Transition metal complexes with sulfur ligands Part CXLIV. Square planar nickel complexes with NiS4 cores in three different oxidation states: Synthesis, X-ray structural and spectroscopic studies

Article abstract of DOI:10.1016/S0020-1693(99)00608-8

The reaction of '(bu)S₂'^2- = 3,5-ditertiarybutyl-1,2-benzenedithiolate(2-) with Ni(ac)₂·4H₂O and subsequently AsPh₄Cl, yielded (AsPh₄)₂[Ni('(bu)S₂')₂] (1).

Full text of DOI:10.1016/S0020-1693(99)00608-8

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Inorganica Chimica Acta 300–302 (2000) 829–836
Transition metal complexes with sulfur ligands
Part CXLIV. Square planar nickel complexes with NiS4 cores in
three different oxidation states: synthesis, X-ray structural and
spectroscopic studiesꢀ
Dieter Sellmann a,*, Herbert Binder b, Daniel Ha¨ußinger a, Frank W. Heinemann a,
Jo¨rg Sutter a
a Institut fu¨r Anorganische Chemie der Uni6ersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 1, D-91058 Erlangen, Germany
b Institut fu¨r Anorganische Chemie der Uni6ersita¨t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
Received 30 November 1999; accepted 10 December 1999
Abstract
The reaction of ‘buS2’2− =3,5-ditertiarybutyl-1,2-benzenedithiolate(2−) with Ni(ac)2·4H2O and subsequently AsPh4Cl, yielded
(AsPh4)2[Ni(‘buS2’)2] (1). Complex 1 is extremely air sensitive and oxidized rapidly to give (AsPh4)[Ni(‘buS2’)2] (2). The anion of
2 could be further oxidized by iodine to give neutral [Ni(‘buS2’)2] (3). Complexes 1, 2 and 3 contain nickel in the formal oxidation
states +2, +3 and +4, and could be completely characterized by X-ray structure determination and spectroscopic methods. The
results indicate that the oxidation of 1“2“3 concerns electrons residing in molecular orbitals having [NiS4] character. No
evidence could be obtained for an involvement of the benzene rings or an electron delocalization beyond the [NiS4] core. This
conclusion corresponds with the reactivity of 3 versus 2 and 1 towards protons and CO. While 3 is inert to the attack of H+ and
CO, 2 and 1 exhibit rapid reactions to give ‘buS2’ꢀH2 and either nickel–chloro complexes or Ni(CO)4. © 2000 Elsevier Science S.A.
All rights reserved.
Keywords: Nickel thiolate complexes; High oxidation states
1. Introduction
ambiguity of these assignments is caused by the formal
redox equilibrium converting an ene-dithiolate into a
dithioketone according to Eq. (1).
Nickel complexes with sulfur coordination spheres
have attracted considerable interest as model com-
pounds for the active centers of nickel sulfur enzymes
[2], e.g. NiFe hydrogenases, as homogeneous and
heterogeneous hydrodesulfurization catalysts [3], and as
compounds exhibiting unusual electronic properties
when the ligands contain sulfur donors connected by
unsaturated C2 bridges. For the latter type of com-
plexes, the nickel dithiolenes, the assignment of (for-
mal) metal oxidation states and ligand charges has been
as controversial as for other metal dithiolenes [4]. The
(1)
Such a redox conversion has principally to be consid-
ered also for 1,2-benzenedithiolate(2−) anions yielding
cyclohexadiene-1,2-dithioketones when binding to high-
oxidation state metal centers. The question is whether
the very stable aromatic system of the benzene ring
prevents such an oxidation, i.e. is 1,2-benzenedithio-
late(2−) an innocent or a so-called ‘non-innocent’
ligand [4c]. Here we address this question with X-ray
structural and spectroscopic results on the series of
[Ni(‘buS2’)2]n− complexes (n=2, 1, 0). These complexes
contain ‘buS2’2− =3,5-ditertiarybutyl-1,2-benzenedithi-
ꢀ For Part CXLIII see Ref. [1].
* Corresponding author. Tel.: +49-9131-852 7360; fax: +49-
9131-852 7367.
E-mail address: sellmann@anorganik.chemie.uni-erlangen.de (D.
