Organoheterobimetallic complexes derived from bis(1-ethoxycarbonyl)-1-cyanoethylene-2,2-dithiolatometalate(II) ion: Synthesis and properties

Article abstract of DOI:10.1016/S0022-328X(00)00267-9

A complex salt [NEt₄]₂[Cu(ecda)₂] and six organoheterobimetallic complexes [Bu₂Sn][Cu(ecda)₂], [Ph₃Sn]₂[Ni(ecda)₂] and [PhHg]₂[M(ecda)₂] (M=Ni(II), Cu(II), Zn(II) or Cd(II); ecda^2-=1-ethoxycarbonyl-1-cyanoethylene-2,2-dithiolate) have been prepared. Their characterization relies on analysis, spectroscopic methods, magnetic and conductivity measurements and cyclic voltammetric studies. The diamagnetic compounds [PhHg]₂[Cu(ecda)₂] and [Bu₂Sn][Cu(ecda)₂] are strongly antiferromagnetically coupled. Unshifted and unbroadened nature of ¹H-NMR signals show that [PhHg]₂[Cu(ecda)₂] remains diamagnetic in solution while [PhHg]₂[Ni(ecda)₂], which is paramagnetic in the solid state becomes square planar-diamagnetic in solution. In the case of [Bu₂Sn][Cu(ecda)₂] the peak broadening shows absence of Cu-Cu interaction due to paramagnetic nature of the compound in solution. All the products exhibited σ_rt in the range of 10^-10-10^-9 S cm^-1. Except [Bu₂Sn][Cu(ecda)₂], [PhHg]₂[Cu(ecda)₂] and [Ph₃Sn]₂[Ni(ecda)₂], which show semi conducting behaviour in the range of 293-343 K, the remaining compounds are electrically insulating over the above temperature range.

Full text of DOI:10.1016/S0022-328X(00)00267-9

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 605 (2000) 102–108
Organoheterobimetallic complexes derived from
bis(1-ethoxycarbonyl)-1-cyanoethylene-2,2-dithiolatometalate(II)
ion: synthesis and properties
Nanhai Singh *, Sanjay Kumar Rai
Department of Chemistry, Faculty of Science, Banaras Hindu Uni6ersity, Varanasi 221 005, India
Received 6 January 2000; received in revised form 18 April 2000
Abstract
A complex salt [NEt4]2[Cu(ecda)2] and six organoheterobimetallic complexes [Bu2Sn][Cu(ecda)2], [Ph3Sn]2[Ni(ecda)2] and
[PhHg]2[M(ecda)2] (M=Ni(II), Cu(II), Zn(II) or Cd(II); ecda2− =1-ethoxycarbonyl-1-cyanoethylene-2,2-dithiolate) have been
prepared. Their characterization relies on analysis, spectroscopic methods, magnetic and conductivity measurements and cyclic
voltammetric studies. The diamagnetic compounds [PhHg]2[Cu(ecda)2] and [Bu2Sn][Cu(ecda)2] are strongly antiferromagnetically
1
coupled. Unshifted and unbroadened nature of H-NMR signals show that [PhHg]2[Cu(ecda)2] remains diamagnetic in solution
while [PhHg]2[Ni(ecda)2], which is paramagnetic in the solid state becomes square planar-diamagnetic in solution. In the case of
[Bu2Sn][Cu(ecda)2] the peak broadening shows absence of Cu–Cu interaction due to paramagnetic nature of the compound in
solution. All the products exhibited |rt in the range of 10−10–10−9 S cm−1. Except [Bu2Sn][Cu(ecda)2], [PhHg]2[Cu(ecda)2] and
[Ph3Sn]2[Ni(ecda)2], which show semi conducting behaviour in the range of 293–343 K, the remaining compounds are electrically
insulating over the above temperature range. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Dithiocomplexes; Organoheterobimetallic complexes; Spectroscopic studies; Semiconducting properties
differ significantly in their behaviour. 1,1-Dithioligands
form four-membered strained chelate ring while 1,2-
dithiolenes form less strained five-membered ring in
their transition metal complexes. Also, the former
shows smaller p-acceptor character as compared to
metal 1,2-dithiolates. These features make huge differ-
ence in the structure and properties of the complexes.
