Synthesis and structural characterization of a few thiocarboxylatonickel(II) complexes

Article abstract of DOI:10.1016/j.ica.2013.11.037

Thiobenzoate complexes of Ni(II), [Ni(dppe)(SCOPh)₂] (1), [Ni(en)(SCOPh)₂] (2), [Ni(opda)(SCOPh)₂] (3), [Ni ₂(SCOPh)₄·EtOH] (4) [Ni₂(SCOPh) ₄·CH₃CN] (5) [where dppe =

Full text of DOI:10.1016/j.ica.2013.11.037

Inorganica Chimica Acta 411 (2014) 119–127
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and structural characterization of a few
thiocarboxylatonickel(II) complexes
⇑
Deepak Kumar Joshi, Subrato Bhattacharya
Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Thiobenzoate complexes of Ni(II), [Ni(dppe)(SCOPh)2] (1), [Ni(en)(SCOPh)2] (2), [Ni(opda)(SCOPh)2] (3),
[Ni2(SCOPh)4ꢀEtOH] (4) [Ni2(SCOPh)4ꢀCH3CN] (5) [where dppe = bis(diphenylphosphino)ethane; en = eth-
ylenediamine; opda = orthophenylenediamine] were synthesized and characterized structurally by X-ray
crystallography. Electronic absorption spectra were recorded for all the complexes and the same has been
computed using time dependent density functional theory for a representative compound (1). Bonding
between Ni–Ni atoms in the dimeric unit has been confirmed in 4 and 5 on the basis of X-ray structural
analyses and magnetic moment measurements. NBO calculations have been carried out to understand
the nature of bonding.
Received 16 September 2013
Received in revised form 25 November 2013
Accepted 27 November 2013
Available online 10 December 2013
Keywords:
Nickel(II)
Thiocarboxylate
Metal–metal bonding
Density functional theory
Magnetism
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
During past several years much attention was attracted by
transition metal chalcogeno complexes due to their structural
diversity [1], relevance to catalysis [2] and biological applica-
tions as models for metalloenzymes [3]. Thiocarboxylates repre-
sent a very interesting class of ligands as they possess both soft
sulfur and hard oxygen donor sites. A variety of thiocarboxylate
complexes are known where either sulfur or oxygen or even
both of them are involved in bonding with metal. Vittal and
co-workers have synthesized a number of mononuclear and
multinuclear thiocarboxylates which include neutral as well as
anionic complexes [4]. Some of these have been found to be
useful as the single source precursors for the synthesis of metal
sulfides [5]. During past few years we have synthesized a few
mono and bimetallic thiocarboxylate complexes [6]. Some of
these exhibited unusual structural features and interesting
properties. The heterobimetallic thiocarboxylates containing
Pb/Cu and Cd/Cu were used to prepare corresponding ternary
oxides and sulfides respectively [7]. A search in the literature
revealed that studies on Ni(II) thiocarboxylates are scanty
though a number of studies have been made on corresponding
carboxylates [8].
2.1. Materials and physical measurements
Solvents used were purified using standard methods. Thiobenzo-
ic acid, diphenylphosphinoethane (Sigma–Aldrich) and NiCl2ꢀ6H2O
(s.d.fine chemicals) were used as such without further purification.
The precursors NiCl2(dppe) [9], NiCl2(en)2 [10], NiCl2(opda) [11]
were prepared by following the methods described in the literature.
IR spectra were recorded using Perkin-Elmer RX-1, FTIR and Varian-
3100 FTIR instruments. Electronic spectra were recorded on a
Shimazdu UV-1700 PharmaSpec spectrophotometer using freshly
prepared solutions of compounds in chloroform or DMSO.
2.2. Synthesis of complexes
2.2.1. Synthesis of [Ni(dppe)(SCOPh)2] (1)
Methanolic solution (10 mL) of thiobenzoic acid (59 lL,
0.5 mmol) was added drop wise to a methanolic solution (10 mL)
of sodium hydroxide (0.020 g, 0.5 mmol) in an ice bath.