Sellmann)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00608-8

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D. Sellmann et al. / Inorganica Chimica Acta 300–302 (2000) 829–836
olate(2−) ligands and nickel in the formal oxidation
states of +2, +3 and +4.
‘buS2’ꢀH2 was prepared as described in the literature
[13]. AsPh4Cl·H2O was purchased from Alfa. Nickel
metal, 86.2% enriched in 61Ni was purchased from
Scientific and Production Association ALFA-LAB,
Moscow, Russia.
Changes in electron content between different oxida-
tion states of transition metal complexes can cause
significant changes in molecular structure. A detailed
analysis of such changes is usually restricted to two
adjacent oxidation states [5]. Completely characterized
and strictly homologous complexes in three or more
different oxidation states are extremely rare [6].
Analogous complexes containing the planar
[Ni(S2C6H4)2]n−, anions with n=2 [7] and n=1 [8]
have been known for some time and could be charac-
terized by X-ray structure determination. Our attempts
to obtain also the neutral species [Ni(S2C6H4)2] failed.
Oxidation of the anionic species with various oxidizing
reagents yielded grey–black products being insoluble
and intractable. Attempts by others to synthesize neu-
tral [Ni(S2C6Cl4)2] by electrochemical oxidation of
(NBu4)[Ni(S2C6Cl4)2] resulted only in a species de-
scribed as (NBu4)0.15[Ni(S2C6Cl4)2] [9].
2.1. Syntheses and reactions
2.1.1. Synthesis of (AsPh4)2[Ni(‘buS2’)2] (1)
Under stirring, ‘buS2’ꢀH2 (1.61 g, 6.33 mmol) was
dissolved in 12.7 ml of 1 N LiOMe in MeOH. After 20
min, solid Ni(ac)2·4H2O (780 mg, 3.13 mmol) was
added. A slightly purple solution resulted, which was
cooled to −30°C. In the course of 5 days, ocher
crystals of Li2[Ni(‘buS2’)2] precipitated, which were sepa-
rated, washed with cold MeOH (−90°C, 30 ml), and
dried in vacuo. These crystals (250 mg, 0.43 mmol)
were dissolved in MeOH (6 ml) and combined with
AsPh4Cl·H2O (380 mg, 0.87 mmol). A small amount of
dark-green crystals of (AsPh4)[Ni(‘buS2’)2] precipitated
and was removed. The remaining dark-yellow solution
was carefully layered with 11 ml of Et2O. In the course
of 8 days, red crystals of (AsPh4)2[Ni(‘buS2’)2]·2Et2O
precipitated which were separated, washed with 4 ml of
a cold (−90°C) 1:1 mixture of MeOH–Et2O and 10 ml
of cold (−90°C) Et2O. Drying these crystals at 60°C in
vacuo (30 h, 10−3 mbar) yielded a brown powder of
solvate free (AsPh4)2[Ni(‘buS2’)2]. Anal. Calc. for
C76H80As2NiS4 (1330.29): C, 68.62; H, 6.06; S, 9.64.
2. Experimental
Unless noted otherwise, all reactions were carried out
under an atmosphere of argon at room temperature
using standard Schlenk techniques. The argon atmo-
sphere was required by the [Ni(‘buS2’)2]2− anion, which
is extremely sensitive towards oxidation and could not
be handled under an atmosphere of dinitrogen. Sol-
vents were dried, distilled and argon saturated before
use. Physical measurements were carried out with the
following instruments: IR, Perkin–Elmer 16 PC FTIR
spectrometer; NMR spectra: Jeol JNM-GX 270 spec-
trometer with the protio-solvent signal used as internal
reference. Chemical shifts are quoted on the l scale
(downfield shifts are positive) relative to tetramethylsi-
lane. Spectra were recorded at 25°C. UV–Vis–NIR
spectra: Shimadzu UV-3101 PC spectrometer; EPR
spectra: Bruker ESP 300 spectrometer (X-band, 9.4
GHz). Spectra were recorded in DMF–CHCl3 (1:1 by
volume) solution at 293 or −120 K, and referenced
towards diphenylpicrylhydrazyl (Bg\ =2.0036(2))
[10]. XPS measurements: Solutions of the probes were
spread on gold plates, the solvents were removed in the
pre-vacuum. Spectra were recorded with an AEI ES 200
B spectrometer (AEI, Manchester, UK), Al Ka radia-
1
Found: C, 68.92; H, 6.25; S, 9.61%. H NMR (269.6
MHz, acetone-d6, l (ppm)): 7.34 (4H, br, C6H2), 1.95
(36H, br, CMe3).
