The chromate ion as a ligand. Physicochemical studies of nickel(II) chromate complexes with 2,2′-bipyridine

Article abstract of DOI:10.1016/S0040-6031(97)00300-6

The preparation of two novel nickel chromate complexes with 2,2′-bipyridine is described. The results of thermal, spectral, magnetic and scanning microscopic studies are analyzed. The data show that the complexes are monomers with pseudooctahedral arrangement around the nickel ion. The coordination of the chromate ion has been found in one of the complexes. The TG-DTG-DTA data show that the thermal decomposition of the complexes occurs in two and three stages in air and four stages in He atmosphere. The decomposition of the complexes has been discussed and compared to that known for other metal chromate complexes with bpy.

Full text of DOI:10.1016/S0040-6031(97)00300-6

Thermochimica Acta 313 (1998) 9±17
The chromate ion as a ligand. Physicochemical studies of
nickel(II) chromate complexes with 2,20-bipyridine
a
A. Repka , M. Cieslak-Golonka , W. Surga , K. Zurowski
b,*
a
a
Ç
Â
a Institute of Chemistry, Pedagogical University, 25-020 Kielce, Poland
bInstitute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University, 50-370 Wrocaw, Poland
Received 28 February 1997; accepted 16 June 1997
Abstract
The preparation of two novel nickel chromate complexes with 2,20-bipyridine is described. The results of thermal, spectral,
magnetic and scanning microscopic studies are analyzed. The data show that the complexes are monomers with
pseudooctahedral arrangement around the nickel ion. The coordination of the chromate ion has been found in one of the
complexes. The TG±DTG±DTA data show that the thermal decomposition of the complexes occurs in two and three stages in
air and four stages in He atmosphere. The decomposition of the complexes has been discussed and compared to that known for
other metal chromate complexes with bpy. # 1998 Elsevier Science B.V.
Keywords: Chromate ion; Nickel chromate complexes; 2, 20-bipyridine; Physiochemical studies
1. Introduction
here the syntheses and physicochemical studies
(thermal decomposition, spectroscopy UV/Vis, IR,
FIR, magnetic and scanning microscopy data) for
two novel nickel chromate complexes with 2,20-
bipyridine (bpy).
One of the possible routs for the detoxication of the
carcinogenic Cr(VI) species is the process of its
complexation [1,2]. This could limit the reduction
of Cr(VI) ion [1] a condition sine qua non for the
mutagenicity of chromium [3], or even completely
abolish it [4]. The coordination model of Cr(VI)
genotoxicity was built on the physicochemical and
biological studies of copper(II) chromate and dichro-
mate complexes with 2,20-bipyridine (bpy) and 1,10-
phenanthroline (phen) [1]. Recently, it has been sup-
ported with the studies on copper(II) complexes with
imidazoles [4].
2. Experimental
2.1. Preparation of the compounds
2.1.1. Green complex: [Ni bpy(H2O)4]CrO4 [A]
A methanolic solution of 2,20-bipyridine (47 cm3,
0.75 M) was added dropwise upon stirring water
solution of K2CrO4 (140 cm3, 0.25 M). The yellow
bipyridinium chromate precipitating upon the bpy
addition disappears when 30 cm3 out of 140 cm3 of
the nickel sulphate (chloride, nitrate) (0.25 M) was
This paper presents the results of further investiga-
tion on (metal ion±bpy±chromate) system. We report
*Corresponding author. e-mail: Golonka@ichn.ch.pwr.wroc.pl
0040-6031/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved
P I I S 0 0 4 0 - 6 0 3 1 ( 9 7 ) 0 0 3 0 0 - 6

