Effect of solute-co-ordinating solvent interactions and temperature on the EPR and electronic spectra of bis(dithiophosphato)copper(II)

Article abstract of DOI:10.1016/S1386-1425(01)00707-7

The self-redox reaction proceeding between two molecules of the complex bis(disubstituted-dithiophosphato)copper(II), Cu^II(R₂-dtp)₂, is studied by EPR and UV-VIS spectroscopy in DMFA, DMSO and pyridine. The effect of temperature and disulphide concentration in the solutions is also evaluated. The EPR spectra show that the g-values of Cu^II(R₂-dtp)₂ increase when it is dissolved in co-ordinating solvents, whereas the copper hyperfine splitting decreases compared to the corresponding values in non-co-ordinating solvents. Under the same conditions, a hypsochromic shift is observed in the maximal absorption at 420 nm of the electronic spectra which corresponds to the ligand-to-metal charge-transfer (LMCT) transition of the complex. The results are explained with the formation of axial or equatorial adducts between Cu^II(R₂-dtp)₂ and the co-ordinating solvents used. On the other hand, the molar absorptivity of the LMCT band and the intensity of the EPR spectrum increase strongly with the nature of the used co-ordinating solvent, the time after dissolution and the quantity of added disulphide. Both also depend on the size and shape of remote ligand substituents and they increase in the order Me2P(S)S-S(S)P(RO)₂] to the Cu^II(R₂-dtp)₂ solution. As a result, the molar absorptivity value at the maximum of the LMCT band of Cu[(i-PrO)₂-dtp]₂ increases from 7.9×10³ m^-1dm³cm^-1 immediately after dissolution to 2.9×10⁴ m^-1dm³cm^-1. In DMSO and pyridine, the intensity of both the EPR signal and LMCT band of Cu^II(R₂-dtp)₂ continuously decrease after the preparation of the solutions. A small increase is only observed immediately after the addition of the corresponding disulphide of dithiophosphate. While DMFA forms stable adducts with Cu[(i-PrO)₂-dtp]₂, adduct formation with DMSO and pyridine destroys the initial complex.

Full text of DOI:10.1016/S1386-1425(01)00707-7

Spectrochimica Acta Part A 58 (2002) 1171–1180
www.elsevier.com/locate/saa
Effect of solute-co-ordinating solvent interactions and
temperature on the EPR and electronic spectra of
bis(dithiophosphato)copper(II)
Nicola D. Yordanov *, Kalina Ranguelova
Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Received 1 July 2001; accepted 1 October 2001
Abstract
The self-redox reaction proceeding between two molecules of the complex bis(disubstituted-dithiophos-
phato)copper(II), CuII(R2-dtp)2, is studied by EPR and UV–VIS spectroscopy in DMFA, DMSO and pyridine. The
effect of temperature and disulphide concentration in the solutions is also evaluated. The EPR spectra show that the
g-values of CuII(R2-dtp)2 increase when it is dissolved in co-ordinating solvents, whereas the copper hyperfine splitting
decreases compared to the corresponding values in non-co-ordinating solvents. Under the same conditions, a
hypsochromic shift is observed in the maximal absorption at 420 nm of the electronic spectra which corresponds to
the ligand-to-metal charge-transfer (LMCT) transition of the complex. The results are explained with the formation
of axial or equatorial adducts between CuII(R2-dtp)2 and the co-ordinating solvents used. On the other hand, the
molar absorptivity of the LMCT band and the intensity of the EPR spectrum increase strongly with the nature of the
used co-ordinating solvent, the time after dissolution and the quantity of added disulphide. Both also depend on the
size and shape of remote ligand substituents and they increase in the order MeBEtBi-Pr. Beer’s law is not obeyed
immediately after dissolution of copper bis-dithiophosphate complexes. However, after standing for 24 h in the dark,
DMFA solutions exhibit linear absorption/concentration dependence with :70% higher molar absorptivity. An
additional increase of the LMCT band and EPR intensity is found after heating the solution up to 50 °C for a short
time, as well as after addition of the corresponding disulphide of dithiophosphate [(RO)2P(S)S–S(S)P(RO)2] to the
CuII(R2-dtp)2 solution. As a result, the molar absorptivity value at the maximum of the LMCT band of Cu[(i-PrO)2-
dtp]2 increases from 7.9×103 m−1 dm3 cm−1 immediately after dissolution to 2.9×104 m−1 dm3 cm−1. In DMSO
and pyridine, the intensity of both the EPR signal and LMCT band of CuII(R2-dtp)2 continuously decrease after the
preparation of the solutions. A small increase is only observed immediately after the addition of the corresponding
disulphide of dithiophosphate. While DMFA forms stable adducts with Cu[(i-PrO)2-dtp]2, adduct formation with
DMSO and pyridine destroys the initial complex. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: bis(Disubstituted-dithiophosphato)copper(II); Molar absorptivity; Pyridine; EPR; Co-ordinating solvents; Solvent inter-
actions
* Corresponding author. Tel.: +359-2-979-2546; fax: +359-2-756-116.
E-mail address: ndyepr@bas.bg, ndyepr@ic.bas.bg (N.D. Yordanov).
1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(01)00707-7