2.1.2. Synthesis of (AsPh4)[Ni(‘buS2’)2] (2)
For 20 min, air was bubbled through a solution of
Li2[Ni(‘buS2’)2] (680 mg, 1.17 mmol) in MeOH (25 ml)
yielding
a deep green solution. A solution of
AsPh4Cl·H2O (525 mg, 1.20 mmol) in MeOH (10 ml)
was added dropwise. Deep green crystals of
(AsPh4)[Ni(‘buS2’)] precipitated, which were separated
after 1 day, washed with 60 ml of MeOH and dried in
vacuo for 1.5 days at 60°C. Yield: 940 mg (85%). Anal.
Calc. for C52H60AsNiS4 (946.95): C, 65.96; H, 6.39; S,
13.54. Found: C, 66.09; H, 6.46; S, 13.42%.
2.1.3. Synthesis of [Ni(‘buS2’)2] (3)
4
tion and referenced towards gold (Eb(Au f7/2)=84.0
An acetone solution of iodine (295 mg, 2.32 mmol,
25 ml) was added to an acetone solution of
(NMe4)[Ni(‘buS2’)2] (700 mg, 1.10 mmol, 20 ml).
((NMe4)[Ni(‘buS2’)2] was prepared from Li2[Ni(‘buS2’)2]
and NMe4Cl analogously to 2.) A red–brown solution
resulted, from which blue crystals of [Ni(‘buS2’)2] precip-
itated. They were separated after 2 days, washed with
MeOH (60 ml) and Et2O (5 ml), dried, and recrystal-
lized from hot xylene. Yield: 565 mg (91%). Anal. Calc.
for C28H40NiS4 (563.60): C, 59.67; H, 7.15; S, 22.76.
eV [11]). The measurements were repeated three times
and yielded a reproducibility of 0.2 eV. Cyclic voltam-
mograms were recorded with an EG&G PAR 264 A
potentiostat, equipped with a ROTEL A three-elec-
trode cell (glassy carbon working electrode, Pt counter-
electrode and Ag/AgCl/KCl reference electrode).
Conducting electrolyte: 0.1 M NBu4PF6; internal stan-
dard Cp2Fe0/+ =0.40 V versus NHE [12]. Elemental
analyses: Carlo Erba EA 1106 or 1108 analyzer.

D. Sellmann et al. / Inorganica Chimica Acta 300–302 (2000) 829–836
831
1
Found: C, 59.75; H, 7.30; S, 22.62%. H NMR (269.6
MHz, C6D6, l (ppm)): 8.23 (2H, s, C6H2), 7.59 (2H, s,
C6H2), 1.78 (18H, s, CMe3), 1.14 (18H, s, CMe3).
13C{1H} NMR (67.7 MHz, C6D6, l (ppm)): 176.1,
167.9, 153.2, 150.7, 122.7 (C6H2), 38.2, 35.7 (CMe3),
31.3, 30.9 (CMe3).
minor component (C241ꢀC243) were refined isotropi-
cally, and no H-atoms were included for this alternative
orientation. Table 1 lists selected crystallographic data.