Ni(dppe)Cl2 (0.132 g, 0.25 mmol) in chloroform (10 mL) was added
drop wise to the reaction mixture. The yellowish solution turned
deep red after the addition. Stirring was continued for about 4 h
followed by evaporation of solvent under reduced pressure. The
red solid obtained was dissolved in chloroform and kept for crys-
tallization. Small rectangular plates were obtained after 3 days.
Yield: (62%). M.P. 192 °C. Anal. Calc. for C40H34Ni1O2P2S2: C,
65.68; H, 4.69. Found: C, 65.73; H, 4.58%. IR(KBr pellet, cmꢁ1):
⇑
Corresponding author. Tel.: +91 542 6702451.
E-mail address: s_bhatt@bhu.ac.in (S. Bhattacharya).
1653 m(C@O), 1197 m(Ph–C), 920 m(C–S), 690 d(SCO).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.11.037

120
D.K. Joshi, S. Bhattacharya / Inorganica Chimica Acta 411 (2014) 119–127
2.2.2. Synthesis of [Ni(en)(SCOPh)2] (2)
2.4. Magnetic measurements
A methanolic solution (10 mL) of thiobenzoic acid (59 lL,
0.5 mmol) was added to a methanolic solution (10 mL) of sodium
hydroxide (0.020 g, 0.5 mmol) in an ice bath. To this a solution of
Ni(en)2Cl2 (0.061 g, 0.25 mmol) (in 10 mL methanol) was added
drop wise. A bright green precipitate started to form during the
addition. After complete addition the reaction mixture was stirred
for further 1 h. The precipitate was then filtered, washed with 5 mL
methanol and dried under vacuum. Green crystals were obtained
upon recrystallization from its chloroform solution. Yield: 0.081 g
Vibrating sample magnetic moment measurement was
conducted on a EV-7 VSM ADE-DMS-Magnetic instrument within
the field range of ꢁ15000 to +15000 Oe at room temperature.
Variable-temperature magnetic susceptibilities were measured
with a Quantum Design MPMS XL SQUID magnetometer in applied
magnetic field of 0.5 Tesla. Measurements were performed in the
temperature range of 5–300 K.
(72%). M.P. 120 °C (dec). Anal. Calc. for
48.88; H, 4.61; N, 7.13. Found: C, 48.69; H, 4.58; N, 7.19%. IR(KBr
pellet, cmꢁ1) 1588
(C@O),1225 (Ph–C), 967 (C–S), 693 d(SCO).
C16H18Ni1N2O2S2: C,
2.5. Density functional calculations
m
m
m
TDDFT [14] and NBO calculations [15] were performed using
the atomic coordinates obtained from X-ray crystallographic
results. The calculations were performed at B3LYP [16] level using
6-31G⁄⁄ as basis set for all the atoms. All the calculations were
carried out using GAUSSIAN 03 program package [17].
2.2.3. Synthesis of [Ni(opda)(SCOPh)2] (3)
Methanolic solution (10 mL) of thiobenzoic acid (59
lL,
0.5 mmol) was added to a methanolic solution (10 mL) of sodium
hydroxide (0.020 g, 0.5 mmol) in an ice bath. The resulting solution
was then added to
a suspension of NiCl2(opda) (0.059 g,
3. Results and discussion
0.25 mmol) in methanol (10 mL). The reaction mixture was stirred
for 2 h. The precipitate was filtered, washed with methanol and
vacuum dried. Recrystallization from a chloroform solution yielded
green thin rod like crystals. Yield: 0.073 g (66%). M.P. 160 °C (dec).
Anal. Calc. for C20H18Ni1N2O2S2: C, 54.45; H, 4.11; N, 6.35. Found: C,
3.1. Synthesis of complexes
Thiobenzoate complexes of Ni(II) were synthesized using
Ni(dppe)Cl2, Ni(en)2Cl2, Ni(opda)Cl2, Ni(H2O)6Cl2 and anhydrous
NiCl2 as precursors. Synthetic routes for all the five complexes
are shown in Scheme 1. The complex 4 was already synthesized
way back in 1970 [18]. For 1–3 the solvent used was methanol
while it was ethanol and acetonitrile for 4 and 5 respectively.