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N.D. Yordano6, K. Ranguelo6a / Spectrochimica Acta Part A 58 (2002) 1171–1180
1. Introduction
formation of adducts with an equatorial co-ordi-
nation with one or two base molecules [8,9]. In
the latter case, the structure and the spectral
properties of the initial complex are dramatically
changed by the formation of a complex with a
new chromophore. Moreover, very strong Lewis
bases reduce copper(II) giving free radical from
the base molecule [11].
All studies reported up to now about the equi-
librium Eq. (2), show that many factors such as
the shape and the size of the remote ligand sub-
stituents, the solvent nature, the temperature, etc.,
can change the electronic structure of the metal
complex. On the other hand, the spectral proper-
ties of CuII(R2-dtp)2 are also affected by the self-
redox reaction Eq. (1) [4] with significant changes
in them. Bearing these effects in mind, we extend
here our studies on the influence of the solute–po-
lar solvents interactions using DMFA, DMSO
and pyridine on the spectral properties of CuII(R2-
dtp)2. Data obtained in toluene and CHCl3 [1] are
used as a basis for comparison. The results can be
of significant importance for the analytical estima-
tion of copper(II) in the form of CuII(R2-dtp)2
[12,13].
In a previous paper from this laboratory [1], we
have reported the influence of the size and shape
of the remote ligand substituents, of non-polar
and non-co-ordinating solvents, of temperature
and the addition of disulphide, (RO)2P(S)S–
S(S)P(RO)2, on the spectral (UV–VIS, EPR)
properties of bis(disubstituted-dithiophosphato)-
copper(II) complexes, CuII(R2-dtp)2. While the
EPR parameters remained unchanged under all
conditions, significant changes were observed in
the intensities of the EPR signal and the ligand-
to-metal charge-transfer (LMCT) band. The ob-
served molar absorptivity was found to vary
between 5×103 and 2×104 m−1 dm3 cm−1 de-
pending on the experimental conditions. These
spectral changes were explained with: (i) an in-
tramolecular electron transfer (self-redox) reac-
tion Eq. (1) [1–4]:
2CuII(dtp)2l[CuII(dtp)2…CuII(dtp)2]
l2CuI(dtp)+ds (ds=dtp−dtp)
(1)
and (ii) specific CuII(R2-dtp)2-solute–solvent inter-
actions affecting the equilibrium reaction Eq. (1).
On the other hand, we have found in previous
studies [5–9] that bis(dithiophosphato)Cu(II)
complexes react with basic molecules and form
1:1 or 1:2 adducts with axial or equatorial co-
ordination of the base molecule towards the
chromophore CuS4 of CuII(R2-dtp)2 (Eq. (2)).
2. Experimental
2.1. Starting materials
CuCl2·2H2O (p.a.) and P2S5 were obtained from
Merck and used without further purification. Dif-
ferent alcohols MeOH, EtOH, i-PrOH were dis-
tilled before use.
CuL2+nBlCuL2Bn
(2)
The main effect of axial adduct-formation on
the electronic and EPR spectra of the metal com-
plex consists as [6,8–10]:
The solvents DMFA, DMSO, pyridine, toluene
and CHCl3 were dried after distillation as de-
scribed [14] and kept in desicator.
1. a bathochromic shift of the d–d bands in the
electronic absorption spectra.
2.2. Synthesis of the ligands, complex and
2. an increase of the g-factor and a decrease of
the hyperfine (hfA) and superhyperfine (shfA)
splitting constants in the EPR spectra due to a
decreased metal–ligand bonds covalence, and
the appearance of a rhombic distortion in the
equatorial plane of the complex.
disulphide preparations
The ligands, [(RO)2-P(S)S]Na, with R=Me, Et,
i-Pr, were synthesized in our laboratory as previ-
ously described [1,15].
The synthesis is carried out in a 500 ml three-
necked round-bottomed flask equipped with a
mechanical stirrer in the central neck, nitrogen
Axial adducts may be stable in time but on the
other hand, they may play a role of precursor in

