Spectral and thermal study on the adduct formation between square planar nickel(II) chelates and some bidentate ligands

Article abstract of DOI:10.1016/j.saa.2005.09.015

Adducts of Ni(II)-square planar complexes [Ni(β-dik)(Me₄en)]⁺, with a series of bidentate ligands (L), where β-dik = acetylacetonate (acac) and benzoylacetonate (bzac), Me₄en = N,N,N′,N′-tetramethylethylenediamine and L = Me₄en, 2,2′-bipyridine (bipy), ethylenediamine (en) and oxalate (C₂O₄^2-) have been synthesized and characterized by spectral, thermal and magnetic measurements. Formation constants of the adducts formed from a series of ternary mixed Ni(II) complexes with the general formula [Ni(β-dik)(diam)]⁺ with 1,10-phenanthroline (phen), 2,2′-bipyridine (bipy) and pyridine were spectrophotometrically determined. Thermodynamic parameters of the adduct formation between nickel(II) square-planar chelates and pyridine (py), 2,2′-bipyridine (bipy) and acetylacetone (acac) were also spectrophotometrically determined in 1,2-dichloroethane. The thermal stability of the isolated adducts was studied using thermogravimetry and the decomposition schemes of the adducts were given.

Full text of DOI:10.1016/j.saa.2005.09.015

Spectrochimica Acta Part A 64 (2006) 1058–1064
Spectral and thermal study on the adduct formation between
square planar nickel(II) chelates and some bidentate ligands
Ahmed A. Solimana,∗, Ali Tahab, Wolfgang Linertc
a Department of Chemistry, Faculty of Science, UAE University, P.O. Box 17551, Al-Ain, United Arab Emirates
b Department of Chemistry, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt
c Institute of Inorganic Chemistry, Technical University of Vienna, Getreidemarkt 9, A-1060 Vienna, Austria
Received 4 May 2005; accepted 16 September 2005
Abstract
Adducts of Ni(II)-square planar complexes [Ni(␤-dik)(Me4en)]+, with a series of bidentate ligands (L), where ␤-dik = acetylacetonate (acac)
and benzoylacetonate (bzac), Me4en = N,N,Nꢀ,Nꢀ-tetramethylethylenediamine and L = Me4en, 2,2ꢀ-bipyridine (bipy), ethylenediamine (en) and
oxalate (C2O42−) have been synthesized and characterized by spectral, thermal and magnetic measurements. Formation constants of the adducts
formed from a series of ternary mixed Ni(II) complexes with the general formula [Ni(␤-dik)(diam)]+ with 1,10-phenanthroline (phen), 2,2ꢀ-
bipyridine (bipy) and pyridine were spectrophotometrically determined. Thermodynamic parameters of the adduct formation between nickel(II)
square-planar chelates and pyridine (py), 2,2ꢀ-bipyridine (bipy) and acetylacetone (acac) were also spectrophotometrically determined in 1,2-
dichloroethane. The thermal stability of the isolated adducts was studied using thermogravimetry and the decomposition schemes of the adducts were
given.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Nickel(II)-complexes; Adducts; Thermal; Formation constant; ␤-Diketones; Diammines
1. Introduction
2. Experimental
2.1. Reagents
Several reports have been found dealing with the thermody-
namic, spectroscopic, quantum mechanics and electrochemical
studies for chromotropic ternary mixed ligand complexes of
[M(␤-dik)(diam)]+, where M = Ni(II) or Cu(II), ␤-dik = ␤-
diketonate and diam = diamines [1–5]. Although the adduct
formation [Ni(␤-dik)(diam)]+ with monodentate ligands
is well known, little is known about their adducts with
bidentate ligands. From this prospective this paper aims
to study the adduct formation between the square planar
[Ni(␤-dik)(diam)(L)]+ and O O and N N bidentate ligands
(Scheme 1).
Chemicals were obtained either from Merck or Aldrich
Chemicals. Solvents were of spectroscopic grade or have been
purified according to standard procedures [6,7].
2.2. Synthesis of the adducts
The mother complexes, Ni(␤-dik)(diam)Bꢀ4 were prepared
as reported earlier [1–5]. The adducts were prepared by adding
one equivalent of bidentate ligand (5 mmol) in 15 mL of EtOH
to the DCE solution of the mother complex, the mixture was
stirred for 30 min. The color was completely changed from red
to green; the green solution was filtered and left overnight. The
precipitated adducts were collected and recrystallized from DCE
where greenish crystals were obtained.
∗
Corresponding author. Permanent address: Department of Chemistry, Fac-
2.3. Measurements
ulty of Science, Cairo University, Giza, Egypt. Tel.: +971 50 7132451;
fax: +971 3 7671291.
Magnetic measurements were made using Jouy method,
Johnson Matthey Alfa products Model MK I Magnetic sus-
E-mail addresses: ahmed.soliman@uaeu.ac.ae,
ahmedsoliman2@hotmail.com (A.A. Soliman).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.09.015

