Table 5
ꢂ
Bond distances (A) and angles (°) in crystals I–III
I
II
III
Pt–Cl
2.299(3)
2.283(4)
2.221(4)
2.067(12)
1.474(10)
1.79(2)
2.298(3)
2.290(3)
2.223(3)
2.059(9)
1.463(9)
1.793(13)
1.33(2)
2.298(2)
2.272(2)
2.221(2)
2.067(6)
1.456(6)
1.782(10)
1.329(9)
1.391(10)
Pt–S
Pt–N
S–O
Fig. 1. Labeled diagram of {trans-Pt(TMSO)Cl2}2(l-pz) (I). (The el-
lipsoids correspond to 30% probability.)
S–C (ave.)
N–C (ave.)
C–C (pz)
Cl–Pt–Cl
S–Pt–Cl
1.34(2)
1.37(2)
1.37(2)
176.6(2)
90.28(13)
92.81(14)
90.1(3)
174.4(2)
93.44(13)
90.84(13)
87.1(3)
175.74(11)
91.62(8)
91.40(8)
88.8(2)
N–Pt–Cl
86.9(3)
88.8(3)
88.5(2)
N–Pt–S
Pt–S–O
175.2(3)
117.7(5)
111.5(6)
120.9(9)
109.1(8)
95.9(8)
177.6(3)
116.2(4)
109.8(5)
121.3(8)
108.5(7)
103.3(7)
117.3(10)
121.2(11)
114.0(11)
112(2)
173.6(2)
118.0(3)
109.1(4)
120.9(5)
108.6(5)
102.5(5)
118.1(6)
121.0(7)
107.0(8)
108(2)
Pt–S–C (ave.)
Pt–N–C (ave.)
O–S–C (ave.)
C–S–C
C–N–C
C–C–N
117.8(12)
121.1(12)
106.2(13)
110(2)
S–C–C (ave.)
C–C–C (ave.)
Fig. 2. Labeled diagram of {trans-Pt(DPrSO)Cl2}2(l-pz) (II)). (The
ellipsoids correspond to 25% probability.)
ꢂ
2.291(4), 2.294(3) and 2.285(2) A for crystals I, II and
III, respectively. The Pt–S bonds are similar for the three
crystals. These values agree with similar distances in the
literature [1,11–13].
The Pt–N bonds are 2.067(12), 2.059(9) and 2.067(6)
A, respectively, for the complexes containing TMSO,
ꢂ
DPrSO and DBuSO. These bonds located in trans po-
sitions to the sulfoxide ligand are longer, since the trans
influence of sulfoxide is quite large. The values agree
well with the bonds observed in trans-Pt(TMSO)
(l-pm)Cl2 [11] and in the pyrimidine-bridged dimers
{trans-Pt(R2SO)Cl2}2(pm) [12]. In the pyrazine ligands,
ꢂ
the ave. N–C bonds are 1.34(2), 1.33(2) and 1.329(9) A
for crystals I, II and III, while the ave. C–C bond lengths
ꢂ
are 1.37(2), 1.37(2) and 1.391(10) A, respectively. These
distances are in agreement with the corresponding dis-
ꢂ
tances determined for free pyrazine, 1.334 and 1.378 A,
Fig. 3. Labeled diagram of {trans-Pt(DBuSO)Cl2}2(l-pz) (III)). (The
ellipsoids correspond to 25% probability.)
respectively [38]. The binding of pyrazine to platinum
through the N atom slightly increases the internal C–N–
C angles, which vary from 117.3(10) to 118.1(6)°. In free
pyrazine, these angles are 115.1° [38]. The other internal
angles are larger, 121.0(7)–121.2(11)° in crystals I–III
and 122.4° in free pyrazine [38].
