Download Notes Synthesis and characterization of a new nickel(II) mixed

Indian Journal of Chemistry
Vol 49, December 2010, pp.1601-1606
Notes
Synthesis and characterization of a new
nickel(II) mixed ligand complex with
2-(2'-pyridyl) benzothiazole
good biological agent and exhibits excellent
hydrogen bonding, making them suitable candidates
in medicine and biochemistry.
R N Patel*, Anurag Singh, K K Shukla, Dinesh K Patel &
V P Sondhiya
Experimental
Nickel(II) nitrate hexahydrate was purchased from
S D Fine-Chemicals, India. All other chemicals used
were of synthetic grade and used without further
purification. Elemental analyses of the complexes were
performed on an Elementar Vario EL III Carlo Erba
1108 analyzer. FAB mass spectra were recorded on a
Jeol SX 102/DA 6000 mass spectrometer using xenon
(6 kV, 10 mA) as the FAB gas. The accelerating voltage
was 10 kV and the spectra were recorded at room
temperature. UV-vis spectra were recorded at 25 °C on a
Shimadzu UV-visible recording spectrophotometer
UV-160 in quartz cells. IR spectra were recorded in KBr
medium on a Perkin-Elmer spectrophotometer.
Magnetic susceptibility measurements were made on a
Gouy balance using mercury(II) tetrathiocynatocobaltate(II) as calibrating agent (χg = 16.44 × 10-6 cgs units).
Cyclic voltammetry was carried out on a BAS-100
Epsilon electrochemical analyzer having an
electrochemical cell with a three-electrode system.
Ag/AgCl was used as the reference electrode, glassy
carbon as the working electrode and platinum wire as
the auxiliary electrode. NaClO4 (0.1 M) was used as
supporting electrolyte and DMSO as solvent. All
measurements were carried out at 298 K under a
nitrogen atmosphere. The in vitro SOD activity was
measured using alkaline DMSO as a source of
superoxide radical (O2-) and nitro blue tetrazolium
chloride (NBT) as O2- scavenger7. Molar conductivities
of the freshly prepared DMSO solutions (2×10-3 M)
were measured on a Systronics conductivity TDS
meter 308.
Crystals suitable for single crystal X-ray analysis
for the complex [Ni(L1)](L2)(H2O])(ClO4) were
grown by slow evaporation of the reaction mixtures
at room temperature. These were mounted on a glass
fiber and used for data collection. Crystal data were
collected on Enraf-Nonius MACH3 diffractometer
using graphite monochromatized Mo-Kα radiation
(λ = 0.71073Å). The crystal orientation, cell refinement
and intensity measurements were made using the
program CAD-4PC performing ψ-scan measure-
Department of Chemistry, APS University,
Rewa 486 003, (MP), India
Email: [email protected]
Received 16 August 2010; revised and accepted 18 November 2010
A new complex, [Ni(L1)(L2)(H2O)]ClO4, (L1 = 2-(2′-pyridyl)benzothiazole), L2 = N-[(1)-pyridin-2-ylmethylidene] benzohydrazone), has been synthesized and characterized by various
physico-chemical techniques. The nickel atom is coordinated in a
distorted octahedral geometry. The metal center is coordinated
through azomethine nitrogen, pyridine nitrogens, carbonyl
oxygen and one nitrogen atom of L1. Additionally, the pyridyl
planes in complex are engaged in intra- and intermolecular
hydrogen bonding interactions.
Keywords: Coordination chemistry, Nickel, SOD activity,
Schiff bases
IPC Code: Int. Cl.9 C07F15/04
Nickel(II) complexes containing sulfur have received
considerable attention due to the identification of a
sulfur-rich coordination environment in biologically
active nickel centres, such as the active sites of certain
ureases, methyl-S-coenzyme-M-methyl reductase and
hydrogenases1. A number of metal complexes with
similar type of Schiff base of 2-aminothiophenol have
been reported from our laboratory2.