10
A. Repka et al. / Thermochimica Acta 313 (1998) 9±17
added. The color of the solution was brown, slightly
turbid. The solution was allowed to stand 24 h and the
resulting light green powder has been ®ltered and
washed with water. The precipitate was dried for ®ve
days over silica gel, after that three extra days in the
vacuum box and ®nally two days in the dry box at
408C.
FIR spectra were obtained in Nujoll mulls sandwiched
between polyethylene plates.
The diffuse re¯ectance, room and liquid nitrogen
temperature absorption spectra were recorded in the
1
wavenumber range of 12000±49000 cm
with a
Hitachi Model 356 UV/Vis spectrophotometer con-
nected to an IBM/PC computer.
The green compound has been also obtained from
NiCl2 and Ni2SO4 in a similar way using Ni : bpy : Cr
(1 : 2 : 1) proportions (Table 2).
The solid-state spectra were measured in Li2CO3
matrix (10% sample concentration). The absorption
spectra were recorded in formamide solutions. The
band positions were determined with the digital ®lter
method [5,6]. The relations between crystal ®eld
parameters and transition energies have been calcu-
lated using computer program LFPOH based on the
Perumareddi data [7].
2.1.2. Red complex: [Nibpy2 CrO4]bpy 8H2O [B]
A methanolic solution of 2,20-bipyridine (100 cm3,
0.75 M) was added dropwise upon stirring to the same
amount of 0.25 M K2CrO4 dissolved in water. The
yellow precipitate which was appearing during the
mixing of the reagents (approximately after adding
30 cm3 of the bpy) was dissolving upon further adding
of the ligand. This was mixed with the 90 cm3 of the
nickel(II) sulphate (chloride, nitride) solutions
(0.25 M). The change of the color of the solution
from yellow to light brown was observed upon the
addition of the nickel salt. The mixture was allowed to
stand for three days in the mother liquid. The solution
was evaporated to approximately half of the initial
volume and left to stand overnight. The obtained
reddish brown crystals were recrystallized from hot
water (60 cm3), ®ltered, washed with water and dried
in the desiccator over silica gel.
Magnetic measurements were carried out by the
Gouy method using Hg[Co(SCN)4] as calibrant.
Scanning electron micrographs were made with
Stereoscan 180 Cambridge apparatus.
3. Results and discussion
The scanning electron micrographs of the substrates
[A] and [B] when magni®ed 3000 and 20 times,
respectively, showed that under the condition of the
experiment, [A] forms very small crystallites and [B]
large crystals (Figs. 1 and 2). Attempts to grow single
crystals are being undertaken, up to now, without
success.
The red compound has been also obtained in a
similar way from Ni(NO3)2 using 1 : 2 : 1 (Ni : b-
py : Cr) proportions.
2.2. Apparatus, methods and calculations
The concentration of the metal ions was obtained
using Perkin±Elmer 180 atomic absorption spectro-
photometer and carbon, hydrogen and nitrogen ana-
lyses using modi®ed Kupman method.
TG and DTA studies were carried out simulta-
neously using Paulik±Paulik±Erdey type derivato-
graph in dynamic air and helium atmosphere over
the temperature range 293±1273 K at heating rates
2.5 deg min 1. The sample weight was 200 mg. The
X-ray patterns of the ®nal residues were obtained on a
DRON-2 diffractometer with Cu-K radiation lamp.
IR spectra were recorded on a Perkin±Elmer 180
(50±3500 cm 1) spectrophotometer in KBr pellets.
Fig. 1. Scanning electron micrograph of [Nibpy(H2O)4]CrO4 [A]
(magnification 3000 times).