1. Introduction
For over 40 years, transition metal 1,2-dithiolene
complexes have been thoroughly investigated by many
research groups [1] and a considerable number of these
complexes have novel and interesting conducting [2–5]
and magnetic properties [6–8]. The 1,1-dithioligand,
isomaleonitriledithiolate (i-mnt2−) which has been re-
cently found useful in molecular electrical conductor
exploration [9] like its 1,2-dithio analogue, mnt2−, is
quite similar to 1-ethoxycarbonyl-1-cyanoethylene-2,2-
dithiolate (ecda2−). Nevertheless, substitution of one of
the CꢀN groups by the ester group in i-mnt2− is
expected to influence the properties of the complexes
[10]. Furthermore, despite some of their properties be-
ing common, 1,1- and 1,2-dithioligands that is where
thiofunctions are on the same or neighbouring carbons
The general interest of chemists in the investigation
of organoheterobimetallics has sparked because of their
significant properties, functions and applications in ar-
eas such as catalysis, material synthesis and
bioorganometallics due to cooperative effect induced by
two metal centers of varying electronic configuration.
Organomercury compounds are important in organic
synthesis on account of generation of radical precursors
* Corresponding author. Tel.: +91-542-318310; fax: +91-542-
317074.
E-mail address: rnsingh@banaras.ernet.in (N. Singh).
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00267-9

N. Singh, S. Kumar Rai / Journal of Organometallic Chemistry 605 (2000) 102–108
103
[11]. Higher propensity of mercurials towards sulfur
2.2.3. [PhHg]2[Ni(ecda)2]
To
(20 cm3) stirring ethanolic solution of
PhHgCH3COO (0.673 g, 2 mmol), was added a (50
cm3) solution of K2[Ni(ecda)2] prepared as described
above.
containing ligands focus research interest because of
topical problems of mercury poisoning while, organ-
otin(IV) mono and di-cation complexes are antitumour
active [12]. The tendency of dithioligands to form heter-
obimetallic complexes is well pronounced [4,5,13–15].
Taking together the chemistry and properties of
organometallic and dithiocomplexes, this paper is
devoted to the study of properties of newly synthesized
organoheterobimetallic dithio complexes, using cationic
organometallic species and anionic transition metal
dithiolates at the border between organometallic and
inorganic chemistry.
a
2.2.4. [PhHg]2[Cu(ecda)2] and [Bu2Sn][Cu(ecda)2]
A dark brown solution of K2[Cu(ecda)2] was pre-
pared in situ by slow addition of a (30 cm3) water–
ethanol (80:20 v/v) solution of Cu(CH3COO)2·H2O
(0.199 g, 1 mmol) to a (20 cm3) aqueous solution of
K2ecda (0.52 g, 2 mmol) with constant stirring. To this
(50 cm3) solution, an ethanolic solution (20 cm3) of
PhHgCH3COO (0.673 g, 2 mmol)–Bu2SnCl2 (0.304 g, 1
mmol) was added with stirring.
2. Experimental
2.2.5. [NEt4]2[Cu(ecda)2]
To a stirred (50 cm3) solution of K2[Cu(ecda)2] pre-
pared as described above, was added (5 cm3) aqueous
solution of NEt4Br (0.420 g, 2 mmol).
In general the reaction mixtures were stirred for
another 30 min and the solid compounds thus formed
in each case were filtered, washed with ethanol–water
mixture followed by ethanol and diethyl ether and dried
in vacuo over CaCl2.
2.1. Materials
Tetraethylammonium bromide, phenylmercuric ac-
etate, triphenyltin chloride, dibutyltin chloride and
ethylcyanoacetate (all Aldrich) were used as received.
The ligand, potassium 1-ethoxycarbonyl-1-cyano-
ethylene-2,2-dithiolate (K2ecda) was prepared accord-
ing to the reported method [16] by reaction of ethyl-
cyanoacetate, KOH and CS2 in dioxane and
characterized by analyses, IR and NMR spectra. All
other metal salts and reagents were of BDH (AR) or
equivalent grades. The solvents were purified by distil-
lation and dried, where necessary, before use. All the
syntheses were performed in open atmosphere.