The synthesis of low molecular weight nickel(II)
complexes mimicking superoxide dismutase (SOD)
activity is a challenge for bioinorganic chemists and
recently some complexes with high catalytic activity
have been reported3. Nickel-containing superoxide
dismutase (NiSOD) has been isolated from several
Streptomyces species4. The enzymatic activity of
NiSOD5 is as high as that of Cu-ZnSOD at about
109 M-1s-1 per metal center. Although numerous
organic, inorganic or coordination compounds with
antiproliferative activity have been reported6, most
of them are excluded from further clinical studies
due to their toxicity and numerous undesirable
side effects. In the present investigation, the Ni(II)
complex of 2-(2′-pyridyl)benzothiazole is a fairly
1602
INDIAN J CHEM, SEC A, DECEMBER 2010
ments. The structures were solved by direct method
using the program SHELXS-978 and refined
by full-matrix least-squares techniques against F2
using SHELXL-979. All non-hydrogen atoms were
refined anisotropically. All the hydrogen atoms
were geometrically fixed and allowed to refine
using a riding model.
For synthesis of the complex, equimolar (1:1)
methanolic solutions (10 mL each) of metal salt
(NiNO3.6H2O 1.0 mmol, 0.291 g) and L1 (1 mmol,
0.212 g) were mixed and stirring was continued for
30 min at room temperature. To this reaction
mixture, ligand L2 (1.0 mmol, 0.225 g) was added
after 1 hr and further stirred for 45 min. Finally, to
this solution one equivalent of sodium perchlorate
was added slowly drop by drop and stirred
further for 30 min. The resultant clear solution was
left for slow evaporation. After 3-4 days, brownish
yellow crystals which were suitable for X-ray
crystallography were collected and washed with
MeOH and diethyl ether. These were dried in air at
room temperature and stored in CaCl2 desiccator,
(yield 65%). The empirical formula given for the
compound C25H18ClN5NiO6S was confirmed by
elemental analysis: Found(calcd): C, 49.01(49.12);
H, 2.71(2.94); N, 11.23(11.46)%.
Results and discussion
By the condensation reaction of 2-pyridinecarboxaldehyde with 2-mercaptoarylamine, a sulfur
containing Schiff base ligand, 2-(2′-pyridyl)benzothiazole, is formed. Sulphur containing
heteroaromatic Schiff bases easily cyclize under
alkaline condition. The Schiff base of such type
can undergo elimination reaction through the
detachment of atom. Miernicka et al.10 have also
reported that the confirmation of detachment of a
molecule can be achieved by thermogravimetric
analysis.
Detachment
of
hydrogen
atom
suggests the cyclization of S containing ligand to
the 2-(2′-pyridyl)benzothiazole.We successfully
performed the cyclization of S containing ligand
to its 2-(2′-pyridyl)benzothiazole. The sequential route
for the synthesis of complex is given in Scheme 1.
X-ray diffraction quality single crystal of the
complex [Ni(L1)(L2)(H2O)]ClO4 was obtained by vapor
diffusion of methanolic solution of the complex. The
ORTEP diagram of complex is shown in Fig. 1 and
the related crystal structure refinement data is given in
Table 1. Selected bond lengths and angles within the
complex are listed in Table 2. Single crystal analysis
of complex reveal that the reddish brown complex
crystallizes in the monoclinic crystal system with
space group P21/c having four molecule in the unit cell.
The X-ray data of the complex show that water molecule
is present in the coordination sphere. It reveals that the
nickel(II) center has a N4O2 coordination moiety and
has distorted octahedral geometry. The two ligands are
coordinated in the meridonial fashion11. L2 undergoes
deprotonation in the presence of metal ion and consequently, the complex of the deprotonated and cyclized
ligand is formed in neutral media. Thus, L2 coordinates
to the nickel atom as a uninegatively charged chelating
agent in its deprotonated hydrazone form via the
ketonic oxygen atom, azomethine nitrogen atom and
the pyridine nitrogen atom.
NH
N
O
N
+
Methanol, at rt
Ni(NO3)2.6H2O
+ NaClO4
N
N
O
H2O
Ni
N
+
N
N
N
S
N
S
Scheme 1
ClO 4
NOTES
1603
Table 1—Crystal data and structural refinement for complex
[Ni(L1)(L2)(H2O)]ClO4
Fig. 1—ORTEP diagram of complex [Ni(L1)(L2)(H2O)]ClO4.
There is extensive delocalization of the negative
charge12, which is generated upon deprotonation of
the ligand, in the C-N-N-C chain as indicated by the
intermediate C(18)-N(4) = 1.271(4) Å, N(4)-N(5) =
1.317(5) Å and N(5)-C(19)= 1.347(19) Å bond distances.