Complexes 1 and 2 were highly soluble in chloroform while
complex 3 was moderately soluble in chloroform. 4 and 5 when
dried were sparingly soluble in ethanol and acetonitrile. The com-
pounds in solid state were found to be quiet stable and no apparent
decomposition occurred in several months on keeping under ambi-
ent conditions. In the IR spectra of these complexes strong bands
due to C@O, Ph–C, C–S stretching vibrations were observed which
are characteristic for thiobenzoate ligands. These IR bands are
important signals for analysing the bonding modes of the thi-
obenzoate ligands [19]. Generally the nitrile stretching band for
acetonitrile appears at 2254 cmꢁ1, however on coordination this
band is known to show a blue shift [20]. In the case of complex 5
54.37; H, 4.15; N, 4.06%. IR(KBr pellet) 1570
968 (C–S), 690 d(SCO).
m(C@O),1224 m(Ph–C),
m
2.2.4. Synthesis of [Ni2(SCOPh)4]ꢀEtOH (4)
An ethanolic solution (10 mL) of thiobenzoic acid (117
lL,
1.0 mmol) was added drop wise to a solution of sodium hydroxide
(0.040 g, 1.0 mmol) in the same solvent (10 mL) in an ice bath.
NiCl2ꢀ6H2O (0.119 g, 0.5 mmol) dissolved in 10 mL ethanol was
then added drop wise to the thiobenzoate solution. A red precipi-
tate appeared soon after the addition was started. After stirring
further for 2 h the red precipitate was filtered and dried under vac-
uum. Recrystallized from hot ethanol. Yield: 0.110 g (62%). M.P.
130 °C (dec). Anal. Calc. for C30H26Ni2O5S4: C, 50.59; H, 3.68. Found:
C, 50.51; H, 3.63%. IR(KBr pellet) 1583
m(C@O),1217 m(Ph–C), 958
m(C–S), 689 d(SCO).
the nitrile stretch appears at 2317 cmꢁ1
.
2.2.5. Synhesis of [Ni2(SCOPh)4]ꢀCNCH3 (5)
A procedure similar to that described for the synthesis of 4 was
adopted, the only change made being the use of anhydrous NiCl2 in
place of NiCl2ꢀ6H2O and acetonitrile as solvent. An almost clear red
brown solution was obtained which was filtered and solvent was
evaporated under vacuum leaving a dark red solid residue. Thin
red coloured crystals were obtained by recrystallization from hot
acetonitrile. Yield: 0.108 g (60%) M.P. 135 °C (dec). Anal. Calc. for
3.2. Structural description of complexes
All the complexes synthesized were characterized by single
crystal X-ray diffraction technique. Complex 1 was crystallized in
orthorhombic system with Pna21 space group. The molecular
structure of the complex is depicted in Fig. 1 along with the se-
lected bond lengths and angles. The geometry around the nickel
atom is square planar. Ni(II) is bonded to two phosphorus belong-
ing to the bis(diphenylphosphino)ethane ligand and other two
bonds are formed with sulfur atoms of the two thiocarboxylate
groups. Ni(II) is slightly displaced off (by 0.026 Å) the plane consti-
tuted by P1, P2, S1 and S2 atoms. The Niꢀ ꢀ ꢀO distances are too long
to have any bonding interactions between these two atoms.
Complex 2 crystallized in orthorhombic crystal system with
Pbcn space group. Recently, a similar complex, Ni(TMEDA) thi-
obenzoate has been reported to crystallize in monoclinic system
with P21/c space group [21] the molecular structure of the complex
is presented in the Fig. 2 along with the selected bond lengths and
angles. Ni(II) is bonded to the nitrogen atoms of the ethylenedia-
mine ligand, four other bonds are formed with the two oxygen
atoms and two sulfur atoms of the two thiocarboxylate ligands.