N.D. Yordano6, K. Ranguelo6a / Spectrochimica Acta Part A 58 (2002) 1171–1180
1173
inlet in one of the side necks and with a Liebig
3. Instrumentation
condenser on the other one. As hydrogen sul-
phide is liberated during the reaction, the upper
part of the condenser is connected to a trap for
H2S. This consisted of a washing vessel filled
with lead acetate. At the beginning of the reac-
tion, the flask is kept in a cold-water bath. The
corresponding dry alcohol (a 1:2 mixture with
benzene) is inserted into the flask via the neck
connected with nitrogen inlet and is saturated
with N2. Under stirring and a continuous weak
stream of N2, a stoichiometric quantity of fine
grinded P2S5 is added to the flask in small por-
tions so as to keep the temperature of the reac-
tion below 40 °C. After the entire quantity of
P2S5 is added, the temperature is increased to
60 °C for 1 h. The heating is then stopped.
When the reaction mixture reaches room temper-
ature, it is filtered and the filtrate containing the
The EPR spectra were taken on an X-band
Bruker ER 200D SRC spectrometer using 100
kHz modulation of the magnetic field. The sam-
ple solutions (0.3 ml) were accommodated in
EPR capillary quartz sample tubes with i.d. 1.2
mm and o.d. 3.5 mm. In order to avoid the
non-linearity in the EPR response due to the
cavity used [16], the sample tubes were symmetri-
cally inserted throughout the standard Bruker
EPR cavity ER 4102ST (mode TE102). In addi-
tion to this, an EPR standard—Mn2+/MgO [17]
was used in order to get semi-quantitative results
from the EPR measurements (as a ratio of the
CuII(R2-dtp)2 versus Mn2+ intensity) and to
compare them with the results from the elec-
tronic spectra. The variable temperature studies
were carried out with an ER 4111VT accessory
in the temperature interval 293–343 K.
benzene
solution
of
the
corresponding
RO2P(S)SH is transferred to a two-necked flask-
motor stirred in the central neck—and the acid
is neutralized by adding of small portions of
water solution of sodium hydroxides under inten-
sive agitation. The quantity of NaOH is calcu-
lated to 60–70% yield of the acid. The
corresponding salt is precipitated, separated,
dried and recrystallized from acetone. The pure
RO2P(S)SK(Na) and specially those with R=
Me, Et, i-Pr are kept in well stoppered vessels
because of the unpleasant odour.
Copper-dithiophosphate complexes with differ-
ent remote ligand substituents were obtained by
mixing of water solutions of corresponding
(RO)2P(S)SNa (R=Me, Et, i-Pr) with water so-
lutions of copper(II) according to the reactions:
The electronic absorption spectra were
recorded on a Specord UV–VIS (Carl-Zeiss
Jena) spectrometer using quartz cells (0.2, 1.0,
2.0 and 5.0 cm path length) in the 300–800 nm
region. A special home-made thermostatic cell
jacket, connected to water thermostat was used
to record the spectra in the temperature interval
293–343 K with an accuracy of 90.1 °C.
All the detected absorbance values were recal-
culated to a 1 cm path length.
The concentration of CuII(R2-dtp)2 was varied
from 5×10−3 (for EPR measurements) to 1×
10−4 M (for spectrophotometric studies), while
the appropriate disulphide concentration was
kept at 5×10−2 M.
In the variable temperature measurements, the
solutions were stored for at least 30 min in the
water thermostat before use.
2(RO)2P(S)SNa+CuCl2
l[(RO)2P(S)S]2Cu+2NaCl
(3)
The formed precipitate was collected and dried
as described in [1,15].
4. Results and discussion
Disulphides of the appropriate dithiophos-
phates were obtained by oxidation of the corre-
sponding (RO)2P(S)SNa dissolved in water, by
the drop wise addition of an alcohol solution of
I2. The precipitate formed was collected, dried
and purified by recrystalization from chloroform.
4.1. Features of the EPR and electronic spectra
of Cu(R2-dtp)2 complexes
All the copper dithiophosphate complexes with
R=Me, Et, i-Pr remote ligand substituents ex-
hibit a brown colour and a strong specific EPR

