Synthesis, NMR spectral and single crystal X-ray structural studies on Ni(II) dithiocarbamates with NiS2PN, NiS2PC, NiS2P2 chromophores: Crystal structures of (4-methylpiperazinecarbodithioato)(thiocyanato-N) (triphenylphosphine)nickel(II) and bis(triphenylphosphine) (4-methylpiperazinecarbodithioato)nickel(II) perchlorate monohydrate

Article abstract of DOI:10.1016/j.poly.2006.10.006

Four Ni(II) dithiocarbamates with NiS₂PX (X = NCS or CN or P) chromophores have been synthesized [[Ni(4-mpzdtc)(PPh₃)(NCS)] (1), [Ni(4-mpzdtc)(PPh₃)(CN)] (2), [Ni(4-mpzdtc)(PPh₃)₂]ClO₄ · H₂O (3) and [Ni(4-mpzdtc)(dppe)]ClO₄ (4) (where 4-mpzdtc = 4-methylpiperazinecarbodithioato anion, PPh₃ = triphenylphosphine and dppe = 1,2-bis((diphenylphosphino)ethane))] from [Ni(4-mpzdtc)₂]. Electronic spectra of the complexes show bands corresponding to d_z² / d_xy → d_{x² - y²} transitions. IR spectra of all the compounds showed characteristic bands due to the dithiocarbamate ligand around 1500 cm^-1 and 990 cm^-1. CV studies showed a higher reduction potential for the parent complex indicating the reluctance to add more electron density to the already electron rich metal centre. ¹H NMR spectra of the complexes showed the deshielding of α-CH₂ protons on complexation. The ¹³C signals show interesting variations for the S₂-¹³C N{2 bonds on right} carbon signals between the neutral [1 and 2] and ionic complexes [3 and 4]. The S₂-¹³C N{2 bonds on right} chemical shifts of the three complexes with PPh₃ follow the order: 3 < 1 < 2 indicating the influence of PPh₃, NCS, CN respectively on the mesomeric drift of electron density towards nickel. The PPh₃ complexes show the following order of increasing ³¹P chemical shifts: 3 (30.9 ppm) > 2 (22.1 ppm) > 1 (20.5 ppm). The observed order clearly establishes the ease with which the back bonding interaction takes place. The back bonding is best observed in the case of the NiS₂P₂ chromophore. Single crystal X-ray structures of [Ni(4-mpzdtc)(PPh₃)(NCS)] and [Ni(4-mpzdtc)(PPh₃)₂]ClO₄ indicate that the central metal atom is in a planar environment for both complexes. The planarity of the molecules is supported by the observed diamagnetism of the complexes. The short S₂-C N{2 bonds on right} bonds indicate the partial double bond character in both cases.

Full text of DOI:10.1016/j.poly.2006.10.006

Polyhedron 26 (2007) 1133–1138
www.elsevier.com/locate/poly
Synthesis, NMR spectral and single crystal X-ray structural studies on
Ni(II) dithiocarbamates with NiS2PN, NiS2PC, NiS2P2
chromophores: Crystal structures of
(4-methylpiperazinecarbodithioato)(thiocyanato-N)
(triphenylphosphine)nickel(II) and bis(triphenylphosphine)
(4-methylpiperazinecarbodithioato)nickel(II) perchlorate monohydrate
a
a,
*
Balasubramaniyam Arul Prakasam , Kuppukkannu Ramalingam
,
a
b
b
Ramalingam Baskaran , Gabriele Bocelli , Andrea Cantoni
a
Department of Chemistry, Annamalai University, Annamalainagar 608002, India
b
IMEM, CNR, Parma, Italy
Received 24 July 2006; accepted 3 October 2006
Available online 14 October 2006
Abstract
Four Ni(II) dithiocarbamates with NiS2PX (X = NCS or CN or P) chromophores have been synthesized [[Ni(4-mpzdtc)(PPh3)(NCS)]
(1), [Ni(4-mpzdtc)(PPh3)(CN)] (2), [Ni(4-mpzdtc)(PPh3)2]ClO4 Æ H2O (3) and [Ni(4-mpzdtc)(dppe)]ClO4 (4) (where 4-mpzdtc = 4-methyl-
piperazinecarbodithioato anion, PPh3 = triphenylphosphine and dppe = 1,2-bis((diphenylphosphino)ethane))] from [Ni(4-mpzdtc)2].