A twofold symmetry axis passes through the molecular center
(NiII). The geometry around Ni(II) is slightly distorted from the
C
30H24Ni2N1O4S4: C, 50.88; H, 3.42; N, 1.98. Found: C, 50.81; H,
3.40; N, 2.02%. IR(KBr pellet) 2317 (C„N), 1601 (C@O),1219
(Ph–C), 959 (C–S), 687 d(SCO).
m
m
m
m
2.3. X-ray data collection and structure refinement
Single crystal X-ray data were collected on a Xcalibur Oxford
EOS diffractometer using graphite monochromated Mo K radia-
a
tion (k = 0.7107 Å). Data collections were carried out at room tem-
perature (293 K). The structures were solved and refined by SHELX
set of software [12] using WINGX (ver. 1.80.05) [13] platform. Non-
hydrogen atoms were refined anisotropically while the hydrogen
atoms were placed at the calculated positions using SHELX default
parameters. Restraints such as DELU, DFIX, ISOR and EADP have
been applied while refining the structures (particularly in the case
of complex 5) where disordered atoms were present. A summary of
crystallographic data and structure solutions are given in Table 1.

D.K. Joshi, S. Bhattacharya / Inorganica Chimica Acta 411 (2014) 119–127
121
Table 1
Crystallographic data collection and structure refinement for 1–5.
1
2
3
4
5
Empirical formula
C
40H34NiO2P2S2
C16H18N2NiO2S2
393.15
293
C20H18N2NiO2S2
441.19
293
C60H50Ni4O10S8
1422.32
293
C62H46N3Ni4O8S8
1452.34
293
M (g molꢁ1
T (K)
)
731.44
293
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Orthorhombic
Pna21
15.807(5)
20.327(6)
10.982(4)
Orthorhombic
Pbcn
Orthorhombic
Pbcn
Triclinic
P1
Triclinic
P1
ꢀ
ꢀ
22.302(2)
8.1017(8)
9.9292(9)
20.8925(10)
10.2027(5)
9.7637(4)
10.410(5)
11.634(5)
12.696(5)
91.321(3)
92.830(5)
92.867(6)
1533.3(12)
1
11.3557(19)
11.4121(18)
13.654(3)
105.029(16)
112.497(18)
91.289(13)
1564.1(6)
1
a
(°)
b (°)
c
(°)
V (Å3)
3529(2)
4
1.377
0.793
1520
1794.1(3)
4
1.456
1.323
816
2081.23(17)
4
1.408
Z
qcalcd. (g cmꢁ1
)
1.540
1.538
1.542
1.509
l
(mmꢁ1
)
1.149
F(000)
Datas/restraints/parameters
Goodness of fit (GOF) on F2
1.006
1.098
1.032
0.955
0.985
R1, wR2 [I > 2
r
(I)]
0.0755, 0.1340
0.746/ꢁ0.515
0.0881, 0.1294
0.556/ꢁ0.591
0.0396, 0.0737
0.265/ꢁ0.313
0.0595, 0.1083
0.791/ꢁ0.729
0.0500, 0.0635
0.400/ꢁ0.518
Largest difference peak/hole (e Åꢁ3
)
Fig. 2. Thermal ellipsoid (at 30% probability level) of complex 2. For clarity
hydrogen atoms are omitted. Selected metric data: Bond lengths (Å): Ni1–N1
2.062(4), Ni1–O1 2.121(3), Ni1–S1 2.448(11). Bond angles (deg): O1Ni1S1 67.61(8),
S1Ni1N1 100.39(12), N1Ni1N1i 83.7(2), N1iNi1S1i 94.39(13), S1iNi1O1 91.81(15).