2.3. Analysis and physical measurements
The organic part of the complexes were decomposed
with concentrated HNO3–aqua-regia. Metals were de-
termined following standard gravimetric procedures
[17]. Sulfur was estimated as BaSO4. Physical measure-
ments dealing with solution and solid state conductivity
and magnetic susceptibility measurements and record-
ing of IR (4000–400 cm−1) spectra were the same as
described earlier [5]. Far IR (400–200 cm−1) as
polyethylene pellets were recorded on a PE 983/PE 781
spectrophotometer. The Raman spectra of selected
samples were recorded on a Bruker IFS 66 V FT IR
2.2. Synthesis of complexes
2.2.1. [Ph3Sn]2[Ni(ecda)2]
A reddish–brown solution of K2[Ni(ecda)2] was gen-
erated in situ by gradual addition (30 cm3) of an
ethanolic solution of Ni(CH3COO)2·4H2O (0.248 g, 1
mmol) to a (20 cm3) ethanol solution of Ph3SnCl (0.77
g, 2 mmol), was filtered (50 cm3) water–ethanol (50:50
v/v) solution of K2ecda (0.52 g, 2 mmol) with vigorous
stirring. To a stirred (10 cm3) solution of K2[Ni(ecda)2]
prepared above. The reaction mixture was stirred for 1
h and then water (20 cm3) was added.
1
FRA 106 Raman module using YAG Laser. H- and
13C-NMR spectra were recorded in DMSO-d6 on a
JEOL FX 90Q spectrometer with l measured relative
to the internal reference Me4Si. The X-band EPR spec-
tra for the powder and DMSO solution of the samples
were obtained at room temperature with a Varian
E-112 spectormeter. Diphenylpicrylhydrazyl (dpph)
powder (g=2.0036) was employed as a field reference.
The FAB mass spectra were recorded on a JEOL SX
102/DA-6000 mass spectrometer using m-nitrobenzyl
alcohol as the matrix. For cyclic voltammetric experi-
ments, a conventional three-electrode system was used
in a single compartment cell. The working electrode
was a platinum wire and the reference electrode was a
saturated sodium chloride calomel electrode (SSCE)
which was separated from the test solution by using
Luggin capillary containing an agar salt (NaCl) bridge.
2.2.2. [PhHg]2 [M(ecda)2]{M=Zn(II) or Cd(II)}
Yellow solution of K2[Zn(ecda)2]–K2[Cd(ecda)2] was
evolved in situ by slow addition of (30 cm3) ethanolic
solution of Zn(CH3COO)2.2H2O–Cd(CH3COO)2·2H2O
(0.219 g, 1 mmol):(0.266 g, 1 mmol) to an (30 cm3)
ethanol–water (80:20 v/v) solution of K2ecda (0.52 g, 2
mmol) with vigorous stirring. To the (60 cm3) stirred
solution of K2[Zn(ecda)2]–K2[Cd(ecda)2], was added a
(10 cm3) ethanolic solution of PhHgCH3COO (0.673 g,
2 mmol).

104
N. Singh, S. Kumar Rai / Journal of Organometallic Chemistry 605 (2000) 102–108
All the potentials were reported with respect to this
electrode. The counter electrode was also a platinum
wire. Cyclic voltammograms were obtained with a
bipotentiostat model-AFRDE 4E, Pine instrument
(USA) and recorded on a Houston Model 2000 X–Y
[Cu(ecda)]+, [HgS]+, [NiS]+–[CuS]+ and [C6H5]+ frag-
ment ions and suggest identical environment around
the metal ions. An intense peak at m/z=432 in both
the complexes may be accounted to the m/2z of the
peak at m/z=865 or [Ni(ecda)2]+–[Cu(ecda)2]+ moi-
eties.
recorder. Concentration of the complexes was 10−3
M
in acetonitrile, containing 0.1 M NaClO4 as a support-
ing electrolyte. The relevant analytical and physico–
chemical data of the complexes are given in Tables 1
and 2.