Electronic spectra of the complexes show bands corresponding to dz2 =dxy ! dx2ꢀy2 transitions. IR spectra of all the compounds showed
characteristic bands due to the dithiocarbamate ligand around 1500 cmꢀ1 and 990 cmꢀ1. CV studies showed a higher reduction potential
for the parent complex indicating the reluctance to add more electron density to the already electron rich metal centre. 1H NMR spectra
of the complexes showed the deshielding of a-CH2 protons on complexation. The 13C signals show interesting variations for the S2–13C
Nꢀ carbon signals between the neutral [1 and 2] and ionic complexes [3 and 4]. The S2–13C Nꢀ chemical shifts of the three complexes with
PPh3 follow the order: 3 < 1 < 2 indicating the influence of PPh3, NCS, CN respectively on the mesomeric drift of electron density
towards nickel. The PPh3 complexes show the following order of increasing 31P chemical shifts: 3 (30.9 ppm) > 2 (22.1 ppm) > 1
(20.5 ppm). The observed order clearly establishes the ease with which the back bonding interaction takes place. The back bonding
is best observed in the case of the NiS2P2 chromophore. Single crystal X-ray structures of [Ni(4-mpzdtc)(PPh3)(NCS)] and
[Ni(4-mpzdtc)(PPh3)2]ClO4 indicate that the central metal atom is in a planar environment for both complexes. The planarity of the mol-
ecules is supported by the observed diamagnetism of the complexes. The short S2–C Nꢀ bonds indicate the partial double bond character
in both cases.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Nickel; Phosphine; Dithiocarbamate; CV; NMR; Single crystal X-ray structure; Thioureide
1. Introduction
The diversity in applications inherent to nickel(II) com-
*
Corresponding author. Tel.: +91 413 2202834; fax: +91 4144 22265.
plexes with simple and chelating phosphines as catalysts
[1–3] and in the medicinal field [4], combined with their
E-mail addresses: krauchem@yahoo.com, krchem@rediffmail.com
(K. Ramalingam).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.10.006

1134
B.A. Prakasam et al. / Polyhedron 26 (2007) 1133–1138
structural novelty has resulted in the synthesis of a series of
compounds with NiPnX (X = halogens or N or C or S)
chromophores. Dithiocarbamates have proven to be versa-
tile metal chelates and in particular nickel(II) dithiocarba-
mates with a planar NiS4 chromophore are found to show
interesting variations in reactivity towards soft Lewis bases
such as phosphines and hard bases such as nitrogenous
ligands. As a continuation of our interest [5,6] in exploring
the synthetic and structural chemistry of transition metal
dithiocarbamates and to understand the influence of phos-
phines and nitrogenous ligands on the NiS2PX (X = C
or N or P) chromophore and on the S2–C Nꢀ bond we
investigated the following compounds: [Ni(4-mpzdtc)-
(PPh3)(NCS)] (1), [Ni(4-mpzdtc)(PPh3)(CN)] (2), [Ni-
(4-mpzdtc)(PPh3)2]ClO4 Æ H2O (3) and [Ni(4-mpzdtc)-
(dppe)]ClO4 (4), where 4-mpzdtc = 4-methylpiperazinecar-
bodithioato anion, PPh3 = triphenylphosphine and dppe =
1,2-bis((diphenylphosphino)ethane). In this paper, we
report the synthesis, spectral and electrochemical proper-
ties of the compounds 1–4 along with the single crystal
X-ray structures of 1 and 3 which crystallized out as single
crystals.
2.1.2. (4-Methylpiperazinecarbodithioato)(thiocyanato-N)-
(triphenylphosphine) nickel(II)
2.1.2.1. [Ni(4-mpzdtc)(PPh3)(NCS)] (1). A mixture of
Ni(4-mpzdtc)2 (205 mg, 0.5 mmol), PPh3 (260 mg,
1 mmol), NiCl2 Æ 6H2O (120 mg, 0.5 mmol) and NH4SCN
(75 mg, 1 mmol) was refluxed for 3 h in an acetonitrile–
methanol solvent mixture (2:1, 75 cm3) and was then con-
centrated to ca. 25 cm3. The dark purple-red solution
obtained was filtered and was kept for evaporation. After
2 days, a solid separated out, was filtered, and then dried
over anhydrous calcium chloride. Single crystals suitable
for X-ray analysis were obtained by recrystallization from
the same solvent mixture (Yield: 75%, dec.: 189 ꢁC). Anal.