Scheme 1. Synthesis of complexes 1–5.
regular octahedron possibly because of the small bite angles
(67.62°) subtended by the thiocarboxylate ligands. The two S
atoms are trans while the two O atoms are cis to each other. The
chelate rings formed by the coordination of the two thiocarboxy-
late ligands are however, almost perpendicular to each other mak-
ing an interplanar angle of 87.70°. The Ni–O and Ni–S distances are
slightly longer than the sum of the respective atomic radii (by 0.21
and 0.25 Å respectively) while the Ni–N distances (2.068 Å) are
nearer to the sum of their covalent radii (1.89 Å).
Complex 3 also crystallized in orthorhombic system with Pbcn
space group. Molecular structure and numbering scheme are
shown in Fig. 3 along with the selected bond lengths and angles.
The molecular core, NiN2O2S2 is structurally comparable to that
of 2. Recently, bis(o-phenylenediamine)nickel(II) carboxylate,
[Ni(opda)2(OAc)2] has been characterized structurally [22]. The
Ni–O bond lengths are 2.126(5) Å while the Ni–N distances are
2.082(7) and 2.099(6) Å which are comparable to the correspond-
ing bond lengths in 3. The notable difference is the mode of binding
of the ligands which are monodentate in [Ni(opda)2(OAc)2] (for
carboxylate) while bidentate in 3 (for thiocarboxylate).
Fig. 1. Thermal ellipsoid (at 30% probability level) of complex 1. For clarity
hydrogen atoms are omitted. Selected metric data: Bond lengths (Å): Ni1–S1
2.230(2), Ni1–S2 2.237(2), Ni1–P1 2.158(2), Ni1–P2 2.150(2). Bond angles (deg):
S1Ni1S2 95.10(8), S2Ni1P1 90.66(8), P1Ni1P2 86.66(8), P2Ni1S1 88.01(8).

122
D.K. Joshi, S. Bhattacharya / Inorganica Chimica Acta 411 (2014) 119–127
Fig. 3. Thermal ellipsoid (at 30% probability level) of complex 3. For clarity
hydrogen atoms are omitted. Selected metric data: Bond lengths (Å): Ni1–O1
2.1026(19), Ni1–S1 2.4552(8), Ni1–N1 2.067(2). Bond angles (deg): O1Ni1S1
67.76(6), S1Ni1N1 93.10(8), N1Ni1N1i 84.48(13), O1iNi1O1 91.80(11).
Fig. 5. Thermal ellipsoid (at 30% probability level) of complex 5. For clarity
hydrogen atoms are omitted. Selected metric data: Bond lengths (Å): Ni1–Ni2
2.5083(18), Ni1–N1 2.055(7), Ni1–O1 2.036(4), Ni1–O2 2.022(4), Ni1–O3 2.044(5),
Ni1–O4 2.009(4), Ni2–S1 2.208(2), Ni2–S2 2.232(2), Ni2–S3 2.224(2), Ni2–S4
2.210(2). Bond angles (deg): N1NI1Ni2 174.95(19), N1Ni1O1 90.5(2), N1Ni1O2
87.0(2), N1Ni1O3 91.7(2), N1Ni1O4 94.0(2), Ni2Ni1O1 89.22(14), Ni2Ni1O2
87.98(14), Ni2Ni1O3 88.48(15), Ni2Ni1O4 91.03(15), O1Ni1O2 91.67(18),
O1Ni1O3 177.4(2), O1Ni1O4 90.79(19), O2Ni1O3 87.10(18), O2Ni1O4 177.3(2),
O3Ni1O4 90.40(19), Ni1Ni2S1 89.22(7), Ni1Ni2S2 89.94(8), Ni1Ni2S3 90.44(7),
Ni1Ni2S4 89.04(7), S1Ni2S2 89.20(8), S1Ni2S3 179.61(11), S1Ni2S4 90.37(8),
S2Ni2S3 90.70(7), S2Ni2S4 178.21(10), S3Ni2S4 89.75(8).