A. Repka et al. / Thermochimica Acta 313 (1998) 9±17
11
monomers [11]. The value of the effective moments
are greater than the spin-only value of 2.83 BM which
is expected for an orbital singlet ground state 3A2g in
Oh symmetry for six-coordinate nickel(II) ion. Thus,
in both complexes the nickel ion is situated in the
pseudooctahedral environment. Table 2 shows the
formulae of the isolating complexes depending upon
the condition of the experiment and Table 3 the IR
data (vide infra).
From the results presented in Table 2, we can
conclude that among four candidates to be bound to
the Ni(II) ion (i.e. bpy, water, the chromate and X ions
(X ˆ Cl , SO24 , NO3 ) the strongest one is the
organic ligand. The most important factor determining
the composition of the coordination sphere around
Ni(II) ion is the amount of the organic ligand. For
the M : L : Cr molar ratio being 1 : 3 : 1, in each
case, we have observed the precipitation of [B]
with total Ni : L ratio ˆ 1 : 3. However, the IR data
show that for some reasons, unlike in Nibpy3SO4 [18]
Fig. 2. Scanning electron micrograph of [Nibpy2CrO4]bpy 8H2O
[B] (magnification 20 times).
Table 1 presents the analyses of the spectra and the
magnetic data. The temperature dependence of the
magnetic susceptibility shows that the complexes are
Table 1
Analyses (%) of compounds and magnetic moments
Compound
Ni
Cr
C
H
N
ꢀ (BM)
[A] [Nibpy(H2O)4]CrO4
Theory
Found
Theory
Found
14.56
15.20
7.45
12.90
13.39
6.60
29.81
30.18
45.76
46.02
4.00
3.52
5.12
5.11
6.95
3.41 (T ˆ 78 K)
3.48 (T ˆ 297 K)
3.37 (T ˆ 78 K)
3.38 (T ˆ 297 K)
6.30
10.67
11.59
[B] [Nibpy2CrO4]bpy8H2O
8.10
6.63
Table 2
The condition of the experiment, i.e. the nickel(II) salts, the molar ratios of the reagents and the resulting solid products (in compound [B] the
water of crystallization has been omitted)
Ni : bpy : Cr(VI)
NiSO4
NiCl2
Ni(NO3)2
1 : 1 : 1
1 : 2 : 1
1 : 3 : 1
[Ni(H2O)4bpy]CrO4 [A]
[Ni(H2O)4bpy]CrO4 [A]
[Nibpy2(CrO4)]bpy [B]
[Ni(H2O)4bpy]CrO4 [A]
[Ni(H2O)4bpy]CrO4 [A]
[Nibpy2(CrO4)]bpy [B]
[Ni(H2O)4bpy]CrO4 [A]
[Nibpy2(CrO4)]bpy [B]
[Nibpy2(CrO4)]bpy [B]
Table 3
The important bands (cm 1) in the IR and FIR spectra for nickel(II) chromate complexes with bpy
Compound
ꢁ1
ꢁ2
ꢁ3
ꢁ4
ꢁNi±N
ꢁasNiOCr
[A]
825ms
305
876vs
894ssh
906s
330w
352ms
365w
363s
240s
268vs
[B]
814w
880vs
895s
262ms
288vs
778vs
384w
906s
Note: s ± strong; ms ± medium strong; vs ± very strong; w ± weak; sh ± shoulder.