3.1. Magnetic moment, EPR and UV–6isible spectra
The veff=2.93 BM for [PhHg]2[Ni(ecda)2] suggests
that geometry around nickel(II) center is not square
planar but involvement of a solid state interaction
through coordinatively unsaturated sulfurs of ecda2−
groups and also the proposed non-linearity for the
SꢁHgꢁPh interactions result into an octahedral environ-
ment around nickel(II). The electronic spectral data for
the complexes are summarized in Table 2. The absorp-
tion band centered at around 600 nm, is most likely
assigned to a d–d transition, characteristic for square
planar environment [19] around nickel(II) and suggests
that solid state interactions responsible for the octahe-
dral environment around nickel(II) are not preserved in
solution. The absorption peak at 450 nm is attributed
to Ni’S(ecda2−) MLCT transition [19] while the high
energy absorptions are mainly associated to the intrali-
gand (p–p*) transitions. The diamagnetism together
with electronic spectral bands at around 580 nm suggest
square planar geometry [19] about nickel(II) in
[Ph3Sn]2[Ni(ecda)2].
Magnetic moment 1.28 BM for [NEt4]2[Cu(ecda)2] is
somewhat low for a single unpaired spin present on
copper(II) center and shows some sort of metal–metal
interaction in the solid state. In DMSO solution this
gives an isotropic spectrum with Aav=56.25 G and
gav=2.060 which conforms to the values reported for
the square planar cupric dithiolates [20]. The trend
gꢀꢀ\gÞ\ge (2.0023) shows that the unpaired electron is
3. Results and discussion
When
an
aqueous–ethanolic
solution
of
K2[M(ecda)2] was allowed to react with an ethanolic
solution of Ph3SnCl–Bu2SnCl2–PhHgCH3COO or an
aqueous solution of NEt4Br in appropriate molar ratio,
the complex salt [NEt4]2[Cu(ecda)2] and the
organoheterobimetallic complexes [PhHg]2[M(ecda)2],
[Ph3Sn]2[Ni(ecda)2] and [Bu2Sn][Cu(ecda)2] were formed
according to the following matathesis reactions:
M(CH3COO)2+2K2ecda
“K2[M(ecda)2]+2CH3COOK
K2[Cu(ecda)2]+2NEt4Br“[NEt4]2[Cu(ecda)2]+2KBr
K2[M(ecda)2]+2PhHgCH3COO
“[PhHg]2[M(ecda)2]+2CH3COOK
K2[Ni(ecda)2]+2Ph3SnCl“[Ph3Sn]2[Ni(ecda)2]+2KCl
K2[Cu(ecda)2]+Bu2SnCl2“[Bu2Sn][Cu(ecda)2]+2KCl
M=Ni(II), Cu(II), Zn(II) or Cd(II)
The complexes melt/decompose in the range of 110–
140°C. Bimetallic complexes [PhHg]2[Ni(ecda)2] and
[Ph3Sn]2[Ni(ecda)2] are soluble in acetone while the
remaining complexes are soluble in acetonitrile and
DMSO. The conductivities (6.7–27.8 cm2 V−1 mol−1
)
present in the d
orbital [20] of copper(II). Com-
2
x
2
of a 10−3 M acetone solution of the complexes (Table
1) show non-electrolytic behaviour [18] of [PhHg]2-
[Ni(ecda)2], [PhHg]2[Cu(ecda)2] and [Ph3Sn]2[Ni(ecda)2]
while [NEt4]2[Cu(ecda)2] registered a conductivity of
108 cm2 V−1 mol−1 in 10−3 M DMSO consistent with
1:2 electrolytes [18]. Molar conductivities (85–90 cm2
V−1 mol−1) for the remaining complexes in DMSO
solution indicate their complete dissociation.