A.A. Soliman et al. / Spectrochimica Acta Part A 64 (2006) 1058–1064
1059
Scheme 1.
culated for the proposed formulae [Ni(␤-dik)(Me4en)(L)]X.
The magnetic susceptibility measurements indicated the para-
magnetic nature of the adduct (Table 1). A nickel(II) ion in
an octahedral field is expected to give a spin-only moment
of 2.83 B.M. However, the magnetic moments of the current
adducts were observed in the range 2.9–3.35 B.M. This could
be attributed to spin-orbit coupling of the 3T2g(F) and 3A2g terms
[9].
ceptibility balance. Thermal analysis was carried out using
a Shimadzu TGA-50 thermal analyzer. A constant flow of
nitrogen (30 cm3 min−1) was maintained. Platinium crucibles
were used and the heating rate of 20 ◦C min−1. The infrared
spectra (400–4000 cm−1) were recorded using a Shimadzu
FTIR 8101 and NICOLET 20F Far IR spectrometers. The
visible absorption spectra for adducts and formation stud-
ies were obtained by means of a Tracor Northen TN-1170
spectralphotometer from Photo-Applied Physics Coorporation
and a Hitachi U-2000 Spectrophotometer using a cell with
a pathlength of 3 cm, thermostated by means of a Haake
F4-Thermostat. The temperature within the cell was measured
before and after recording the spectra. To a 3 × 10−3 molar
stock solution of the reactive square planar Ni-complex DCE
solution the bidentate ligand diluted/dissolved with DCE were
added in a course of titration procedure. The obtained spectral
titration curves were fitted by means of a combined Marquardt
Newton method [8] to evaluate the formation constants (log K).
In the infrared spectra of the isolated adducts, the fre-
quencies of the asymmetric stretching vibration combination
(νasym
C
O + νasym
C
C) [10,12] have been shifted to higher
frequencies than those of their mother chelates. Hence the posi-
tion of the perturbed carbonyl bond is influenced by the inter-
action of the mother chelate with the bidentate ligand. This
enhanced the ␲-interactions within the adduct complexes as a
result of hyperconjugation which result in an increase of the
C O bond strength. The same reasons can be applied to the
shift in stretching Ni–O to lower frequency compared to those
assigned to the mother chelates.
The square planar [Ni(␤-dik)(diam)]+ complexes showed a
3. Results and discussion
1
strong band near 490 nm which assigned to a 1A2g ← A1g (i.e.
dx2−y2 ← dxy) transition [13,14], this band was found in the
3.1. Characterization of adducts
spectra of the adducts near 950 nm indicating an octahedral
3
3
Ni complexes. This band can be assigned to a T2g ← A2g
transition (corresponding directly to dx2−y2 ← dxy). The small
Table 1 shows the analytical data of the prepared adducts.
Elemental analyses were in good agreement with those cal-
Table 1
Analytical dataa and magnetic moments of the adduct complexes
No.
Adduct complex
Color
% Yield
F.W.
C%
H%
N%
∝
(B.M.)
νC–Ob
νNi–Oc
eff
1
2
3
4
5
[Ni(acac)(Me4en)(bipy)]Bꢀ4·2H2O
[Ni(acac)(Me4en)(en)]Bꢀ4
[Ni(acac)(Me4en)2]Bꢀ4
Green
Green
Green
Green
Blue
40
43
49
54
32
786.28
654.3
710.28
847.33
638.56
68.63 (68.68)
67.79 (67.92)
68.33 (69.33)
70.95 (70.88)
51.30 (45.14)
6.06 (7.05)
7.31 (7.86)
8.05 (8.37)
5.99 (6.78)
5.79 (6.95)
5.38 (5.60)
8.84 (8.56)
7.19 (7.89)
6.53 (6.61)
5.04 8.77)
3.35
2.91
1.67
2.99
2.41
1600
1600
1603
1600
1625
615
612
608
615
608
[Ni(bzac)(Me4en)(bipy)]Bꢀ4·2H2O
[Ni2(acac)2(Me4en)2(C2O4)]
a
b
c
Calculated values are given in parenthesis.
νC–O: for [Ni(acac)(Me4en)]Bꢀ4 (1578 cm−1), for [Ni(bzac)(Me4en)]Bꢀ4 (1585 cm−1), for C2O4 (1661 cm−1).
νNi–O: for [Ni(acac)(Me4en)]Bꢀ4 (630 cm−1), for [Ni(bzac)(Me4en)]Bꢀ4 (628 cm−1).