12
A. Repka et al. / Thermochimica Acta 313 (1998) 9±17
and Nibpy3Cl2 C6H4O6 [19] one bpy molecule is out
of the coordination sphere and at the same time the
chromate ion is coordinated to the nickel(II) ion. It
seems to be a stronger ligand than the sulphate ion
and/or water.
4. Spectral studies
4.1. Infrared spectra
Table 3 shows the band positions assigned to the
Cr±O vibrations in the nickel chromate complexes
with bpy. The analysis of the spectra in the region of
the Cr±O activity show that the chromate ion in these
complexes is bound in a different way. In contrast to
[A], the [B] compound shows both splittings of the
bands and the appearing of very strong, new band at
778 cm 1. The latter is the most characteristic feature
of the chromates coordination to the metal ion [8]. It is
associated with the M±O±Cr bridge or ring [8]. The ꢁas
mode is very strong and was found to be approxi-
For the 1 : 2 : 1 M : L : Cr we observe `the compe-
tition' between the chromate ion and water molecules.
Besides the organic ligand, in this competition the
counter anions i.e. sulphate, chloride and nitrate seem
to play a key role. The sulphate and chloride seem to
favor water molecules to be coordinated whereas the
nitrate anions promotes the chromate anion. For the
1 : 1 : 1 ratio, in each case, the chromate ion is out of
the coordination sphere.
Such results indicate that for the chromate ion to be
coordinated to the nickel ion there must be:
1
mately at 770 cm in various chromate complexes
[8].
1
ꢀ the appropriate concentration of the organic ligand
ꢀ the nitrate rather than chloride or sulphate anions
must be present.
Generally, the splittings in the 770±1000 cm
absorption region (i.e. the region of the Cr±O stretch-
ing mode vibrations) seem to be explicable only in
Table 4
Electronic spectra (cm 1) resolved under digital filtration 250/20/100 (see the text) and crystal field parameters for the complexes. (The values
in the parentheses are obtained from the calculation)
Compound
Diff. reflectance
spectra
Room temp.
formamide solution
Liquid nitrogen
temperature a
Assign
A
(9600)
15600
21300
24900
26200sh
32500
34400
38400
41900br
(9700)
16200
22200
27000
29400
33500
34800
35800sh
(9900)
16300
22300
26800
27700
d±d
d±d
Cr±O
Cr±O
d±d
Ligand
Ligand
Cr±O
Ligand
Mean values for Dq and B: Dq ˆ 980 cm 1; B ˆ 970)
B
(1220)
18300
(1084)
14400?
15400?
17400
21800
27000
28400sh
33250
34450
40770
41070
(1250)
19100
d±d
d±d
22700
26000
28000sh
32400
34500
39500
22200
Cr±O
Cr±O
d±d
L
26800
28300sh
L
Cr±O
L
41400br
1
Mean values for Dq ˆ 1180 cm and B ˆ 720
a Liquid nitrogen measurements were made only for the visible part of the spectra.

A. Repka et al. / Thermochimica Acta 313 (1998) 9±17
13
terms of the coordinated CrO4 group [8±10]. In [B]
5. Thermal studies
this coordination is seen mainly through Ni±Cr±O
vibration. If this is the case, as it has been found
elsewhere [14,23] the various function of the organic
ligand can be proposed [B], i.e. the coordinating and
non-coordinating one. On the basis of the elemental
analysis, the arguments given above as well as elec-
tronic spectral data (Table 4 vide infra) we propose the
formula [Nibpy(H2O)4]CrO4 and [Nibpy2CrO4]bpy
8H2O for [A] and [B], respectively.
The TG, DTG and DTA curves for both complexes
are given in Figs. 3±6 and the data characteristic of the
thermal decomposition processes are collected in
Tables 5 and 6. Whereas the thermolysis of nickel(II)
bpy complexes in a vacuum results in stoichiometric
products [22], the thermal decomposition of these
complexes in air or inert atmosphere proceeds often
through the non-stoichiometric products [15].
4.2. Electronic spectra
5.1. [Nibpy(H2O)4]CrO4 [A]
A considerable number of Ni(II) complexes with
bpy have been reported so far [11,12]. Literature data
point to a wide variety of Ni(II)±bpy complexes of the
mono-, bis- and tris-type, respectively [12].
The DTA curve in air indicate two main energetic
effect which occur in the range 25±235 and 235±
4258C, respectively. Both are well determined. The
The band maxima resolved with the digital ®lter
method of both compounds together with the spectro-
scopic parameters 10Dq and B are presented in
Table 4. The d±d bands in the electronic re¯ectance,
solution and low-temperature spectra and magnetic
moments indicate that the nickel(II) ion is in a pseu-
dooctahedral environment. The very detailed analysis
of the electronic spectra will be published separately
[20]. The spectra are formed by the spin-allowed d±d
3
3
transitions from A2g to T2g, T1g and T1g(P) states,
respectively.
Both the ligand ®eld splitting parameter and values
of the Racah parameter B are between the pure
2‡
NiꢁH2O† (10Dq ˆ 8500 cm 1, B ˆ 881 cm 1) [13]
6
2‡
1
and Niꢁbpy† (10Dq ˆ 12790 cm 1 , B ˆ 710 cm
)
3
values [12]. The respective values for the green com-
plex are higher than for hexaaquanickel(II) ion
whereas for the red one are lower than tris bpy
nickel(II) ion (Table 4). These results support the
IR data for the existence of the [NiO4N2] and
[NiO2N4] chromophores in the [A] and [B] com-
plexes, respectively. It is well known [14] that bpy
can exist both in inner and outer coordination spheres.
Our results support the foundings of Czakis et al. [14]
who observed that in some Ni(II) complexes with 2,40-
bpy the nitrate anions are non-coordinating and some
of the organic ligand are outside the coordination
sphere.
The nephelauxetic parameters 0.93 [A] and 0.69 [B]
are in the range of pseudooctahedral Ni(II) complexes
[13].
Fig. 3. TG, DTG and DTA traces for [Nibpy(H2O)4]CrO4 [A] in
air.