3. Results
2.1.4. Acid hydrolysis of (AsPh4)2[Ni(‘buS2’)2] (1) and
(AsPh4)[Ni(‘buS2’)2] (2)
3.1. Syntheses and reactions
The respective complexes (2 mmol) were stirred in a
mixture of CH2Cl2 (20 ml) and concentrated hydrochlo-
ric acid (25 ml) for 30 min. The CH2Cl2 phase was
separated, and the green aqueous phase was extracted
with CH2Cl2 (20 ml). The CH2Cl2 phases were com-
bined, washed with water, dried over Na2SO4, and
evaporated. The remaining colorless oil was identified
According to Scheme 1, the red NiIIcomplex
(AsPh4)2[Ni(‘buS2’)2] (1) formed from Ni(ac)2·4H2O, the
dilithium salt of the ligand and AsPh4Cl. The yellow
MeOH solution containing the Li2[Ni(‘buS2’)2] salt
proved extremely air-sensitive, and even traces of air
led to an immediate color change to light-green. When
air was bubbled through such a solution, the color
changed to dark green. Addition of AsPh4Cl yielded
(AsPh4)[Ni(‘buS2’)2] (2). The further oxidation of 2
proved more difficult. A considerable number of unsuc-
cessful experiments were carried out before we found
that the analogous (NMe4)[Ni(‘buS2’)2] could be readily
oxidized by iodine in acetone to give the neutral com-
plex [Ni(‘buS2’)2] (3). Complex 3 is deep blue and solu-
ble in toluene.
1
as ‘buS2’ꢀH2 by H NMR spectroscopy. [Ni(‘buS2’)2] (3)
remained unchanged under identical conditions for 24
h.
2.1.5. Reaction of (AsPh4)2[Ni(‘buS2’)2] (1) and
(AsPh4)[Ni(‘buS2’)2] (2) with CO
THF solutions (30 ml) of the respective complexes
were treated with 120 bar CO in an autoclave for 24 h.
The IR spectra of the resulting solutions showed a
strong w(CO) band at 2041 cm−1, which was assigned
to Ni(CO)4. [Ni(‘buS2’)2] remained unchanged for 72 h
under analogous conditions, and no Ni(CO)4 band
could be detected in the IR spectrum.
Concerning the reactions of [NiFe] hydrogenases in
different oxidation states, which are assumed to occur
at the nickel centers [2], the reactivity of 1, 2 and 3
towards protons and CO was of particular interest.
Somewhat surprisingly, the neutral complex 3 proved
completely inert towards protons and is stable for days
even towards concentrated hydrochloric acid. A reac-
tion with CO could neither be observed at 1 bar nor at
120 bar pressure in an autoclave. In contrast, and as
observed with other anionic nickel thiolate complexes
[15], 1 and 2 almost instantaneously dissociated when
treated with hydrochloric acid to give ‘buS2’ꢀH2 and
nickel–chloro species. When treated with CO, complex
1 and 2 decomposed in THF to give Ni(CO)4 and
organosulfur compounds which were not characterized
in detail. Ni(CO)4 was detected by its IR band at 2041
2.2. X-ray structure analyses of
(AsPh4)2[Ni(‘buS2’)2]·2Et2O (1·2Et2O),
(AsPh4)[Ni(‘buS2’)2] (2) and [Ni(buS2)2] (3)
Single crystals were grown as follows: red cubes of
1·2Et2O from an Et2O solution which was layered with
MeOH, dark green prisms of 2 from an acetone solu-
tion which was layered with Et2O, and dark-blue
cuboids of 3 from a toluene solution which was layered
with MeOH. Suitable single crystals were sealed in glass
capillaries. Data were collected with a Siemens P4
diffractometer using Mo Ka radiation (u=71.073 pm),
a graphite monochromator and ꢀ-scan technique (scan
speed 3.0–30° min−1). The structures were solved with
cm−1
.
Hydride sources such as LiBEt3H reduced 3 to give
successively the anions of 2 and 1, which could be
precipitated by addition of AsPh4Cl. Monitoring these
1
direct methods (SHELXTL
squares refinements on F2 were carried out with
NT 5.1 [14]. Non-hydrogen atoms were refined
-PLUS) [14]. Full-matrix least-
reactions by H NMR spectroscopy did not yield any
evidence for nickel–hydride species.