Calc. for C25H26N3NiPS3: C, 54.16; H, 4.72; N, 7.58.
Found: C, 54.01; H, 4.59; N, 7.44%.
2.1.3. (Cyano)(4-methylpiperazinecarbodithioato)-
(triphenylphosphine) nickel(II)
2.1.3.1. [Ni(4-mpzdtc)(PPh3)(CN)] (2). A mixture of
Ni(4-mpzdtc)2 (205 mg, 0.5 mmol), PPh3 (260 mg, 1 mmol),
NiCl2 Æ 6H2O (120 mg, 0.5 mmol) and KCN (65 mg,
1 mmol) was refluxed for 3 h in an acetonitrile–methanol
solvent mixture (2:1, 75 cm3) and then concentrated to ca.
25 cm3. The pale orange-red solution obtained was filtered
and kept for evaporation. After 2 days, an orange-red solid
separated out. It was dried over anhydrous calcium chloride
and recrystallized from the same solvent mixture (Yield:
65%, dec.: 180 ꢁC). Anal. Calc. for C25H26N3NiPS2: C,
57.49; H, 5.01; N, 8.04. Found: C, 57.38; H, 4.90; N,
7.94%.
2. Experimental
All the reagents and solvents employed were commer-
cially available analytical grade materials and were used
as supplied, without further purification. IR spectra were
recorded on an ABB Bomem MB 104 spectrophotometer
(range 4000–400 cmꢀ1) as KBr pellets. The UV–Vis spec-
tra were recorded in CH2Cl2 on a HITACHI U-2001
spectrophotometer. Cyclic voltammetric studies were car-
ried out using an ECDA-001, Basic Electrochemistry sys-
tem. A three-electrode configuration was used, comprising
of glassy carbon as the working electrode, a platinum wire
as the counter electrode and AgjAgCl as the reference
electrode. Experiments were carried out at room tempera-
ture using 0.01 M tetrabutylammonium perchlorate as the
supporting electrolyte under a nitrogen atmosphere.
NMR spectra were recorded on a Bruker 400 MHz spec-
trometer at room temperature, using CDCl3 as solvent.
13C NMR spectra were recorded in the proton decoupled
mode.
2.1.4. Bis(triphenylphosphine)(4-methylpiperazine-
carbodithioato)nickel(II) perchlorate monohydrate
2.1.4.1. [Ni(4-mpzdtc)(PPh3)2]ClO4 Æ H2O (3). A mixture
of Ni(4-mpzdtc)2 (205 mg, 0.5 mmo1), PPh3 (520 mg,
2 mmol), NiCl2 Æ 6H2O (120 mg, 0.5 mmol) and NH4ClO4
(115 mg, 1 mmol) was refluxed for about 2 h in a metha-
nol–dichloromethane solvent mixture (1:1, 50 cm3), and
then concentrated to ca. 25 cm3. The purple-red solution
obtained was filtered and left for evaporation. The sepa-
rated solid was filtered and dried, and single crystals suitable
for X-ray analysis were obtained by repeated recrystalliza-
tion from the same solvent mixture (Yield: 65%, dec.:
183 ꢁC). Anal. Calc. for C42H43ClN2NiO5P2S2: C, 57.58;
H, 4.94; N, 3.19. Found: C, 57.42; H, 4.84; N, 3.10%.
2.1. Preparation of the complexes
2.1.5. 1,2-Bis((diphenylphosphino)ethane)(4-methyl-
piperazinecarbodithioato) nickel(II) perchlorate
2.1.1. Bis(4-methylpiperazinecarbodithioato) nickel(II)
2.1.1.1. [Ni(4-mpzdtc)2]. A mixture of 1-methylpiperazine
(100 mg, 1 mmol) and CS2 (76 mg, 1 mmol) in ethanol
(50 mL) was kept under ice cold conditions (ꢀ5 ꢁC) for
10 min and to this pale yellow solution NiCl2 Æ 6H2O
(120 mg, 0.5 mmol) in water (25 mL) was added slowly
with vigorous stirring. The dark green precipitate obtained
was filtered, washed with ether and water. It was kept for
evaporation and dried over anhydrous calcium chloride
(Yield: 95%, dec.: >250 ꢁC).