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N.D. Yordano6, K. Ranguelo6a / Spectrochimica Acta Part A 58 (2002) 1171–1180
Table 2
spectrum in solution. However, they are pale-yel-
low and EPR silent in the solid state as reported
previously [2,4], because of the self-redox reaction
Eq. (1) [4]. The absorption band, appearing at 420
nm in the electronic spectra in non-co-ordinating
solvents, is assigned to a LMCT band. Its posi-
tion is independent of the shape and size of the
remote ligand substituents, the temperature (293–
343 K) and the presence of disulphide in solution.
Experimental data for molar absorptivity (m) obtained 1 h after
preparing of the solutions
Compl./Solv. DMFA
(×103)
DMSO
Pyridine
(×103)
(×103)
Cu(i-Pr2-dtp)2 7.90
11.20
7.20
Cu(Et2-dtp)2 4.50
Cu(Me2-dtp)2 2.90
6.50
4.90
4.00
2.60
Fig. 2. Dependence of the absorbance (at 420 nm) of CuII(R2-
dtp)2, R=i-Pr, Et, Me, in DMFA, as a function of the
concentration of the complexes. The electronic spectra are
taken 1 h after preparation of solutions. The detected ab-
sorbances are recalculated to 1 cm quartz cell.
All the CuII(R2-dtp)2 complexes studied in
DMFA, DMSO, toluene and CHCl3 solutions,
are also characterized by an absorption band at
420 nm, which is strongly hypsohromically shifted
and peaks at 345 nm in pyridine. This effect arises
because DMFA and DMSO are weak Lewis bases
slightly affecting the equilibrium Eq. (2), whereas
pyridine is strongly shifts the equilibrium towards
adduct formation. These conclusions are sup-
ported by the parameters of the recorded EPR
spectra (vide infra).
Fig. 1. EPR spectra of CuII(i-Pr2-dtp)2 complex dissolved in
the used solvents (1, CHCl3; 2, DMFA; 3, DMSO; 4, Pyri-
dine). Mn2+/MgO is used as a standard for position of the
spectra on the magnetic field axis.
Table 1
EPR parameters of the Cu(R2-dtp)2 in used solvents
The shape of the EPR spectra of CuII(R2-dtp)2
dissolved in co-ordinating solvents at 293 K ex-
hibits all the features of the chromofore CuS4
[15,18] (Fig. 1). It is characterized by a hyperfine
splitting arising from the interaction of copper(II)
unpaired electron with 63,65Cu nuclei (I=3/2). In
addition, the EPR spectra of CuII[(RO)2-dtp]2 ex-
Solvent
g (90.001)
hfA (90.5 G)
Pyridine
DMSO
DMFA
Toluene or
CHCl3
2.067
2.057
2.053
2.049
56.5
64.8
68.2
74.2

N.D. Yordano6, K. Ranguelo6a / Spectrochimica Acta Part A 58 (2002) 1171–1180
1175
toluene(CHCl3)BDMFABDMSOBPyridine
hibit a superhyperfine (shf) splitting. Its intensity
(4)
ratio of 1:2:1 indicates the interaction of the cop-
per(II) unpaired electron with two equivalent 31P
nuclei from the ligands and is unambiguous evi-
dence for the presence of the CuII(R2-dtp)2. In
some co-ordinating solvents the shf splitting is not
resolved due to the broadening of the EPR lines.
The isotropic EPR parameters are independent of
the shape and size of the remote ligand sub-
stituents, the temperature (293–343 K) and the
presence of disulphide in the solution. However,
they change in the co-ordinating solvents used
because of specific interaction with them. Fig. 1
shows the shift of the EPR spectra to lower
magnetic field values. The isotropic EPR parame-
ters are given in Table 1 (toluene or CHCl3 [1] are
given for comparison as a non-co-ordinating sol-
vents). As seen from Table 1, the g-factors of
CuII(R2-dtp)2 complexes increase hfA decrease in
the following order (4) of solvents:
These changes in the EPR parameters may be
considered as evidence for adduct formation [7–9].
The order (4) suggests the weakest adduct forma-
tion to be with DMFA and strongest with pyridine.
4.2. Effect of sol6ent and time after dissolution
on the intensity of the EPR and electronic spectra
of CuII(R2-dtp)2 and their stability
One hour after the dissolution of equimolar
quantities of CuII(R2-dtp)2 in the solvents the
magnitudes of the absorption band in the elec-
tronic spectra are found to be solvent dependent
(Table 2). On the other hand, Fig. 2 shows a
non-linear dependence of A420 on concentration,
measured 1 h after the preparation of solutions in
DMFA, so that Beer’s law is not obeyed. On
keeping DMFA solutions of CuII(R2-dtp)2 for
Fig. 3. Dependence of the absorbance (at 420 nm) of CuII(R2-dtp)2, R=i-Pr, Et, Me, in DMFA, as a function of the concentration
of the complexes. The electronic spectra are taken 24 h after preparation of solutions. The detected absorbances are recalculated to
1 cm quartz cell.