SHELXTL
-
anisotropically. The hydrogen atoms were located in
Fourier synthesis difference maps. For all hydrogen
atoms both the positional parameters and a common
isotropic displacement parameter were restricted during
3.2. X-ray structure determination of
(AsPh4)2[Ni(‘buS2’)2]·2Et2O (1·2Et2O),
(AsPh4)[Ni(‘buS2’)2] (2) and [Ni(‘buS2’)2] (3)
refinement. Complex
2
contains two independent
All three complexes 1, 2 and 3 yielded single crystals
upon recrystallization. Complex 1 was obtained as the
solvate 1·2Et2O. The X-ray crystal structure determina-
tions showed that all three compounds contain discrete
cations, anions or molecules. Intermolecular interac-
molecules in the unit cell; in one molecule one tertiary
butyl group is disordered. Two different orientations
were refined (C231ꢀC233 and C241ꢀC243), with site
occupancies of 0.83(1) and 0.17(1). The C atoms of the

832
D. Sellmann et al. / Inorganica Chimica Acta 300–302 (2000) 829–836
Table 1
Crystal and data collection a parameters of (AsPh4)2[Ni(‘buS2’)2]·2Et2O (1·2Et2O), (AsPh4)[Ni(‘buS2’)2] (2) and [Ni(‘buS2’)2] (3)
Compound
1·2 Et2O
2
3
Chemical formula
Formula weight
Crystal system
Space group
a (pm)
b (pm)
c (pm)
h (°)
i (°)
C84H100As2NiO2S4
1478.43
triclinic
C52H60AsNiS4
946.87
triclinic
C28H40NiS4
563.55
triclinic
(
(
(
P1
P1
P1
1001.4(2)
1354.7(3)
1617.6(3)
71.34(2)
87.21(2)
69.57(2)
1.943(1)
1
1314.8(3)
1447.4(3)
1537.1(3)
87.03(2)
68.01(2)
64.81(2)
2.434(1)
2
589.4(1)
938.5(3)
1399.5(4)
70.56(2)
85.65(2)
79.65(2)
0.718(1)
1
k (°)
V (nm3)
Z
Dcalc (g cm−1
)
1.25
12.47
293
1.29
12.78
293
1.30
9.81
293
v (cm−1
T (K)
)
Crystal size (mm)
q Range (°)
0.7×0.5×0.4
1.7–27.1
9980
8539
5184
421
0.738
341/−300
3.3; 6.8
0.6×0.4×0.3
1.6–27.1
12 991
10 691
5325
539
0.754
900/−1103
4.6; 10.9
0.7×0.4×0.4
1.5–27.1
4068
3161
2407
151
0.983
365/−518
3.5; 10.3
Measured reflections
Independent reflections
Observed reflections b
Reflection parameters
Goodness-of-fit
Dz (e nm−3
R1 b; wR2 (%)
)
a All data collected with graphite-monochromatized Mo Ka radiation (u=71.073 pm).
b [I\2|(I)].
tions or stacking phenomena as found with other nickel
dithiolenes were not observed.
graphically required, (AsPh4)[Ni(‘buS2’)2] (2) has two
independent molecules (1) and (2) in the unit cell. One
of the two tertiary butyl groups of the anion (2) is
disordered with site occupancies of two possible sites
being refined to 0.83(1) and 0.17(1). Fig. 1 depicts the
molecular structure of the [Ni(‘buS2’)2] entities found in
all three compounds 1, 2 and 3, Table 2 lists and
compares relevant distances and angles of the individ-
ual complexes.
The nickel complexes of 1, 2 and 3 contain cen-
trosymmetric planar [Ni(‘buS2’)2] entities. Crystallo-
Table 1 shows that the NiꢀS distances decrease in the
order 1\2\3 with the formal increase of the nickel
oxidation states from +2 to +4. Likewise, the CꢀS
distances decrease. However, the decrease of NiꢀS and
CꢀS distances covers only a very small range. In all
three complexes the NiꢀS distances are relatively short,
in particular, when compared with the NiꢀS distances
Fig. 1. Molecular structure of the [Ni(‘buS2’)2] entities in
(AsPh4)2[Ni(‘buS2’)2]·2Et2O, (1·2Et2O), (AsPh4)[Ni(‘buS2’)2] (2) and
[Ni(‘buS2’)2] (3) (H atoms omitted).
Scheme 1. Syntheses and reactions of 1, 2 and 3.