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A.A. Soliman et al. / Spectrochimica Acta Part A 64 (2006) 1058–1064
Table 2
Equilibrium constants (log K) for the formation of adducts of [Ni(ß-dik)(diam)]+
with mono- and bidentate ligands (L) at 293 K in DCE
No.
Chelate
Pya
bipy
phen
1
2
3
4
5
6
7
8
9
[Ni(acac)(Me4en)]+b
[Ni(bzac)(Me4en)]+
[Ni(dbm)(Me4en)]+
[Ni(dipm)(Me4en)]+
[Ni(acac)(Me3en)]+
[Ni(acac)(Et4en)]+
[Ni(acac)(Et3en)]+
[Ni(acac)(dipe)]+
3.27
3.23
3.17
2.50
4.78
3.16
3.52
4.18
3.51
3.15
3.19
3.21
3.08
3.26
3.11
3.15
3.27
3.08
3.15
3.20
3.18
3.11
3.27
3.10
3.15
3.28
3.09
[Ni(acac)(medach)]+
a
Taken from Ref.[1].
log K for its adducts with: en (7.60), Me4en (2.54), Sal (0.85), Hacac (2.63).
b
The resulting formation constants (K) values are listed in
Table 2. These values indicates that the N N bidentate ligand
(L) gives more stable adducts than O O ligands. The predomi-
nant order of formation (in terms of log K) of the adducts was:
py > bipy ≈ phen. This order reflects the role of sterric hindrance
on the adduct formation. Adduct of sal and acac bidentate
ligand (L) with [Ni(acac)(Me4en)]+ showed that acac adduct
(log K = 2.63) is more stable than sal (log K = 0.84) adduct, indi-
cating again the role of sterric factor.
The plot of log K values of bipy adducts against the corre-
sponding values of phen adducts interactions gave a straight
line, log K bipy = 0.01 + 0.995 log K phen, r = 0.97. The linear-
ity, slope and intercept indicate that the nature and mode of
the mutual interactions among three species namely bidentate
ligand, mother chelate and six-coordinated adduct complex in
such a group of reactions are similar to each other. On the other
hand, a comparison between log K of bidentate adducts with
the log K1K2 of monodentate adducts (two steps) such as Py
and DMSO, shows that log K1K2 for mono-dentate adducts [1]
are higher than the corresponding log K of bidentate adducts.
This indicates that the octahedral adduct which was produced
from the addition of monodentate ligand such as Py to the
mother chelate is more favorable than that produced from the
bidentate one. This could be attributed to the presence of two
different mechanisms for the interaction during the adduct for-
mation form bidentate and monodentate adducted ligands. In
other words, the addition of bidentate ligand requires rearrange-
ment in the position of the ligands on the square planar mother
chelate to permit the entering of the new bidentate ligand dur-
ing the formation of adduct. In addition, the relationship is also
linearly correlated, log K(bipy) = 2.82 + 0.098 log K1K2 (Py),
r = 0.92 for Ni(acac)(diam)+ mother chelates and log K(bipy) =
5.20 + 0.62 log K1K2 (Py), r = 0.96 for Ni(␤-dik)(Me4en)+
mother chelates. Moreover, log K(bipy) = 2.96 + 0.048 log K1K2
(DMSO) [1], r = 0.96 Ni(acac)(diam)+ mother chelates
log K(bipy) = 2.69 + 0.097 log K1K2 (Py), r = 0.96 for Ni(␤-
dik)(Me4en)+ mother chelates. These relationships suggest that,
the effect of ␤-dik on the mother chelate is more pronounced
than diam ligands as indicated from slope values.
Fig. 1. Effect of o-phenanthroline (phen) on absorption spectra of
Ni(acac)(Me4en)Bꢀ4 solution in DCE at 25 ◦C. Nickel chelate conc.
1.5 × 10−3 M; phen conc. 1 M; non: 2, 0.14: 3, 0.28: 4, 0.41: 5, 0.54: 6, 0.67: 7,
0.80: 8, 0.91: 9, 1.03: 10, 1.14 M.
absorption band observed for the octahedral complexes at
800 nm may be assigned to a 1Eg ← A2g transition.
1
3.2. Formation constant studies on the adduct formation
The d–d band positions and the corresponding molar absorp-
tions are shifted by the change of the concentration of the
bidentate ligand as well as by change in substituents at the ␤-
diketonate or diamine moieties on the mother chelate. Fig. 1
represents typical effects appeared in the absorption spec-
tra caused by the addition of the bidentate ligand, 1,10-
phenanthroline (phen), to the square planar nickel(II) chelate,
Ni(acac)(Me4en)Bꢀ4 in DCE. The maximum absorbance of
the square planar species decreases on the successive addition
of the bidentate ligand (phen) and this has been used to evalu-
ate the respective formation constants. The isosbestic point has
been shown near 600 nm holds for all mother chelates and for
all adducted investigated. This fact as well as the least-square
evaluation of the titration curves show that the coordination
of bidentate ligand molecule proceeds to form six-coordinated
species:
The relationship between log K values of bipy adducts and
the frequencies of the observed d–d absorption bands of mother
K
[Ni(β-dik)(diam)]+ + Lꢀ[Ni(β-dik)(diam)L]+