Fig. 4. Thermal ellipsoid (at 30% probability level) of complex 4. For clarity
hydrogen atoms are omitted. Selected metric data: Bond lengths (Å): Ni1–Ni2
2.5012(12), Ni1–O1 2.009(3), Ni1–O2 2.037(3), Ni1–O3 2.004(3), Ni1–O4 2.064(3),
Ni1–O5 2.054(3), Ni2–S1 2.2221(16), Ni2–S2 2.2281(16), Ni3–S3 2.2317(16), Ni2–
S4 2.2255(17). Bond angles (deg): Ni2Ni1O1 90.17(9), Ni2Ni1O2 87.70(10),
Ni2Ni1O3 90.14(10), Ni2Ni1O4 90.07(9), Ni2Ni1O5 178.40(10), O1Ni1O2 90.38(4),
O1Ni1O3 179.68(13), O1Ni1O4 90.63(13), O1Ni1O5 88.47(13), O2Ni1O3 89.71(13),
O2Ni1O4 177.56(14), O2Ni1O5 91.45(13), O3Ni1O4 89.28(13), O3Ni1O5 91.22(13),
O4Ni1O5 90.79(12), Ni1Ni2S1 88.38(4), Ni1Ni2S2 89.34(5), Ni1Ni2S3 88.64(5),
Ni1Ni2S4 91.37(4), S1Ni2S2 92.20(5), S1Ni2S3 89.18(5), S1Ni2S4 178.43(6),
S2Ni2S4 89.35(6), S3Ni2S4 89.26(5).
Complexes 4 and 5 crystallized in triclinic crystal system with
P1 space group. Figs. 4 and 5 depict the molecular structures of 4
ꢀ
Fig. 6. The dimeric (tetranuclear) structure of 4 which is supposed to be due to the
interaction of Ni1 of one unit with S1 of other unit.
and 5 respectively along with the selected bond lengths and angles.
The complexes 4 and 5 are structurally similar to each other in
every aspect other than the coordinated solvent which is ethanol
in 4 and acetonitrile in 5. Further, the unit cell of 5 contains a dis-
ordered molecule of solvent acetonitrile in the lattice.
Solid state structures of 4 and 5 revealed the existence of a di-
meric assembly of dinuclear (two Ni atoms) complexes. The two
dinuclear units are joined to each other by bridging through S
atoms (Fig. 6). Two molecules of ethanol are coordinated to two
Ni(II) centres of the dimer from opposite sides. Each dinuclear unit
has a four bladed paddlewheel structure and the coordinated eth-
anol appears like a shaft. Coordination environment of the two Ni
of a monomeric unit are distinctly different from each other. Ni1 is
surrounded by five oxygens (four from the thiocarboxylates and
one from ethanol) while Ni2 is coordinated by five sulfurs. Though
the bridging, Ni2–S1 bond is longer [2.811(2) Å] than the other Ni–
S bonds yet it is significantly shorter than the sum of van der Waals
radii of the two atoms (3.26 Å). The sixth coordination site of Ni1 is
occupied by Ni2 and vice versa. Thus, the geometry around each
Ni(II) centre is octahedral. Notably, the distance between the two
nickel atoms, 2.500(1) Å is very close to the Ni–Ni single bond
length (2.42 Å). Very recently, the structure of a nickel(II) carboxyl-
ate, Ni2(O2CPh)4(NITpPy)2 has been reported [23]. In this complex

D.K. Joshi, S. Bhattacharya / Inorganica Chimica Acta 411 (2014) 119–127
Fig. 7. Selected molecular orbitals for 1 (orbital contour value = 0.05).
Fig. 8. The M (H) curve at room temperature for complex 4.
123
the two Ni(II) are bridged by four benzoate ligands forming a pad-
dle wheel type of cage and the Ni–Ni distance is 2.645(4) Å. Anti-
ferromagnetic coupling has been observed in this complex at
lower temperatures. A few other dinuclear carboxylates have also
been reported earlier [8(c)] in which the Ni–Ni distances vary be-
tween 2.70 and 2.75 Å which are significantly longer than the
Ni–Ni distance in 4 and 5. The Ni–Ni distance in dithiocarboxylate
complex, Ni2(S2CCH2Ph)4 is also slightly larger being 2.551(3) Å
[24].