plete quenching of−pyaramagnetism at room temperature
for [Bu2Sn][Cu(ecda)2] and [PhHg]2[Cu(ecda)2] is indica-
tive of strong antiferromagnetic interaction between
uncoupled spins at copper(II) centers. Features of RT
powder EPR spectra for the diamagnetic [PhHg]2[Cu-
(ecda)2] and [Bu2Sn][Cu(ecda)2] belong to Cu(II), S=
1/2 monomeric impurities possibly incorporated during
preparation. The complex salt [NEt4]2[Cu(ecda)2] and
organoheterobinuclear complexes containing copper(II)
show a weak broad d–d band at around 600 nm,
characteristic of square planar geometry [21] about
copper(II) while the intense high energy absorptions at
around 300–350 nm and 398–450 nm are assigned to
p–p* intraligand and Cu’S(ecda2−) charge transfer
transitions.
In the positive ion FAB mass spectra of [PhHg]2[Ni-
(ecda)2]–[PhHg]2–[Cu(ecda)2], the peaks at m/z (I%),
1091(15)/1021(40), 865(40/100), 743(100), 588(25/80),
467(10)/471(5), 244(25)/248(40), 232(30/15), 89(60)/
94(20) and 77(75/40) correspond to {[PhHg]2[Ni-
(ecda)2]+S}+–{[PhHg]2[Cu(ecda)2]+S}+, {(PhHg)2-
ecda+2C2H5SH}+, [(PhHg)2ecda]+, [(PhHg)2+S]+,
[Ni(ecda)2+H2S]+–[Cu(ecda)2+H2S]+, [Ni(ecda)]+–

Table 1
Analytical data, molar conductance and 1H-NMR chemical shifts, l ppm (proton/J) of the complexes a
Complex empirical
formula
Colour (% yield)
M.p. (decomp.)
(°C)
\
(cm2 V−1
Analysis, % found (calculated)
1H-NMR
M
mol−1) in 10−3
DMSO–acetone
M
Hg/Sn
M
S
OCH2CH3
4.22 (m, 4H)
OCH2CH3
C6H5
[Bu2Sn][Cu(ecda)2]
C20H28N2O4SnCuS4
[PhHg]2[Cd(ecda)2]
C24H20N2O4Hg2CdS4
[PhHg]2[Ni(ecda)2]
C24H20N2O4Hg2NiS4 (72)
[Ph3Sn]2[Ni(ecda)2]
C48H40N2O4Sn2CuS4 (78)
[PhHg]2[Cu(ecda)2]
Red (60)
115
87.4
85.0
21.2
27.8
6.7
17.71 (17.89)
38.08 (38.19)
40.44 (41.05)
20.00 (19.81)
40.15 (40.78)
40.16 (40.25)
9.40 (8.99)
10.75 (10.89)
5.91 (5.90)
5.13 (5.09)
6.30 (6.59)
6.54 (6.49)
9.10 (8.99)
19.09 (19.09)
12.24 (12.30)
12.90 (12.89)
11.21 (11.29)
13.84 (13.99)
12.81 (12.81)
18.35 (18.30)
(0.88–1.77)
Yellow (65)
Yellowish green
135
4.20 (q, 4H,
J=7.5 Hz)
4.33 (q, 4H,
J=6.6 Hz)
4.26 (q, 4H,
J=7.0 Hz)
4.22 (q, 4H)
1.26 (t, 6H, 7.46 (m, 10H)
J=7.5 Hz)
1.53 (t, 6H, 7.60 (m, 10H)
J=6.6 Hz)
1.24 (t, 6H, 7.50 (m, 30H)
J=7.0 Hz)
(140)
110
Reddish brown
Yellowish brown
(130)
129
1.24(t, 6H)
7.50 (m, 10H)
/
C24H20N2O4Hg2CuS4 (70)
[PhHg]2[Zn(ecda)2]
C24H20N2O4Hg2ZnS4
[NEt4]2[Cu(ecda)2]
Yellow (65)
90.5
108.5
4.22 (q, 4H,
J=7.5 Hz)
1.24 (t, 6H, 7.50 (m, 10H)
J=7.4 Hz)
Brown
155
C28H50N4O4CuS4
a Signals for –CH3 of ecda and Bu groups are merged.