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Fig. 4. Dependence of the absorbance (at 420 nm) of CuII(R2-dtp)2, R=i-Pr, Et, Me, in DMFA as a function of the time. The
detected absorbances are recalculated to 1 cm quartz cell.
4.3. Effect of remote ligand substituent (R) on the
24 h at room temperature, a linear dependence
of A420 on concentration is observed with the
EPR and electronic spectra of CuII(R2-dtp)2
absorbance values being almost twice as high as
those obtained 1 h after dissolution (Fig. 3).
Thereafter the absorption remains constant for a
period of up to 20 days (Fig. 4). The calculated
The comparison of the electronic spectra of
equimolar solutions of CuII(R2-dtp)2 with differ-
ent remote ligand substituents (R=Me, Et, i-Pr)
dissolved in DMFA (Figs. 2 and 3) clearly shows
the following order of decreasing absorbance as
a function of the remote ligand substituent:
values of the effective molar absorptivities (mt=24
)
are shown in Table 3. The highest molar absorp-
tivity value of CuII(R2-dtp)2 in DMFA compared
to the other solvents can be explained by the
formation of axial adducts. In DMSO and pyri-
dine, the absorbance decreases with time after
dissolution (Tables 2 and 3, Fig. 5). The reason
is that the axial adduct in these solvents is a
precursor of the equatorial co-ordination of the
solvent molecule, followed by the destruction of
the complex. The sets of EPR spectra given in
Figs. 6 and 7 show the decrease and disappear-
ance of the EPR signal several days after prepa-
ration of the solution and this corresponds to the
changes in the molar absorptivity data given in
Tables 2 and 3.
Cu(i-Pr2-dtp)2\Cu(Et2-dtp)2\Cu(Me2-dtp)2
(5)
Table 3
Experimental data for molar absorptivity (m) obtained after 24
h staying of the solutions
Compl./Solv. DMFA
(×103)
DMSO
Pyridine
(×103)
(×103)
Cu(i-Pr2-dtp)2 13.3
10.50
5.90
Cu(Et2-dtp)2 7.40
Cu(Me2-dtp)2 4.70
6.10
4.60
3.30
2.10

N.D. Yordano6, K. Ranguelo6a / Spectrochimica Acta Part A 58 (2002) 1171–1180
1177
Fig. 5. Dependence of the absorbance (at 420 nm) of CuII(R2-dtp)2, R=i-Pr, Et, Me, dissolved in DMSO as a function of the time.
The detected absorbances are recalculated to 1 cm quartz cell.
The intensity of the EPR spectra follows the same
trend.
preparation were recorded at different tempera-
tures in the interval 20–50 °C. The molar absorb-
tivity values increased by :50% on heating the
DMFA solutions up to 50 °C (Table 4). The
amplitudes remain unchanged for 4–5 days after
cooling the solution to room temperature. The
same changes are observed in the EPR spectra.
Table 4 shows a different behaviour in DMSO
and pyridine. Now, the intensity of the electronic
and EPR spectra decreases by :10% in DMSO
and 30% in Pyridine. After cooling the solutions
to room temperature both intensities continue to
decrease.
This effect cannot be explained by changes in
the electronic structure of the complexes due to
the different shapes and sizes of the remote ligand
substituents, since the recorded EPR parameters
(g, hfA, shfA) of all the complexes dissolved in the
appropriate solvent are the same [15,18]. From
our point of view, this observation may be ex-
plained by the steric effect of the remote ligand
substituents since the bulkier is R, the more equi-
librium Eq. (1) is shifted to CuII(R2-dtp)2 [4].
Therefore, the higher absorbance of CuII(i-Pr2-
dtp)2 may be explained by its higher concentra-
tion compared to Cu(Me2-dtp)2 and Cu(Et2-dtp)2.
4.5. Effect of added disulphide on the intensity of
the electronic and EPR spectra of CuII(R2-dtp)2
solutions
4.4. Effect of temperature on the intensity of the
EPR and electronic spectra of CuII(R2-dtp)2 solu-
tions
Studies of the electronic absorption and EPR
spectra of CuII(R2-dtp)2 solutions (kept after
preparation for at least 24 h in the dark) were
carried out after the addition of portions of the
The electronic absorption and EPR spectra of
CuII(R2-dtp)2 solutions kept at least 24 h after