The coordination around the Pt atoms is square
planar. In order to reduce the steric hindrance, the O
atom of the sulfoxide ligand is also not very far from the
Pt plane, with deviations of 0.026(13), 0.34(1) and
ꢂ
0.136(8) A for crystals I, II and III, respectively. The
The bond distances and angles in the different sulf-
angles around the Pt atoms are close to the expected
square planar values. The N–Pt–Cl angles are slightly
smaller (ave. 88.4°) than the S–Pt–Cl (ave. 91.73°) in
order to slightly reduce the steric hindrance caused by
the sulfoxide ligands.
oxide ligands are as expected. The S–O bonds are
ꢂ
1.474(10) (I), 1.463(9) (II) and 1.456(6) A (III). The S–C
bond distances sometimes depend on the lengths of the
alkyl chains, especially in bulky compounds, but it is
constant in the three studied crystals, probably because
the bulkiness is less important in trans isomers. The
sulfur atom is in an approximate tetrahedral environ-
The Pt–Cl distances are normal for bonds in trans
position to chloro ligands. The average values are

2174
J.R.L. Priqueler, F.D. Rochon / Inorganica Chimica Acta 357 (2004) 2167–2175
ment. The C–S–C angles (95.9(8), 103.3(7) and 102.5(5)°
for TMSO, DPrSO and DBuSO, respectively) are
smaller than the other angles as observed for similar
compounds [39–50]. In the TMSO complex (Fig. 1), the
conformation of the five-membered ring is of the enve-
lope type. The four atoms S, C11, C12 and C14 are in
The inverse polarization of the sulfoxide p-bond
dꢂS Odþ was suggested to interpret some of the NMR
results. This phenomenon would be more important in
the DPhSO dinuclear compound, since it contains two
electron attracting groups located directly on the
bonding atom. It would increase the electron density on
the S atom and its neighbouring atoms. Attempts will be
made in the future to include other aromatic sulfoxides.
The dibenzylsulfoxide complex is too insoluble for so-
lution studies.
ꢂ
the plane (mean deviation ¼ 0.008(10) A), while C13 is
ꢂ
out of the plane by 0.49(4) A. The four atom plane is
perpendicular to the platinum plane with a dihedral
angle of 90.0(5)°.
Planar ligands such as pyridine derivatives are often
perpendicular to the platinum plane in order to reduce
the steric hindrance, especially for crowded ligands. The
orientation of the planar rings is particularly important
in cis isomers, where steric factors are usually more
important. The dihedral angles between the platinum
and the pyrazine planes were calculated and they are
58.9(4), 52.8(5) and 49.4(2)° for crystals I, II and III,
respectively. These angles vary between 44.4(4) and
62.0(6) for the pyrimidine-bridged dimers [12]. The steric
hindrance in these trans compounds is not important,
especially for pyrazine where the two bonding atoms of
the bridging ligand are in para position. Packing forces
are probably the more determining factors in the ori-
entation of the pyrazine rings, when there is no steric
hindrance.
The crystallographic results have shown that the trans
influence of pyrazine is very similar to the one of
pyrimidine. Results on cis compounds are needed to
obtain more information on its order in the trans
influence series and we are continuing the project in this
direction.
5. Supplementary material
The CIF tables of the three crystal structure deter-
minations have been deposited at the Cambridge Data
File Centre. The deposit numbers are: CCDC 223651–
223653.
Acknowledgements
The authors are grateful to the Natural Sciences and
Engineering Research Council of Canada for financial
support of the project.
4. Conclusion
The aqueous reaction of K[Pt(R2SO)Cl3] with pyra-
zine in a 2:1 ratio gives the dimeric complexes trans,-
trans-Pt(R2SO)Cl2(l-pz)Pt(R2SO)Cl2. The compounds
were obtained pure and IR spectroscopy suggested a
trans geometry for the six compounds studied in the
solid state. The coupling constants with 195Pt also sug-
gested a trans configuration for the five compounds
studied in solution by NMR. We have not found a
solvent for the DBzSO compound. The trans,trans
configurations of three complexes (TMSO, DPrSO and
DBuSO) were confirmed by crystallographic methods.