14
A. Repka et al. / Thermochimica Acta 313 (1998) 9±17
Fig. 4. TG, DTG and DTA traces for [Nibpy(H2O)4]CrO4 [A] in
inert atmosphere.
Fig. 5. TG, DTG and DTA traces for [Nibpy2CrO4]bpy [B] in air.
®rst one is endothermic and the second one is very
high and exothermic in character. The latter one splits,
so it is evidently composed of more than one effect.
In the ®rst step the molecule loses two water
molecules. The next step is composed with the loses
of the remaining water and the decomposition of the
bpy and the reduction of chromium(VI). The ®nal
products obtained is a mixture of NiO and NiCr2O4.
Fig. 7 shows the grains of NiCr2O4 covered with
`dust' of NiO. (Pure nickel chromite can be obtained
from a double salt (NH4)2Ni(CrO4)2 pyrolysis at
12008C [22]).
(235±4258C) is exothermic. In the ®rst step the com-
plex loses a part of water molecule. In the second one
both the remaining water and partial deamination
takes place. On further heating (III step) the next
portion of bpy is eliminated. The last step is connected
with the loss of the remaining organic molecule
together with the ®nal decomposition of the complex.
The X-ray diffraction points to the mixture of NiO,
Cr2O3 and NiCr2O4. The decomposition and the oxi-
dation of the organic ligand are at cost of the chromate
oxygens.
DTA curve in helium indicates three well resolved
peaks and the fourth one without unequivocal ener-
getic character. The ®rst (20±2358C) and the third
(425±5708C) peaks are endothermic, while the second
5.2. [Nibpy2CrO4]bpy 8H2O [B]
The thermoanalytical data obtained show that the
thermal decomposition of [B] proceeds in three and

A. Repka et al. / Thermochimica Acta 313 (1998) 9±17
15
Fig. 7. Scanning electron micrograph of NiCr2O4 and NiO ± final
residue of [A] and [B] decomposition in air.
four stages in air and inert atmosphere, respectively.
In air the complex loses all water molecules in the
®rst stage (25±1608C). In the second step the deami-
nation of bpy molecule outside coordination sphere
takes place. Finally, the asymmetric exothermic DTA
peak show on the deamination along with the redox
process resulting in the formation of NiO ‡ NiCr2O4
mixture.
In argon atmosphere the third step, i.e. the decom-
position of Nibpy2CrO4 is evidently divided into
formation of the non-stoichiometric [Nibpy1.4CrO4]
Fig. 6. TG, DTG and DTA traces for [Nibpy2CrO4]bpy [B] in inert
atmosphere.
Table 5
Thermal decomposition of green nickel chromate complex with 2,20-bpy [A]
Reaction
Temperature
range (8C)
Decomposition
stage
DTA
peak
Weight
loss (%)
Found
Calcd.
[A] in air:
[Nibpy(H2O)4]CrO4
#
23±235
I
Endo
Exo
10.20
46.90
8.94
[Nibpy(H2O)2]CrO4
#
235±425
II
47.70
NiCr2O4 ‡ NiO
[A] in inert atmosphere:
[Nibpy(H2O)4]CrO4
#
20±235
235±425
425±570
570±715
I
Endo
Exo
10.52
10.57
19.64
18.15
11.17
10.52
19.38
15.50
[Nibpy(H2O1.5]CrO4
#
II
[Ni(bpy)0.9]CrO4
#
III
IV
Endo
Exo
[Ni(bpy)0.4]CrO4
#
NiO ‡ NiCr2O4 ‡ Cr2O3