Table 2
Selected IR [Raman] bands (cm−1) and UV–visible spectral data of the complexes
605
Complex
w(CꢀN)
w(CꢂO)
w(CꢂCS2)
w(CꢁS)
w(SnꢁC)–w(HgꢁC)
w(MꢁS)
w(SnꢁS)–w(HgꢁS)
Solvent
umax (nm) (m (dm3 mol−1 cm−1))
(
[Bu2Sn][Cu(ecda)2]
2210
1684
1458
1175
596, 460
375
280
Acetonitrile
Acetone
230, 330, 432(1196), 600 (342)
430 (1435)
102
[PhHg]2[Cd(ecda)2]
[PhHg]2[Ni(ecda)2]
2190 [2193]
2200
1695 [1690]
1689
1430 [1421]
1430
1141 [1158]
1140
670, 460 [660, 450]
675, 452
350 [360]
380
260 [255]
260
108
Acetonitrile
Acetone
600
345 (25,000), 370 (30,100), 398
(23,840), 450 (1757), 600 (156)
220, 340, 440, 580
340, (13,560), 350 (13,550), 405
(5990), 442 (8934)
[Ph3Sn]2[Ni(ecda)2]
2190
1620
1429
1175
600, 450
375
300
Acetonitrile
Acetone
[PhHg]2[Cu(ecda)2]
[PhHg]2[Zn(ecda)2]
[NEt4]2[Cd(ecda)2]
2195
2200 [2193]
2192
1665
1690 [1689]
1675
1438
1430 [1421]
1460
1140
1158 [1158]
1170
660, 450
670, 455 [660, 460]
370
360 [375]
370
255
255 [250]
Acetonitrile
DMSO
240, 330, 450
342, 395, 620

106
N. Singh, S. Kumar Rai / Journal of Organometallic Chemistry 605 (2000) 102–108
Fig. 2. Cyclic voltammogram of 1 mM [NEt4]2[Cu(ecda)2] in acetoni-
trile at Pt electrode recorded at different scan rates starting from 40
mV s−1 (curve I) and increasing in steps of 20 mV s−1
.
w(CꢂC) and a little variation (ꢀ15 cm−1) in w(CꢁS)
from the free ligand, K2ecda spectrum to that of the
complexes. (b) The occurrence of SnꢁC and HgꢁC
stretching vibrations at 596–670 and 450–460 cm−1 re-
spectively in the organoheterobimetallic complexes [22].
(c) Appearance of a single sharp band near 2200 cm−1 in
the complexes because of their higher symmetry com-
pared to the splitted bands at 2160 and 2190 cm−1 in the
ligand K2ecda. In the lower frequency region (400–200
cm−1), the absorptions occurring in 350–380 and 250–
300 cm−1 region have been assigned to w(M/SnꢁS)
modes, respectively [23].
Fig. 1. Temperature dependence of the electrical conductivities of the
complexes.
3.2. Vibrational spectra
Apart from the NEt+4 and coordinated Ph as well as
Bu groups [22], the complexes exhibit strong IR absorp-
tions (Table 2) near 2190–2210, 1660–1690, 1420–1460
and 1140–1175 cm−1 corresponding to w(CꢀN), w(CꢂO),
w(CꢂC) and w(CꢁS) modes characteristic of the ecda2−
in a (S,S) chelating–chelating-bridging manner [10,14].
A comparison of the infrared and Raman spectra ob-
tained for [PhHg]2[Zn(ecda)2] and [PhHg]2[Cd(ecda)2]
shows that Raman bands typically agree well with those
obtained by infrared spectroscopy for w(CꢀN), w(CꢂO),
w(CꢂC), w(CꢁS), w(SnꢁC), w(HgꢁS) and w(MꢁS) [24] fre-
quencies. Both in the IR and Raman spectra, peaks cor-
responding to the phenyl group and its monosubstituted
analogue [22] have been found at around 1500, 1600, 727
The three important observations of the spectra are:
(a) The high energy shift (52–92 cm−1) of the strong
and 674 cm−1
.