660 nm was observed. For complex 5 a broad band around
730 nm was obtained.
The broad nature of the spectral bands often poses a problem
for their unambiguous assignment. Time dependent density func-
tional calculations are quiet useful to understand the spectral fea-
tures. However, these calculations are limited to molecules with a
closed shell. TD DFT calculations were performed on complex 1 as
it is the only molecule having a closed electronic shell. The results
revealed presence of three bands in the visible region at 507, 472
and 463 nm. All the three bands have merged in a single broad
band centered at 467 nm in the experimentally observed spec-
trum. Bands below 400 nm are generally associated with the ligand
to metal or ligand to ligand charge transfers. Fig. 7 shows the orbi-
tals involved in transitions for the above given respective bands.
3.3. Electronic absorption spectra
The electronic absorption spectra of compounds 1 and 2 were
recorded in chloroform solutions whereas those of 3, 4 and 5 were
recorded in DMSO (Fig. S1–S5; Supplementary data). For complex 1
only one prominent band centered on 467 nm was observed. For
complex 2 one prominent band centered at 630 nm was observed.
The complex 3 exhibited three bands of which two peaks were cen-
tered at 778 and 552 nm. A third one centered at 306 nm is due to
charge transfer (LLCT). For complex 4 a broad band centered at
3.4. Magnetic studies
The complexes 4 and 5 were binuclear nickel complexes and in
both of them the Ni–Ni distance is in covalent bonding range. VSM
magnetic moment measurements were conducted for complex 4 at

124
D.K. Joshi, S. Bhattacharya / Inorganica Chimica Acta 411 (2014) 119–127
Fig. 9. Molar and inverse magnetic susceptibilities of complex 4 as a function of temperature.
Fig. 10. The effective magnetic moment of complex 4 in Bohr magnetons as a function of temperature.
room temperature varying field from ꢁ15000 Oe to +15000 Oe.
Though a magnetic moment of 4.89 B is expected for a system
of two Ni(II) atoms (with 4 unpaired electrons) the experimentally
observed value at the maximum field was found to be only 2.66
revealing the presence of only two unpaired electrons in the dinu-
clear unit.
Fig. 8 represents the M (H) curve for complex 4. The curve
clearly shows that the magnetisation (M) depends linearly on
magnetic field (H) at room temperature which is an indication
of the complex being paramagnetic in behaviour at room temper-
ature. Although at very low fields region an ‘‘S’’ shaped M (H)
curve is obtained. The enlarged view of ‘‘S’’ shaped curve however
depicts hysteresis with very little coercivity (Hc) ꢂ66 Oe and rem-
anence magnetisation (Mr) ꢂ1.313 ꢃ 10ꢁ3 emu/g. The hysteresis
curve is shown in the inset of Fig. 8. The observed low magnetic
we performed SQUID magnetic measurements for a temperature
range of 5–300 K at a field of 0.5 T. A plot of molar magnetic sus-
ceptibility and inverse magnetic susceptibility for the complex 4
in the solid state as a function of temperature is shown in
Fig. 9. The value of molar susceptibility is 0.0030 emu/mol at
300 K which increases smoothly till 75 K as temperature is re-
duced and then rises sharply reaches a maximum of 0.035 emu/
mol at around 9.8 K and then again dips down as further temper-
ature is reduced reaching a value of 0.032 emu/mol at 5 K. Fig. 10
shows the plot of effective magnetic moment as a function of
l
lB
temperature. At 300 K, the effective magnetic moment leff
2.49 B which reaches a maximum of about 2.55 B at around
154 K. From 52 K onwards leff falls down very sharply to a mini-
mum of 1.07 B at 5 K. To a plot of inverse of molar susceptibility
=
l
l
l
versus temperature (Fig. 11) linear fit was applied to the
Curie–Weiss law which gave a h value of ꢁ2.8 K. The value of J
calculated from the equation J/2k = h is ꢁ3.91 cmꢁ1. The value of
J was also obtained using theoretical equation of R. L. Carlin
moment (2.66 lB per dinuclear unit possessing two Ni(II) centers)
is indicative of the existence of an electron pair bond between
two Ni centers. It may be noted that Ni–Ni bonds are not gener-
ally expected in such systems [25]. This fact prompted us to per-
form detailed magnetic study for the complex 4. In that context
[26] by plotting
a graph between molar susceptibility and
temperature (Fig. 12), using the following equation.