2.1.5.1. [Ni(4-mpzdtc)(dppe)]ClO4 (4). A mixture of
Ni(4-mpzdtc)2 (410 mg, 1 mmol), dppe (790 mg, 2 mmol),
NiCl2 Æ 6H2O (240 mg, 1 mmol) and NH4ClO4 (230 mg,
2 mmol) was refluxed for about 4 h in a methanol-dichloro-
methane solvent mixture (1:1, 50 cm3). The solution was
then concentrated to ca. 25 cm3. The purple-red solution
obtained was filtered and kept for evaporation. After 2
days, a purple-red solid separated out and it was recrystal-
lized from the same solvent mixture (Yield: 65%,

B.A. Prakasam et al. / Polyhedron 26 (2007) 1133–1138
1135
Table 1
Crystal data, data collection and refinement parameters for 1 and 3
Complex
1
3
Empirical formula
FW
C25H26N3NiPS3
554.3
C84H86Cl2N4Ni2O10P4S4
1752.0
Crystal dimensions (mm)
Crystal system
Colour
0.19 · 0.23 · 0.24
0.22 · 0.25 · 0.29
monoclinic
monoclinic
purple red
orange red
Habit
needle
column, irregular
Space group
P21/c
P21/c
˚
a (A)
7.86(2)
31.819(3)
˚
b (A)
31.011(3)
11.033(2)
11.323(3)
24.464(3)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90
90.31 (2)
90
2689(7)
90
105.31(3)
90
8501(3)
3
˚
U (A )
Z
4
8
Dc (g cmꢀ3
)
1.290
9.53
1088
Mo Ka (0.71073)
1.367
7.38
3640
Mo Ka (0.71069)
l (cmꢀ1
F(000)
k (A)
)
˚
h Range (ꢁ)
Scan type
1.31–27.53
x–2h
3.03–28.01
x–2h
Index ranges
ꢀ3 6 h 6 3, ꢀ 40 6 k 6 40, ꢀ1 6 l 6 14
0 6 h 6 42, 0 6 k 6 14, ꢀ32 6 l 6 31
Reflections collected
Observed reflections Fo > 4r(Fo)
Weighting scheme
2649
1474
20311
6357
2
2
W ¼ 1=½r2ðF 2oÞ þ ð0:0470PÞ þ 0:00Pꢁ, where
W ¼ 1=½r2ðF 2oÞ þ ð0:0051PÞ þ 0:00Pꢁ, where
P ¼ ðF 2o þ 2F 2c Þ=3
402
P ¼ ðF 2o þ 2F 2c Þ=3
1355
Number of parameters refined
Final R, Rw (observed, data)
Goodness-of-fit
0.0354, 0.1033
1.009
0.0660, 0.0598
0.877
Table 2
dec.: 193 ꢁC). Anal. Calc. for C32H35ClN2NiO4P2S2: C,
52.51; H, 4.82; N, 3.82. Found: C, 52.41; H, 4.70; N, 3.71%.
Selected bond distances and bond angles for 1 and 3
1
3
Bond distances
Ni(1)–N(31)
Ni(1)–S(2)
Ni(1)–P(12)
Ni(1)–S(3)
S(2)–C(4)
S(3)–C(4)
C(4)–N(5)
N(8)–C(11)
N(31)–C(32)
C(32)–S(33)
2.2. X-ray crystallography
1.859(6)
2.1809(19) Ni(1A)–P(11A)
Ni(1A)–S(2A)
2.1937(15)
2.2149(16)
2.2333(16)
2.2272(16)
1.710(5)
1.707(5)
1.306(5)
1.471(8)
1.493(6)
Details of the crystal data, data collection and refine-
ment parameters for 1 and 3 are summarized in Table 1.
Intensity data were collected at ambient temperature
(295 K) on Bruker AXS (with CCD) (for 1) [7] and Philips
PW 1100 (for 3) diffractometers [8], using graphite mono-
2.207(3)
2.213(4)
1.714(8)
1.706(7)
1.316(8)
1.477(13)
1.152(7)
1.619(7)
Ni(1A)–P(30A)
Ni(1A)–S(1A)
S(1A)–C(3A)
S(2A)–C(3A)
C(3A)–N(4A)
N(7A)–C(10A)
N(4A)–C(9A)
N(4A)–C(5A)
˚
chromated Mo Ka radiation (k = 0.71073 A and
˚
0.71069 A for 1 and 3, respectively). All the non-hydrogen
1.486(6)
atoms were refined anisotropically and all the hydrogen
atoms were refined isotropically. The absorption correc-
tions were performed with the method in SHELXTL-NT V5.1
[7] for 1 and with the North, Phillips and Mathews method
for 3 [9]. The ORTEP diagram was drawn with the ORTEP-3
program [10]. Selected bond distances and bond angles are
given in Table 2.