Although the cis geometry should be thermodynami-
cally more stable, the trans,trans dinuclear complexes
did not isomerize in common organic solvents, unlike
their pyrimidine analogues [12]. These pyrazine-bridged
compounds are quite insoluble and their solution spec-
tra were difficult to obtain. Our results seem to indicate
a potential p-backbonding from Pt to pyrazine, but to a
much smaller extent than with sulfoxides.
References
[1] S. Komeda, G.V. Kalayda, M. Lutz, A.L. Spek, Y. Yamanaka, T.
Sato, M. Chikuma, J. Reedijk, J. Med. Chem. 46 (2003) 1210.
[2] S. Komeda, M. Lutz, A.L. Spek, M. Chikuma, J. Reedijk, Inorg.
Chem. 39 (2000) 4230.
[3] S. Komeda, M. Lutz, A.L. Spek, Y. Yamanaka, T. Sato, M.
Chikuma, J. Reedijk, J. Am. Chem. Soc. 124 (2002) 4738.
[4] P.M.G. Calvert, A.N. Hughes, E.R. Plummer, A.S.T. Azzabi,
M.W. Verrill, M.G. Camboni, E. Verdi, A. Berareggi, M.
Zuchetti, A.M. Robinson, J. Carmichael, A.H. Calvert, Clin.
Cancer Res. 5 (1999) 3796.
[5] Y. Qu, H. Rauter, A.P.S. Fontes, R. Bandarage, L.R. Kelland, N.
Farrell, J. Med. Chem. 43 (2000) 3189.
[6] A. Hegmans, Y. Qu, L.R. Kelland, J.D. Roberts, N. Farrell,
Inorg. Chem. 40 (2001) 6108.
[7] M. Caluccia, A. Nassi, F. Loseto, A. Boccarelli, M.A. Mariggio,
D. Giordano, F.D. Intini, P. Caputo, G. J. Natile. Med. Chem. 36
(1993) 510.
The project is continuing in order to synthesize
monomeric species. Preliminary works have shown
that pure monomeric complexes are not easy to syn-
thesize. We have obtained mixtures and the purifica-
tion will be difficult because of the insolubility of the
products.
[8] K.J. Barnham, M.I. Djuran, P.S. Murdoch, J.D. Ranford, P.J.
Sadler, Inorg. Chem. 35 (1996) 1065.
[9] N. Farrell, D.M. Kiley, W. Schmidt, M.P. Hacker, Inorg. Chem.
29 (1990) 397.
[10] M. van Beusichem, N. Farrell, Inorg. Chem. 31 (1992) 634.
ꢀ ꢀ
[11] N. Nedelec, F.D. Rochon, Inorg. Chim. Acta 319 (2001) 95.
ꢀ ꢀ
[12] N. Nedelec, F.D. Rochon, Inorg. Chem. 40 (2001) 5236.

J.R.L. Priqueler, F.D. Rochon / Inorganica Chimica Acta 357 (2004) 2167–2175
2175
ꢀ ꢀ ꢁ ꢀ
[30] C. Bensimon, M.Sc. thesis, Universite du Quebec a Montreal,
ꢀ
[13] A. Albinati, F. Isaia, W. Kaufmann, C. Sorato, L.M. Venanzi,
Inorg. Chem. 28 (1989) 1112.
Montreal, Canada, 1986.
[14] P.S. Hall, D.A. Thornton, G.A. Foulds, Polyhedron 6 (1987) 85.
[15] A.R. Siedle, K.R. Mann, D.A. Bohling, G. Filipovich, P.E. Toren,
F.J. Palensky, R.A. Newmark, R.W. Duerst, W.L. Stebbings,
H.E. Mishmash, K. Melancon, Inorg. Chem. 24 (1985) 2216.
[16] XSCANS and SHELXTL (PC, version 5) Programs, Bruker
Analytical X-Ray Systems, Madison, WI, 1995.
[17] Yu.N. Kukushkin, Yu.E. VyazÕmenskii, L.I. Zorina, Russ. J.
Inorg. Chem. 13 (1968) 1573.
[31] D.G. Cooper, J. Powell, Inorg. Chem. 16 (1977) 142.