The two Ni-N distances (~2.074Å) are close to the
sum of the non-polar covalent radii (2.15 Å)
(ref. 13), indicating strong nickel-nitrogen interaction.
However, the Ni-N(4)(azo) bond distance (1.991(2) Å)
in the present complex is shorter than the
Ni-N(2)(azo) distance (2.116(2) Å). The stronger
coordination of the nickel atom to the azomethine
nitrogen of L2 as compared with the azomethine
nitrogen atom of L1 is consistent with the higher
basicity of the azomethine nitrogen atom and also a
consequence of the atom being shared by the adjacent
5-membered chelate rings. The Ni-N distance in the
complex lies within a close range (2.074 Å) and is
consistent with the corresponding values of Ni(II)-N
system14 but is considerably shorter than the van der
waals radii (4.0 Å) (ref. 15).
In the octahedral geometry, N3O atoms are
involved in the plane whereas a pair of NO atoms is in
the apical position. The values obtained in
this study are comparable with those obtained
previously16. However, the mutual disposition of the
ligands deviates noticeable from perpendicularity
which is indicated by decreased values of the trans
angle, N(4)-Ni-N(1) 173.89°(10), N(3)-Ni-O(1)
155.26°(9) and N(2)-Ni-O(2) 167.69°(9). Individual
bond distances and angles within the coordination
polyhedron compare well with those found in other
octahedral complexes of nickel(II) involving N donor
atoms17. The ranges of the cisoid and the transoid
angles reflect the degree of distortion from the ideal
octahedron geometry. The distortion of the geometry
Empirical formula
Formula weight
Temperature (T) (K)
Wavelength (Mo-Kα)(Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å 3)
Z
Calc. density (mg m-3)
Abs. coeff. (mm-1)
F(000)
Crystal size (mm)
θ Range for data collection
Limiting indices
Reflections collected /
Unique reflections
Completeness to theta
Abs. correction equiv.
Max. and min. transmission
Refinement method F2
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I > 2σ (I)]
R indices (all data)
Largest diff. peak and
hole (e.A-3)
C25H18ClN5NiO6S
610.66
120(2)
0.71073
Monoclinic
P21/c
12.3537(11)
13.5555(9)
15.459(3)
90
100.266(11)
90
2547.3(5)
4
1.592
1.001
1248
0.33 × 0.28 × 0.22
3.07 to 24.99
-14≤ h ≤ 14, -16 ≤ k ≤ 16, -18 ≤ l ≤ 18
19667
4482 [R(int) = 0.0529]
24.99 99.8 %
Semi-empirical from
0.8099 and 0.7336
Full-matrix least squares on
4482 / 0 / 352
0.916
R1 = 0.0347, wR2 = 0.0725
R1 = 0.0626, wR2 = 0.0758
0.583 and -0.367
Table 2—Bond lengths and angles of complex Ni(L1)(L2)(H2O)]ClO4
Bond lengths (Å)
Ni(1)-N(4)
1.991(2)
Ni(1)-N(1)
2.085(2)
Ni(1)-O(2)
2.091(2)
Ni(1)-N(3)
2.104(2)
Ni(1)-O(1)
2.081(2)
Ni(1)-N(2)
2.116(2)
Bond angles(°)
N(4)-Ni(1)-O(2)
92.94(8)
O(1)-Ni(1)-N(2)
89.63(9)
N(4)-Ni(1)-O(1)
76.63(9)
N(1)-Ni(1)-N(2) 79.27(10)
O(2)-Ni(1)-O(1)
88.61(8)
N(3)-Ni(1)-N(2)
95.05(9)
N(4)-Ni(1)-N(1) 173.89(10)
O(2)-Ni(1)-N(3)
91.63(9)
O(2)-Ni(1)-N(1)
89.78(9)
O(1)-Ni(1)-N(3) 155.26(9)
O(1)-Ni(1)-N(1)
108.93(9)
N(1)-Ni(1)-N(3)
95.81(9)
N(4)-Ni(1)-N(3)
78.65(10)
N(4)-Ni(1)-N(2) 98.50(10)
Symmetry transformations used to generate equivalent atoms.
is also evidenced from the rms deviations and
dihedral angles of the planes. The dihedral angle
formed by the two least square planes is equal
to 88.49° (0.07), which confirms the non-coplanarity
of the two rings. The deviation from octahedral
geometry is due to the restricted bite size of the
ligand. The C-S bond distance (1.746 Å) being similar
1604
INDIAN J CHEM, SEC A, DECEMBER 2010
to that in the free ligand, is indicative of
non-coordination of S atom in the complex. Similarly,
the N(4)-N(5) bond distance (1.377(3) Å) is closer to
the single bond length (1.45 Å) rather than a double
bond length(1.25 Å) (ref. 18).