Bond angles
N(31)–Ni(1)–
P(12)
92.15(19)
97.35(8)
S(2A)–Ni(1A)–P(11A)
91.02(6)
99.82(6)
S(2)–Ni(1)–P(12)
P(11A)–Ni(1A)–
P(30A)
N(31)–Ni(1)–S(3)
S(2)–Ni(1)–S(3)
C(4)–S(2)–Ni(1)
C(4)–S(3)–Ni(1)
N(5)–C(4)–S(3)
N(5)–C(4)–S(2)
S(3)–C(4)–S(2)
C(32)–N(31)–
Ni(1)
92.2(2)
78.86(8)
86.3(2)
S(2A)–Ni(1A)–S(1A)
P(30A)–Ni(1A)–S(1A)
C(3A)–S(1A)–Ni(1A)
C(3A)–S(2A)–Ni(1A)
N(4A)–C(3A)–S(1A)
N(4A)–C(3A)–S(2A)
S(1A)–C(3A)–S(2A)
C(3A)–N(4A)–C(9A)
78.44(5)
90.72(6)
85.31(18)
86.42(17)
123.7(4)
126.5(4)
109.8(3)
122.5(5)
85.5(3)
125.3(6)
125.4(6)
109.3(4)
168.6(6)
3. Results and discussion
3.1. Electronic spectra
N(31)–C(32)–
S(33)
179.5(9)
C(3A)–N(4A)–C(5A)
124.1(5)
The d–d transition bands are observed at 483, 465, 483
and 467 nm for complexes 1, 2, 3 and 4, respectively. The

1136
B.A. Prakasam et al. / Polyhedron 26 (2007) 1133–1138
bands are assigned to dz2 =dxy ! dx2ꢀy2 transitions [11]. The
cyanide ion exerts a larger LFSE and hence the lowest kmax
value is associated with 2. The observed red shift in 2 when
compared to similar complexes with a NiS2PN chromo-
phore [6] is significant.
ical reduction. Lower reduction potentials observed for
complexes 1–3 indicate the ease of electron addition in
the mixed ligand complexes and this is because of the pres-
ence of PPh3, NCSꢀ and CNꢀ around the nickel atom [5].
3.4. NMR spectral studies
3.2. IR spectral studies
NMR chemical shifts of the synthesized complexes are
summarized in Table 3.
For complexes 1, 2, 3 and 4, the mC–N (S2–C'Nꢀ) bands
are observed at 1547, 1529, 1534 and 1519 cmꢀ1, respec-
tively. The increase in wavenumber for the mixed ligand
complexes compared to that observed for the parent com-
plex [Ni(4-mpzdtc)2] (1503 cmꢀ1) is attributed to the meso-
meric drift of electron density towards the nickel atom
through the S2–C'Nꢀ bond from the dithiocarbamate.
The mC–S stretching vibrations are observed around
1000 cmꢀ1 (1–4) without any splitting, supporting the
bidentate coordination of the dithiocarbamate ligand.
The observed stretching frequencies at 2093 cmꢀ1 for com-
plex 1 and at 2115 cmꢀ1 for complex 2 are attributed to the
‘N’-coordinated thiocyanate and ‘C’-coordinated cyanide
anions, respectively. The intense signals observed in the fin-
gerprint region for complexes 3 and 4 [1095 cmꢀ1 for 3 and
1
3.4.1. H NMR
For all the four mixed ligand complexes (1–4), the aro-
matic protons resonate in the region 7.31–7.91 ppm. The
a(methylene) protons are greatly deshielded and are
observed in the downfield region due to complexation.
However, the b-CH2 and N–CH3 proton signals are
observed closely and the assignments are made on the basis
of proton integral values obtained for the corresponding
signals. For complex 4, the signal due to the methylene pro-
tons of the chelating phosphine appears at 2.67 ppm.