[32] F.D. Rochon, L. Gruia, Inorg. Chim. Acta 306 (2000) 193.
[33] T.B.T. Ha, J.P. Souchard, F.L. Wimmer, N.P. Johnson, Polyhe-
dron 9 (1990) 2647.
[34] C. Tessier, F.D. Rochon, Inorg. Chim. Acta 295 (1999) 25.
[35] C. Tessier, F.D. Rochon, Inorg. Chim. Acta 322 (2001) 37.
[36] F.D. Rochon, V. Buculei, Inorg. Chim. Acta (accepted).
[37] W. McFarlane, J. Chem. Soc., Dalton Trans. (1974) 324.
[38] P.J. Wheatley, Acta Cryst. 10 (1957) 182.
[18] J.D. Simmons, K.K. Innes, J. Mol. Spectrosc. 14 (1964) 190.
[19] J.H. Price, A.N. Williamson, R.F. Schramm, B.B. Wayland,
Inorg. Chem. 11 (1972) 1280.
[39] R. Melanson, F.D. Rochon, Can. J. Chem. 53 (1975) 2371.
[40] R. Melanson, F.D. Rochon, Acta Cryst.
1125.
[41] F. Caruso, R. Spagna, L. Zambonelli, Acta Cryst. B 36 (1980)
B 34 (1978)
[20] V.Yu. Kukushkin, V.K. Belsky, V.E. Konovalov, G.A. Kirakos-
yan, L.V. Konovalov, A.I. Moiseev, V.M. Tkachuk, Inorg. Chim.
Acta 185 (1991) 143.
713.
[42] B. Viossat, P. Khodadad, N. Rodier, Acta Cryst. C 47 (1991)
1316.
[21] Yu.N. Kukushkin, V.N. Spevak, Russ. J. Inorg. Chem. 18 (1973) 240.
[22] Yu.N. Kukushkin, I.V. Pakhomova, Russ. J. Inorg. Chem. 16
(1971) 226.
[43] V.K. Belsky, V.E. Konovalov, V.Yu. Kukushkin, Acta Cryst. C
47 (1991) 292.
[23] B.F.G. Johnson, R.A. Walton, Spectrochim. Acta 22 (1966) 1853.
[24] Yu.N. Kukushkin, N.V. Ivannikova, K.A. Khokhryakov, Russ. J.
Inorg. Chem. 15 (1970) 1595.
[44] A. Cornia, A.C. Fabretti, M. Bonivento, L. Cattalini, Inorg.
Chim. Acta 255 (1997) 405.
[45] R. Melanson, F.D. Rochon, Acta Cryst. B 33 (1977) 3571.
[25] K. Nakamoto, P.J. McCarthy, J. Fujita, R.A. Condrate, G.T.
Behnke, Inorg. Chem. 4 (1965) 36.
ꢂ
€
[46] K. Lovqvist, A. Oskarsson, Acta Cryst. C 48 (1992) 2073.
[47] V.K. Belsky, V.E. Konovalov, V.Yu. Kukushkin, A.I. Moiseev,
Inorg. Chim. Acta 169 (1990) 101.
[26] F.D. Rochon, S. Boutin, P.C. Kong, R. Melanson, Inorg. Chim.
Acta 264 (1997) 89.
[27] F.D. Rochon, R. Boughzala, R. Melanson, Can. J. Chem. 70
(1992) 2476.
[48] F.D. Rochon, P.C. Kong, R. Melanson, Inorg. Chim. Acta 216
(1994) 163.
[49] R. Melanson, F.D. Rochon, Acta Cryst. C 40 (1984) 793.
[28] P.S. Pregosin, Coord. Chem. Rev. 44 (1982) 247.
[29] F.D. Rochon, M. Doyon, I.S. Butler, Inorg. Chem. 32 (1993)
2717.
ꢁ
[50] R. Melanson, C. de la Chevrotiere, F.D. Rochon, Acta Cryst. C
41 (1985) 1428.