The packing of the molecule in a unit cell is shown
in Fig. 2. The unit cell is viewed down the ‘c’-axis
and four molecules of the complex are arranged in the
unit cell. It is evident from the figure that the complex
exhibit a 2D supramolecular architecture arising from
two types of hydrogen bonding interactions (Table 3).
The solvent water molecule (O2W), amine nitrogens
(N(4), N(5) and N(1), enolic sulfur and carbonyl
oxygen (O1) atom play a vital role in the formation of
the 2D sheet through intra-molecular hydrogen bonding
interactions. Four neighboring mononuclear units are
held together through inter-molecular interactions
generated by anionic oxygen atoms. These two types
Fig. 2—Unit cell packing diagram of complex Ni(L1)(L2)(H2O)]ClO4.
of hydrogen bonding are interwoven and facilitate the
formation of a 2D supramolecular sheet in the c-axis
(Fig. 2). The C-N (2.845 Å) and C-O (2.793 Å)
distances between hydrogen bonded groups are indicative of the presence of moderate directional hydrogen
bonding, which may be used in crystal engineering19.
The paramagnetic nickel(II) complexes are
characterized by two main absorptions bands at
about 300 and 400 nm. These bands are present
in [Ni(L1)(L2)(H2O)](ClO4) complex. The complex
shows three strong absorption and one weak
absorption [3A2g→3T2g, (11499 cm-1 (0.2)υ1),
3
A2g→3T1g (F) (14286 cm-1 (0.8)υ2), 3A2g→3T1g (P)
(26316 cm-1 (3.0) υ3)], characteristic of regular
octahedral nickel(II) complex16.
The medium intensity band around 1610 cm-1 due
to ν(C=N)16 of L2 shifts to 1590-1600 cm-1 in the
complex, indicating the azomethine coordination to
the metal through nitrogen. The coordination of the
azomethine nitrogen is further evidenced by the
presence of a new band in the range 450-490 cm-1,
assignable to ν(Ni-N) for this complex20. Two bands
are clearly observed at 3420 cm-1 s (ν(OH)assym) and
3330 cm-1 mw (ν(OH)sym) for the coordinated water
molecule21. The presence of bands at ~ 1100 cm-1
and ~ 625 cm-1 indicates that the Td symmetry
of ClO4- is maintained in the complex. This
therefore, suggests the presence of ClO4- outside
the coordination sphere in the complex22.