A.A. Soliman et al. / Spectrochimica Acta Part A 64 (2006) 1058–1064
1061
Table 3
Thermodynamic data for adduct formation of [Ni(acac)(Me4en)]+ with mono- and bidentate ligands in DCE
Chelate
log K
−ꢁG (K cal. mol−1
)
−ꢁH (K cal. mol−1
)
ꢁS (cal. mol−1 deg−1
)
10 ◦C
20 ◦C
30 ◦C
40 ◦C
[Ni(acac)(Me4en)(py)2]+
[Ni(acac)(Me4en)bipy]+
[Ni(acac)2(Me4en)]
3.59a
3.19
2.81
3.34
3.15
2.63
2.85
3.09
2.45
–
3.05
2.29
4.48
4.29
3.49
17.01
0.10
0.34
−42.82
14.05
10.55
a
t = 13.5 ◦C.
chelates in DCE (which is non-donor solvent) [1] is linearly cor-
related, log K(bipy) = −2.71 + 0.00029 ν/cm−1 (DCE), r = 0.90
(except medach, dipe and dipm mother chelates. The positive
slope indicates that in accord with the increase in the log K value,
the frequency of the d–d transition band of the mother chelate
increases.
among the other factors affecting ꢁS. The sterric requirement
for the adduct of bidentate ligands cause the mother chelate
to rearrange its chelate ring [15], and this structural rearrange-
ment together with the chelate effect, makes the values of ꢁS of
bidentate adducts more positive than that of the corresponding
monodentate (Py) adducts.
The position of the IR absorption frequency associated with
the νC=O [11] is linearly related to the formation constants of
bipy adducts, log K(bipy) = −1.77 + 0.0031 νC=O, r = 0.99. This
indicates, with increasing the strength of C=O in the mother
chelate the developing adduct formation is favored.
The apparent formation constants at various temperature
were obtained for the adducts of [Ni(acac)(Me4en)]+ with py,
bipy and Hacac as representative examples of the N N and O O
bidentateligands. The thermodynamicparametersdeterminedin
the current study are shown in Table 3. The data suggest that the
adduct formation is exothermic and more favorable for N N
than O O bidentate ligands as indicated from their ꢁG values.
ꢁH value of bipy adduct was significantly smaller than the cor-
responding Py or Hacac adducts. ꢁS values of the adducts of
bipy or Hacac which are bidentate are positive, however, the
corresponding value of Py adduct(s), which is monodentate lig-
and is negative. It suggests that chelate effect is predominant
3.3. Thermal analysis
Thermal analysis becomes an important tool not only for the
study of the thermal stability and the decomposition patterns of
complexes but also it provides important information about their
structural features and the preferential ligation of the different
groups directly attached to and/or associated with metal ions
[16–20].
The adduct complexes are stable up to 100 ◦C. The decom-
position schemes mostly have a common feature that they start
with the elimination of one of the diamine ligands, at the tem-
perature range 100–200 ◦C, to attain a less sterically hindered
Scheme 4.
Scheme 2.
Scheme 5.
Scheme 6.
Scheme 3.

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A.A. Soliman et al. / Spectrochimica Acta Part A 64 (2006) 1058–1064
square planar geometry. Then eliminates a second diamine lig-
and at higher temperature range and finally the elimination of
the acac in the last step leaving NiO as a residue in all cases.
[Ni(acac)(Me4en)(bipy)]Bꢀ4·2H2O decomposes in four
steps at the temperature range 70–680 ◦C (Scheme 2). The
first step, 70–145 ◦C, is a dehydration step with a mass loss
of 36 (4.58%) corresponding to the elimination of two water
molecules. The second step observed at the temperature range
145–171 ◦C is due to elimination of the relatively weakly bonded
amine which is Me4en compared with the aromatic biby moi-
ety. This can be evidenced from the higher stability constant of
[Ni(acac)(Me4en)(bipy)] (log K = 3.15) compared with those of
[Ni(acac)(Me4en)2] (log K = 2.54). The biby is eliminated at a
higher temperature step. The Bipy is eliminated along with the
tetraphenyl borate at a temperature range of 171–398 ◦C. The
final step observed at higher temperature (500–680 ◦C) ended
with the formation of Ni as the metallic residue with decompo-
sition and elimination of the acac moiety.
Fig. 2. TG and DTG plots of adduct complexes.