D.K. Joshi, S. Bhattacharya / Inorganica Chimica Acta 411 (2014) 119–127
125
Fig. 11. Inverse magnetic susceptibility as a function of temperature along with the fit to Curie–Weiss law.
Fig. 12. Molar magnetic susceptibility as a function of temperature along with the fit to R. L. Carlin equation.
Computations at B3LYP level using 6-31G⁄⁄ basis set were carried
out to understand the nature of bonding in 4 and 5. The atomic
coordinates obtained from X-ray diffraction measurements were
used for the computations. As mentioned (in Section 3.4) above
the Ni1–Ni2 distance in 4 is 2.500 Å which is comparable to the
Ni–Ni single bond length. It is normally expected that a M–M bond
will not be stable enough to hold two square planar Ni units as 16
electrons (from the two Ni centers) will fill four bonding and four
antibonding molecular orbitals equally. One may thus argue that
the close proximity of the two Ni atoms is dictated by the bite span
of the thiocarboxylate ligands. However, In our recent studies on
heterobimetallic thiocarboxylate complexes we have seen that
the thiocarboxylate ligands can span to quite a larger extent if
there is no M–M bonding [7]. An important question came forth
to start DFT calculations on 4. Should the Ni atoms be considered
as spin paired or with (one or two) unpaired spins. Single point
energy calculation on the molecule with different electronic con-
figurations revealed that the one with 1 unpaired electron on each
Ni(II) is most stable. (However, the energy of molecule with a
vm ¼ Ng2b2=kT ꢀ expð2J=kTÞ þ 5expð6J=kTÞ=1 þ 3expð2J=kTÞ
þ 5expð6J=kTÞ
The best fitting for the experimental data gives a value of
g = 2.24 and J = ꢁ3.76 cmꢁ1 the latter is indicative of a weak anti-
ferromagnetic coupling in the system. Here
have their usual meanings.
vm, C, T, h, k, N, b, g
3.5. Density functional calculations
The antiferromagnetic coupling in the carboxylate complexes
have been explained by Bencini et al. who proposed dz2 —dz2
(r
bonding) and dx2 —dx2ꢁy2 (d bonding) between two Ni centers
ꢁy2
[27]. Absence of a direct Ni–Ni bond in these complexes was
however, suggested by the fact that the M–M distances are signif-
icantly longer than the same in Ni2(S2CCH2Ph)4. Rakitin et al. pro-
posed the possibility of two super exchange channels, the
r path
(dx2 o–d ) and the path (dxy o–dxy) [28].
–roꢁrC–r
p
–po–pC–p
ꢁy2
x2ꢁy2

126
D.K. Joshi, S. Bhattacharya / Inorganica Chimica Acta 411 (2014) 119–127
Appendix A. Supplementary material
CCDC 888478 (1), 888476 (2), 888475 (3), 888479 (4), 888477
(5) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. Electronic absorption spectra of all the complexes are
286 given in Figures S1-S5. Supplementary data associated with
this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2013.11.037.
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Acknowledgements
Financial assistance from the Council of Scientific and Industrial
Research, New Delhi (Scheme number 01(2680)/12/EMR-II) and
University Grants Commission (Project number: F-40-62/2011SR)
are gratefully acknowledged. Authors are grateful to the coordina-
tors at IIT, Kanpur and IIT, Roorkee for magnetic susceptibility
measurements using VSM and SQUID respectively. Thanks are due
to Mr. Bikash Shaw, IACS, Kolkata for his help in certain
calculations.

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