The molar conductivity of the complex was
measured in DMSO solution (3 × 10-3M). The molar
Table 3—Hydrogen bonding interactions (Å and °) for complex [Ni(L1)(L2)(H2O)]ClO4
D---H....A
Intra-molecular
C1 -H1 ...O2 (0)
C4 -H4 ...S1 (0)
C11 -H11 ...N4 (0)
C11 -H11 ...N5 (0)
C13 -H13 ...N1 (0)
C21 -H21 ...N5 (0)
C25 -H25 ...O1 (0)
C4 -H4 ...O1 (3)
Inter-molecular
C10 -H10 ...O444 (0)
C1 -H1 ...O111 (1)
C25 -H25 ...O222 (1)
C14 -H14 ...O333 (2)
C13 -H13 ...O444 (2)
C8 -H8 ...O111 (4)
C15 -H15 ...O333 (6)
D - H (Å)
H...A (Å)
D...A (Å)
∠ D - H...A (°)
0.950(.003)
0.950(.003)
0.950(.003)
0.950(.003)
0.950(.003)
0.950(.003)
0.950(.003)
0.950(.003)
2.532(.002)
2.802(.101)
2.627(.002)
2.592(.002)
2.848(.002)
2.543(.003)
2.493(.002)
2.543(.002)
3.086(.004)
3.179(.004)
3.361(.004)
3.476(.004)
3.353(.004)
2.845(.004)
2.793(.004)
3.279(.003)
117.30( 0.20)
104.68( 0.21)
134.36( 0.21)
155.09( 0.21)
114.34( 0.20)
98.59 ( 0.22)
98.29 ( 0.20)
134.36( 0.20)
0.950(.004)
0.950(.003)
0.950(.003)
0.950(.004)
0.950(.003)
0.950(.003)
0.950(.003)
2.782(.002)
2.793(.002)
2.690(.003)
2.838(.003)
2.523(.003)
2.711(.002)
2.597(.003)
3.395(.004)
3.372(.004)
3.367(.004)
3.392(.004)
3.309(.004)
3.385(.004)
3.314(.004)
123.03( 0.21)
120.19( 0.20)
128.76( 0.20)
118.21( 0.21)
140.14( 0.20)
128.47( 0.21)
132.56( 0.21)
0 = x,y,z; 1 = x,+y-1,+z; 2 = x,-y+1/2,+z-1/2; 3 = -x+1,-y,-z+1; 4 = -x+1,-y+1,-z+1; 6 = -x,+y-1/2,- z+1/2;
NOTES
Fig. 3—Cyclic voltammogram of the ligands, L1 (1) and L2 (2), and,
the Ni(II) complex (3) in DMSO. [Conc.: 0.003 mol-1 dm-3;
supporting electrolyte: 0.1M NaClO4; scan rate: 200 mV/s].
conductivity value (121 ohm-1cm2mol-1) indicates
the presence of 1:1 electrolyte23, suggesting the
presence of deprotonated coordination of L2 in
solution. The room temperature effective magnetic
moment (µeff) of the complex is 2.89 BM. The
L1 and L2 along with a water molecule may
provide a octahedral geometry around nickel(II).
The observed magnetic moment values favor an
octahedral geometry.
The cyclic voltammogram of complex was
recorded in DMSO, with sodium perchlorate as
supporting electrolyte. The cyclic voltammogram
for nickel(II) complex exhibits metal centered
electrochemical process (Fig. 3). The complex
shows two reduction peaks corresponding to
Ni(II)-Ni(I) and Ni(I)-Ni(0). Although the
voltammogram shows two reduction peaks and
two oxidation peaks, the oxidation peak of the
second redox couple is ill-defined. This may be
due to the width of the peak24. The first redox couple
is obtained at Epc = -0.431 V and Epa = -0.30 V which
may be due to ligand based reduction, whereas the
second redox couple is obtained at Epc = -0.768 V and
Epa = -0.27 V. Both originate from an irreversible
process with a peak-to-peak separation, ∆Ep = -495 V
and Ipa/Ipc ratio is less than unity (Ipa/Ipc = 0.45)
confirming the irreversible process.
The superoxide dismutase activity (SOD) for the
present complex was measured. Superoxide was
1605
enzymatically supplied from alkaline DMSO and the
SOD activity was evaluated by the NBT assay1
following the reduction of NBT to MF+ kinetically at
560 nm. The observed IC50 value of the present
complex was compared with the earlier reported values
(43-58 µmol dm-3) for nickel(II) complexes17. The
catalytic activity of NiSOD5, however, is on the same
high level as that of Cu-ZnSOD at about 109 M-1 s-1 per
metal center. The IC50 data of the SOD activity assay of
the present complex is 65 µmol dm-3 while the kinetic
catalytic constant, kMccF is1.462 ×104 M-1 s-1.
In the above study, the synthesis and characterization of [Ni(L1)(L2)(H2O)]ClO4 and L1 is described.
L1 undergoes cyclization in basic medium whereas
L2 deprotonates in presence of metal and coordinates
with the nickel atom as uninegative charged NNO
tridentate chelating agent. Both ligand and water
molecules bind to the nickel(II) ion and show a
distorted octahedral geometry around the nickel
center. The various bonding parameters indicate
that substantial delocalization occurs within the
chelate rings of L2 mediated via in-plane-π-bonding
along with σ-donation, thus resulting in an increased
structural stability. Metal centered process that is
strongly influenced by the nature of binding does not
seems to dominate the redox behavior of the complex.
It is due to the non coordination behavior of S atom.