A.A. Soliman et al. / Spectrochimica Acta Part A 64 (2006) 1058–1064
1063
Table 4
[Ni(acac)(Me4en)(en)]Bꢀ4 decomposes in three successive
steps in the temperature range of 100–650 ◦C (Scheme 3).
The diamine (Me4en) was removed in the first step at
100–190 ◦C to form a less sterrically hindered square pla-
nar complex [Ni(acac)(en)]Bꢀ4. This is in accordance with
the higher stability of the [Ni(acac)(Me4en)(en)] complex
(log K = 7.60) compared with that of the [Ni(acac)(Me4en)2]
complex (logK = 2.54). The en was then removed at higher tem-
perature range (190–300 ◦C) along with the Bꢀ4 moiety. The
complex was finally decomposed to Ni with removing of the
acac moiety in the temperature range 300–650 ◦C.
Comparison of the X-ray powder diffraction data of Ni
Angle 2θ
Observed
Referencea
b
˚
˚
˚
d-Value (A)
I/I0
d-Value (A)
I/I0
ꢁd (A)
43.96
51.28
75.84
2.06
1.78
1.25
100
51
37.8
2.03
1.78
1.25
100
100
100
0.03
0.00
0.00
a
JCPDS-ICDD 3-1043.
b
ꢁd = dobsd. − dref.
.
with a mass loss of 148 (17.26) could be assigned to the removal
of the bzac moiety, which is not the case with the acac moi-
eties in the other complexes where it was removed in the final
step. This may be explained in terms of bulkiness of the bzac
compared with acac, which facilitate its removal in lower tem-
perature range. The second diamine (Me4en) is removed in the
final step (510–690 ◦C).
[Ni(acac)2(Me4en)2(OX)] decomposes in two simple steps
at the temperature range 100–650 ◦C (Scheme 5). The two
diamine moieties (Me4en) were removed in the first step,
110–250 ◦C, while the two acac were removed in the second
step, 250–650 ◦C. The bridged oxalate moiety (OX) was also
removed in the final step as carbon dioxide leaving Ni as the
metallic residue Scheme 6.
[Ni(acac)(Me4en)2)]Bꢀ4·2H2O decomposes in four steps in
the temperature range 91–125 ◦C (Scheme 4). The first step at
91–125 ◦C, is a dehydration step with a mass loss of 36 (4.84%)
corresponding to the elimination of two water molecules. One
of the diamine (Me4en) is completely removed in the second
decomposition step within the temperature range 126–175 ◦C.
The second diamine (Me4en) is removed partially in the second
step and finally in the final step along with the acac leaving Ni
as the metallic residue. The tetraphenyl borate is removed in the
third step within the temperature range 175–275 ◦C.
[Ni(bzac)(Me4en)(bipy)]Bꢀ4·2H2O Decomposes also in
four steps at the temperature range 70–690 ◦C (Scheme 5). The
first step at 70–110 ◦C, is a dehydration step with a mass loss
of 36 (4.25%) corresponding to the elimination of two water
molecules. The biby as the added diamine was removed in the
second step within the temperature range 110–290 ◦C. This is
may be due to the fact that biby is not sterrically favored like
Me4en as a second ligand with the bzac in the square planar moi-
ety. The third step observed at the temperature range 290–510 ◦C
3.4. XRD analysis
The metallic residue for all complexes obtained at 800 ◦C
were grounded and subjected to XRD analysis. All metallic
Fig. 3. XRD powder diffractogram of the metallic residue of the complexes.

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A.A. Soliman et al. / Spectrochimica Acta Part A 64 (2006) 1058–1064
residues had the same diffraction pattern; this indicates that
they reach the same metallic moiety at high temperature. The
XRD data are listed in Table 4. It is clear from the table that
the final decomposition product of all complexes was Ni metal
whose XRD data matches the reference data for metallic nickel
(JCPDS-ICDD 3-1043). This conclusion is consistent with the
results of the TG analysis (Schemes 2–6) (Figs. 2 and 3).
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