3.4.2. 13C NMR
For all the four complexes, the N–CH3 carbon signal is
observed around 45.0 ppm and this deshielding is due to
the minor deshielding effect of the nearby electronegative
nitrogen. The b-CH2 carbon signals are observed around
53.0 ppm with a small upfield shift of about 3 ppm com-
pared to that of the corresponding carbon signal in N-
methylpiperazine [56.4 ppm]. Signals due to a-carbons are
only slightly deshielded. Aromatic carbon signals are
observed in the downfield region of 126.9–137.3 ppm for
all the four synthesized complexes. The N–13CSꢀ and
13CNꢀ carbon signals are observed at 143.9 and
ꢂ131 ppm for complexes 1 and 2, respectively (for complex
2, the 13CNꢀ signal at 131 ppm, is merged with the aro-
matic carbon signals). For complex 4, the methylene car-
bon present in the chelating phosphine is observed at
1089 cmꢀ1 for 4] are due to the presence of mClO stretching
ꢀ
4
in both cases.
3.3. Cyclic voltammetric studies
The reduction potential for the parent [Ni(4-mpzdtc)2]
complex is observed at ꢀ1548 mV. All the mixed ligand
complexes showed lower one electron reduction potentials.
(ꢀ1189, ꢀ972 and ꢀ754 mV for complexes 1, 2 and 3
respectively). For the chelating phosphine complex 4,
quasi-reversible reduction potentials are observed at
ꢀ998ðEcpÞ= ꢀ 790ðEapÞ mV and it is due to the chelating nat-
ure of the phosphine, which is not completely delinked
from the NiI-species formed as a result of the electrochem-
Table 3
NMR chemical shifts of the complexes (in ppm)
a
a
b
Compound
NMR
a-CH2
3.78 (t)
46.6
b-CH2
2.46 (t)
53.2
d-CH3
PPhx
ꢁN 13CS2
S–13C–Nꢀ
143.9
13CNꢀ
1
1H
13C
31P
2.17 (s)
45.2
7.46–7.70 (m)
128.8–134.9
20.5
202.8
2
3
1H
3.74 (t)
46.0
2.43 (t)
53.7
2.30 (s)
45.5
7.40–7.81 (m)
128.6–134.1
22.1
13C
31P
204.2
199.2
201.7
ꢂ131
1H
13C
31P
3.73 (t)
46.3
2.63 (t)
53.4
2.43 (s)
45.3
7.31–7.66 (m)
128.8–137.3
30.9
4
1H
13C
31P
3.89 (t)
46.3
2.63(t)
53.3
2.40 (s)
45.1
7.52–7.91 (m)c
126.9–132.8d
61.8
a
Chemical shifts due to a, e protons.
Where x = 3 for 1, 2 and 3 and 2 for 4.
b
c
The (ꢁP–CH2–)2 signal is observed at 2.67 ppm.
d
The (ꢁP–13CH2–)2 signal is observed at 26.4 ppm.

B.A. Prakasam et al. / Polyhedron 26 (2007) 1133–1138
1137
26.4 ppm. Finally, the most important 13C NMR signals of
the S2–13C'Nꢀ carbons are observed at 202.8, 204.2,
199.2 and 201.7 ppm for complexes 1, 2, 3 and 4, respec-
tively with a very weak intensity characteristic of the qua-
ternary carbon signals. The mesomeric shift of electron
density from the dithiocarbamate moiety towards the metal
centre contributes to the upfield shift from the normal
chemical shift values of Ni(dtc)2 complexes (206–
210 ppm) [12]. The S2–13C'Nꢀ chemical shifts of the three
complexes with PPh3 follow the order: 3 < 1 < 2, indicating
the influence of PPh3, NCS and CN, respectively on the
mesomeric drift of electron density towards nickel.
ported by the observed diamagnetism of the complex.
Delocalization of p electron density over the S2CN moiety
leads to a decrease in the C–N [S2–C'Nꢀ] distance
˚
[1.316(8) A] and also an increase in the S–C–N angles
[125.3(6)ꢁ and 125.4(6)ꢁ] over the S–C–S angle [109.3(4)ꢁ].
The observed difference in Ni–S distances [2.213(4) and
˚
2.1809(19) A] is attributed to the difference in the trans
influencing properties of PPh3 and NCSꢀ. PPh3 being a
good p acceptor has a greater trans influence and hence
the Ni–S bond trans to PPh3 is longer than the other
Ni–S bond.