In general, the oxidation process which occurs at
lower potential due to sulfur atom does not favor the
coordination through sulfur atom.
Supplementary data
CCDC 766598 and 766597 contain the
supplementary crystallographic data for the ligand
L1 and its complex [Ni(L1)(L2)(H2O)]ClO4. These
data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK;
Fax: (+44) 1223-336-033; Email: deposit@
ccdc.cam.ac.uk.
Acknowledgement
Our grateful thanks are due to the National
Diffraction Facility, X-ray Division and RSIC
(SAIF), IIT Bombay, Mumbai, for single crystal
data collection. The Head RSIC (SAIF), Central
Drug Research Institute, Lucknow, is also thankfully
acknowledged for providing analytical and
spectral facilities. Financial assistance from UGC
1606
INDIAN J CHEM, SEC A, DECEMBER 2010
[Scheme no.36-28/2008 (SR)] and DRDO [Scheme
no. ERIP/Er 0603574/m/01/1118], Delhi is also
thankfully acknowledged.
References
1
2
3
4
5
6
7
8
9
10
Naik A D, Annigeri S M, Gangadharmath U B, Revankar V
K & Mahale V B, J Mol Struct, 616 (2002) 119.
Patel R N, Gundla V L N & Patel D K, Indian J Chem, 47A
(2008) 353.
Patel R N, Kesharwani M K, Singh A, Patel D K & Choudhary M, Trans Met Chem, 33 (2008) 733.
Stroupe M E, DiDonato M & Tainer J A, Handbook of Metalloproteins, Vol 2, edited by A Messerschmidt, (Wiley, UK)
2001, p. 941.
Youn H D, Kim E J, Roe J H, Hah Y C & Kang S O, J
Biochem, 318 (1996) 889.
Ziegler C J, Silverman A P & Lippard S J, J Bioinorg Chem,
5 (2000) 774.
Patel R N, Shukla K K, Singh A, Patel D K, NiclosGutierrez J & Choquesillo-lazarte D, J Coord Chem, 63(20)
(2010) 3648.
Sheldrick G M, Acta Crystallo, A46 (1990) 467.
Sheldrick G M, SHELXL-97. Program for the Refinement of
Crystal Structures, (University of Goettingen, Germany) 1997.
Miernicka M, Szulawska A, Czyz M, Lorenz I-P, Mayer P,
Karwowski B & Budzisz E, J Inorg Biochem, 102 (2008) 157.
11 Garcia I, Bermejo E, El Sawaf A K, Castineiras A & West D
X, Polyhedron, 21 (2002) 729.
12 Akbar Ali M, Mirza A H, Ravoof T B S A & Bernhardt P V,
Polyhedron, 23 (2004) 2031.
13 Sanderson R T, Inorganic Chemistry (Reinhold, New York)
1967, p. 174.
14 Niu D Z, Sun B W, Lu Z S, Wang Z M & Yan C H, Acta
Crystallogr, E57 (2001) M590.
15 Bondi A, J Phys Chem, 68 (1964) 441.
16 Patel R N, Shukla K K, Singh A, Choudhary M, Patel D K,
Niclós-Gutiérrez J & Choqusillo-Lazarte D, Trans Met
Chem, 23 (2009) 239.
17 Patel R N, Gundla V L N & Patel D K, Polyhedron, 26
(2007) 757.
18 Huheely J E, Keiter E A & Keiter R L, Inorganic Chemistry,
Principles of Structure and Reactivity, 4Th Edn, (Harper
Collins College, New York) 1993.
19 Rahaman S H, Bose D, Chowdhary H, Mostaffa G, Fun H-K
& Ghosh B K, Polyhedron, 24 (2005) 1837.
20 Hathaway B J & Underhill A E, J Chem Soc, (1961) 3091.
21 Vallarino L M, Goedken V L & Quagliano J V, Inorg Chem,
12 (1973) 102.
22 Ali M A, Hossain S M G, Majumdar S M M H, Nazimuddin
M & Tarafder M T H, Polyhedron, 6 (1997) 1653.
23 Geary W J, Coord Chem Rev, 78 (1971) 81.
24 Tabbi G, Dressen W L, Reedijk J, Bonomo R P, Veldman N
& Spek A L, Inorg Chem, 36 (1997) 1168.