Fig. 3. (a) Cyclic voltammogram of 1 mM [Bu2Sn][Cu(ecda)2] in acetonitrile at Pt electrode recorded at different scan rates (cathodic region)
starting from 40 mV s−1 (curve I) and increasing in steps of 20 mV s−1. (b) Scan in anodic region (20 mV s−1).

N. Singh, S. Kumar Rai / Journal of Organometallic Chemistry 605 (2000) 102–108
107
3.3. NMR spectra
All the complexes show well resolved 1H-NMR
signals in the aliphatic and aromatic region (Table 1)
and integrate well for the corresponding number of
protons. As compared to the free ligand K2ecda, the
OCH2 and CH3 protons are marginally downfield
shifted in the complexes. Consistent with UV–visible
1
spectra, appearance of narrow and unshifted H-NMR
resonances show diamagnetic, square planar coordina-
tion around nickel(II) in [PhHg]2[Ni(ecda)2] in the
solution. This indicates that the solid state interactions
bringing octahedral geometry about nickel(II) exist no
more in the solution. Consistent with magnetic be-
haviour and appearance of unbroadened and unshifted
1H-NMR resonances for [PhHg]2[Cu(ecda)2] clearly
indicates that dominant Cu–Cu interaction present in
the solid state is preserved in the solution while the
peak broadening for [Bu2Sn][Cu(ecda)2] shows that
strong Cu–Cu interaction, responsible for its diamag-
netism in the solid is not retained in the solution.
The 13C-NMR spectra of the complexes show signals
for CH3 (18.4–23.68), OCH2 (52.4), CꢂC (90), CꢀN
(120.8–122.4), C6H5 (128.8–129.6) and CꢂO (l 174
ppm) carbons of the ecda2− and phenyl group.
3.4. Solid state electrical conducti6ity
The conductivity measurements of the pressed pellets
were made in the temperature range of 293–343 K
using conventional two probe technique (on a Keithely
236 SMU). The room temperature conductivity, |rt=
1.78×10−9 to 1.41×10−10 S cm−1 for the com-
pounds is low. In the case of [Bu2Sn][Cu(ecda)2],
[Ph3Sn]2[Ni(ecda)2] and [PhHg]2[Cu(ecda)2], the con-
ductivity increases with the increase in temperature
(Fig. 1) with energy band gaps, Ea, 0.39, 0.25 and 0.20
eV, respectively, showing their semiconducting
behaviour. Interestingly organoheterobimetallic dithio
complexes containing Ni(II) and Cu(II) which show
greater tendency to form square planar p-delocalized
extended S…S stacked structure, exhibit semicon-
ducting behaviour compared to those electrically
insulating materials containing Zn(II), Cd(II), or Hg(II)
which possess preferential tetrahedral/octahedral
geometry where a columnar stacked arrangement is less
feasible.
Fig. 4. Suggested structure of the complexes.
cess. The second reduction near −0.7 V which is much
stronger than the first reduction is probably ligand
based since dithiolene complexes are well known to
exhibit such processes [25]. In Fig. 3a the reduction
peak at −0.90 V and its reoxidation peak at −0.53 V
are too widely separated to be accounted due to one
species. Probably, some chemical reaction must have
occurred following the reduction at −0.90 V. The
oxidation peak at +0.80 V and its corresponding
reduction at +0.65 V seems to have involved metal
3.5. Cyclic 6oltammetry
The electrochemical behaviour of [NEt4]2[Cu(ecda)2]
and [Bu2Sn][Cu(ecda)2] was measured by cyclic voltam-
metry in acetonitrile solution using NaClO4 as support-
ing electrolyte. The cyclic voltammogram (Fig. 2) for
[NEt4]2[Cu(ecda)2] shows that this salt undergoes a
quasi reversible reduction at −0.1 V for Cu(II/I) pro-

108
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EPR and UV–visible spectra, these findings indicate
that the copper is present as Cu(II), d9 in these
complexes.
Despite our best efforts, we could not grow single
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characterization. On the basis of foregoing discussion
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Acknowledgements
One of the authors, Sanjay Kumar Rai is thankful to
UGC, New Delhi for financial assistance in the form of
JRF.
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