˚
The Ni–N distance [1.859(6) A] is quite short, showing
the effective bonding between the nickel atom and the thio-
cyanate-N. The C–N bond distance in N–C–S [1.152(7) A]
3.4.3. 31P NMR
˚
31P chemical shifts are observed at 20.5, 22.1, 30.9 and
61.8 ppm for complexes 1, 2, 3 and 4, respectively. A good
extent of deshielding of about 75 ppm is observed for the
31P signal of complex 4, compared to the free chelating
dppe 31P signal observed at ꢀ13 ppm [13]. In the case of
complexes 1, 2 and 3 signals are also observed in the
deshielded region compared to the free PPh3 signal at
ꢀ5 ppm [14]. A very high deshielding of signals observed
in all the cases is attributed to the movement of electron
density from the phosphorus towards the nickel atom on
complexation. The PPh3 complexes show the following
order of increasing 31P chemical shifts: 3 (30.9 ppm) > 2
(22.1 ppm) > 1 (20.5 ppm). The observed order clearly
establishes the ease with which the back bonding interac-
tion takes place. The back bonding electron donation is
the best in the case of 3, containing the NiP2 chromophore.
is significantly shorter than the S2–C'Nꢀ N–C bond dis-
˚
tance of 1.316(8) A. The C–S bond length in N–C–S is
˚
1.619(7) A, which is similar to the C@S distance of
˚
1.69 A. The N–C–S bond is [179.5(9)ꢁ] and is almost linear.
In the dithiocarbamate part, the piperazine ring is in a ste-
rically favoured chair conformation with the normal C–C
and C–N bond lengths. The N-methyl group in the termi-
nal position is equatorially oriented to avoid the 1,3-diaxial
steric interactions with the axial hydrogens. Phenyl groups
in triphenylphosphine show normal bond parameters.
[Ni(4-mpzdtc)(PPh3)2]ClO4 Æ H2O is discrete and mono-
meric. One molecule of water is co-crystallized with the
complex. Eight formula units are present in the unit cell.
Each asymmetric unit has two molecules and the bond
parameters of one molecule are listed. The ORTEP dia-
gram of 3 is shown in Fig. 2. Distortions with respect to
the idealized geometry are observed in the coordination
angles: the P–Ni–P angle is very large [99.82(6)ꢁ] due to ste-
ric interactions between two PPh3 groups, while the S–Ni–S
bite angle is low, 78.44(5)ꢁ as observed in similar com-
pounds [6]. The observed shortening of the ꢁN–CS2 C–N
distance is in line with the bond parameters observed for
[Ni(dtc)(PPh3)2]+ (where, dtc = dmdtc or dedtc or pipdtc)
complexes [6]. Among the mixed ligand complexes, those
3.5. Structural analysis
The single crystal X-ray structure of 1 contains four dis-
crete molecules per unit cell. The ORTEP diagram of the
molecule is shown in Fig. 1. The observed distortion of
the square planar coordination around nickel is attributed
to the small bite angle of 78.86(8)ꢁ. The planarity is sup-
Fig. 1. Thermal ellipsoid plot of [Ni(4-mpzdtc)(PPh3)(NCS)] (30%
probability).
Fig. 2. Thermal ellipsoid plot of [Ni(4-mpzdtc)(PPh3)2]ClO4 Æ H2O (30%
probability) (H2O and the ClO4 ion are omitted for clarity).
ꢀ

1138
B.A. Prakasam et al. / Polyhedron 26 (2007) 1133–1138
of the NiS2P2([Ni(dtc)(PPh3)2]+) chromophore show very
short S2–C'Nꢀ distances due to the p-acidic nature of
the triphenylphosphine. The two S–C–N angles [123.7(4)ꢁ
and 126.5(4)ꢁ] are observed to be larger than the S–C–S
angle [109.8(3)ꢁ]. Ni–P distances are very similar
Appendix A. Supplementary material
CCDC 263874 and 263875 contain the supplementary
crystallographic data for 1 and 3. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.poly.2006.10.006.
˚
[2.2333(16) and 2.2149(16) A]. The two Ni–S and S–C dis-
tances are comparable, showing that the negative charge of
the bidentate ligand is equally distributed over the two
donor atoms. The perchlorate ion shows thermal disorder
as reported earlier [5]. The disordered perchlorate ion
˚
shows extensive short contacts ranging from 2.5 to 2.9 A
with H atoms of the piperazine ring and similarly with
the oxygen atom of the water molecule O1W in the same
range. Two hydrogen atoms of the water molecule attached
to O2W could not be located inꢀthe Fourier map. Also, the
disorder associated with ClO4 has been modelled such
that each oxygen atom is positioned at two coordinate
positions of 50% site occupancy at distances of 0.552,
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