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N.D. Yordano6, K. Ranguelo6a / Spectrochimica Acta Part A 58 (2002) 1171–1180
Table 5
Experimental data for (m) obtained after adding of disulphide
Compl./Solv. DMFA
(×104)
DMSO
Pyridine
(×103)
Cu(i-Pr2-dtp)2 2.9
22.0
13.1
9.4
9.60×104
5.30×104
3.40×103
Cu(Et2-dtp)2
1.62
Cu(Me2-dtp)2 1.03
the EPR intensity after the addition of the appro-
priate disulphide. This proves the increase of
CuII(R2-dtp)2 concentration. This effect of disul-
phides on the absorption and EPR intensity of
CuII(R2-dtp)2 can be explained by a shift of equi-
librium Eq. (1) to the left. The obtained ab-
sorbance and EPR intensity remain unchanged in
DMFA but they decrease in DMSO and pyridine
(Fig. 8) because of complex destruction.
Fig. 6. Decreasing of the EPR signal of CuII(i-Pr2-dtp)2 dis-
solved in DMSO as a function of the time. Mn2+/MgO is
used as a standard for intensity calibration.
appropriate disulphide, dissolved in the same sol-
vent. Significantly higher molar absorbtivity val-
ues were obtained after correction for dissolution
(Table 5). The maximum value found is 2.9×104
m
−1 cm−1 (for i-Pr derivative in DMFA). The
EPR measurements carried out under the same
experimental conditions show a similar increase in
Table 4
Experimental data for (m) obtained after heating the solutions
till 50 °C
Compl./Solv. DMFA
(×103)
DMSO
Pyridine
(×103)
(×103)
Cu(i-Pr2-dtp)2 2.2×104
9.6
4.5
Fig. 7. Decreasing of the EPR signal of CuII(i-Pr2-dtp)2 dis-
solved in pyridine as a function of the time (1–3) and its
increasing (4) after adding of disulphide. Mn2+/MgO is used
as a standard for intensity calibration.
Cu(Et2-dtp)2 12.1×103
Cu(Me2-dtp)2 7.7×103
5.6
4.2
2.5
1.6

N.D. Yordano6, K. Ranguelo6a / Spectrochimica Acta Part A 58 (2002) 1171–1180
1179
Fig. 8. Decreasing of the absorbance of CuII(R2-dtp)2, R=i-Pr, Et, Me, dissolved in DMSO as a function of the time after adding
of disulphide.
5. Conclusions
effect of solvent, remote ligand substituent, tem-
perature and disulphide on the molar absorptivity
we have to consider very carefully the results of
analytical estimations carried out with these
complexes.
The dependence of molar absorptivity and EPR
intensity of CuII(R2-dtp)2 complexes on the nature
of the co-ordinating solvents are reported. It is
found that these solvents form adducts with the
complexes, axially (in DMFA) or equatorially (in
DMSO and pyridine) co-ordinated with respect to
the plane of the chromophore CuS4. The effect of
the shape and size of the remote ligand sub-
stituent, the temperature and the quantity of the
added disulphide on the molar absorptivity of the
LMCT band and EPR intensity is discussed.
Beer’s law is not obeyed immediately after disso-
lution because of the self-redox reaction of
CuII(R2-dtp)2 yielding CuI(R2-dtp) and disulphide.
After keeping the solutions for several hours,
heating up to 50 °C and addition of disulphide
the molar absorptivity is increased up to 2.9×104
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