16
A. Repka et al. / Thermochimica Acta 313 (1998) 9±17
Table 6
Thermal decomposition of red nickel chromate complex with 2,20-bpy [B]
Reaction
Temperature
range (8C)
Decomposition
stage
DTA
Weight
loss (%)
Found
Calcd.
[B] in air
[Nibpy2CrO4]bpy 8H2O
#
25±160
160±250
250±345
I
Endo
Endo
Exo
19.02
23.25
40.36
18.30
19.84
39.67
[Nibpy2CrO4]bpy
#
II
III
[Nibpy2CrO4]
#
NiCr2O4 ‡ NiO
[B] in inert atmosphere:
[Nibpy2CrO4]bpy 8H2O
#
20±115
215±250
250±330
560±830
I
Endo
Endo
Exo
16.03
22.27
11.46
Ð
16.01
22.13
11.90
15.46
[Nibpy2CrO4]bpy H2O
#
II
[Nibpy2CrO4]
#
III
IV
[Ni(bpy)1,4CrO4]
#
Exo
[NiO ‡ Ni ‡ C ‡ Cr2O3]
and its decomposition into the mixture of NiO ‡
Ni ‡ C ‡ Cr2O3.
mite AgCrO2 formed at 260±4508C decomposes
into mixture Ag ‡ Cr2O3 at higher temperatures
[15,21].
5.2.1. Thermal decomposition of the metal chromate
complexes with bpy. (M- Ag(I), Cu(II) and
Ni(II))
The ®nal products for nickel complexes (i.e the
mixture of metal oxide and corresponding metal
chromite) are in accordance with the results obtained
for the thermal decomposition of the zinc and cad-
mium chromates [17].
The situation in inert atmosphere is different. The
mechanisms are more complicated and more sensitive
toward the metal ion, the amount of the organic ligand
and the physical conditions. For example in the case of
silver complexes the products was dependent upon the
heating rate showing even the presence of substrate at
10008C [15]. Evidently, the redox processes have been
operating in the wide temperature range.
Despite some differences in the chemical composi-
tion in air of the ®nal products at 10008C, i.e.
Ag ‡ Cr2O3 for Ag2bpy2CrO4 [15]; Cu(I)CrO2 for
[Cubpy2Cl]2CrO4[16] and NiO ‡ NiCr2O4 for the
complexes obtained in this work, the mechanism of
the decomposition reveals some common feature for
all the complexes. On the literature data and the results
of this work the process of the thermal decomposition
in air of the metal chromate complexes with bpy can
be divided into two or three steps:
± the water loses (if the complex possesses it)
(100±2008C)
5.3. Conclusion
± the decomposition of the organic part of the
molecule with the formation of the stoichiometric
or non-stoichiometric intermediate (200±4008C)
± the final loses of the rest of the organic fragment
which accompanies the chromium(VI) reduction
and the formation of the metal chromites (400±
10008C) (for the silver complexes). The metachro-
A synthetic method has been successfully employed
to obtain two Ni(II) chromate complexes with bpy.
The coordination of the chromate ion in [B] signi®-
cantly in¯uences the spectral characteristics and the
decomposition schemes especially in inert atmo-
sphere.

A. Repka et al. / Thermochimica Acta 313 (1998) 9±17
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17
Acknowledgements
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Pergamon Press, Oxford±New York±Sydney, (1987) and the
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Â
One of the author, M. Cieslak-Golonka, is indebted
to Technical University of Wrocøaw for ®nancial
support (grant No. 341±695).
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