D. Sellmann et al. / Inorganica Chimica Acta 300–302 (2000) 829–836
833
Table 2
Selected distances (pm) and angles (°) of 1, 2 and 3
Compound
Ni(II)
Ni(III) (1) a
Ni(III) (2) a
Ni(IV)
Trend b
Ni(1)ꢀS(1)
Ni(1)ꢀS(2)
S(1)ꢀC(10)
S(2)ꢀC(15)
C(15)ꢀC(10)
C(11)ꢀC(10)
C(15)ꢀC(14)
C(12)ꢀC(11)
C(14)ꢀC(13)
C(13)ꢀC(12)
dCC(Aryl)
217.50(8)
217.15(9)
177.0(2)
175.5(2)
140.1(3)
142.6(3)
140.2(3)
139.8(3)
138.3(3)
139.0(3)
140.0(3)
214.37(11)
213.53(11)
175.4(4)
174.5(4)
140.7(5)
142.5(5)
139.1(5)
138.7(5)
136.7(5)
141.2(5)
139.8(5)
214.46(12)
214.46(12)
175.1(4)
174.3(4)
140.7(5)
143.4(5)
139.1(5)
137.8(5)
138.9(5)
140.1(5)
140.0(5)
212.40(8)
212.67(8)
173.1(2)
172.3(2)
141.9(3)
142.9(3)
140.9(3)
137.5(3)
137.3(3)
142.2(3)
140.5(3)
¡
¡
¡
¡
o
o
o
n
n
n
o
S(1)ꢀNi(1)ꢀS(2)
89.63(3)
90.37(3)
116.2(2)
119.9(2)
125.2(2)
118.6(2)
119.2(2)
120.8(2)
91.01(4)
88.99(4)
116.3(3)
119.8(3)
125.4(3)
118.3(3)
118.7(3)
121.5(3)
90.96(4)
89.04(5)
117.0(3)
119.3(3)
124.6(3)
118.4(3)
119.2(3)
121.5(4)
91.57(3)
88.43(3)
116.3(2)
119.9(2)
124.3(2)
119.3(2)
118.8(2)
121.3(2)
S(1)ꢀNi(1)ꢀS(2a)
S(1)ꢀC(10)ꢀC(15)
S(2)ꢀC(15)ꢀC(10)
S(1)ꢀC(10)ꢀC(11)
C(11)ꢀC(10)ꢀC(15)
S(2)ꢀC(15)ꢀC(14)
C(14)ꢀC(15)ꢀC(10)
¡
o
o
n
n
o
o
a Two independent molecules in the unit cell.
b ¡=decreasing value, =increasing value, o=identical within three e.s.d., n=not identical, but no clear trend.
3.3. Electrochemistry, NMR, EPR and UV–Vis
spectra
of up to 240 pm in paramagnetic nickel thiolate com-
plexes which have electrons in antibonding nickel–lig-
and molecular orbitals [16]. The CꢀC distances within
the benzene rings do not show any significant changes.
However, the individual CꢀC distances exhibit rather
large deviations from the mean CꢀC distance value.
This is certainly caused by the bulky tertiary butyl ring
substituents introducing considerable asymmetry in the
benzene system. The main point is that the CꢀC ben-
zene distances do not yield any evidence for an oxida-
tive formation of o-quinoid or 1,2-dithiocyclohexadike-
tone structures according to Eq. (2), when
[Ni(‘buS2’)2]2− is oxidized to [Ni(‘buS2’)2]0.
Corresponding to the chemical redox transforma-
tions of 1 to 3, the cyclic voltammograms (CV) of 1, 2
and 3 are identical and each exhibits one quasi-re-
versible redox wave in the anodic and one in the
cathodic region. They can be assigned to the
[Ni(‘buS2’)2]−/0 and the [Ni(‘buS2’)2]1−/2− couples. As
an example, Fig. 2 shows the CV of (AsPh4)[Ni(‘buS2’)2]
(2).
The redox potentials of the [Ni(‘buS2’)2] couples lie in
the same range as observed for other planar nickel
complexes with four sulfur donors, irrespective of the
question whether the bidentate sulfur ligands are con-
sidered dithiolene type ligands or rather ‘normal’ dithi-
olates, e.g. [Ni(S2C2(4-ClC6H4)2]0/1− [4], [Ni(S2C6-
H4)2]2−/1− [8], or [Ni(S2-norbornane)2]2−/1− [19].
(2)
The NiꢀS distances of 1 and 2 correspond with those in
the parent complexes (NMe4)2[Ni(S2C6H4)2] [8] and
(NMe4)[Ni(S2C6H4)2] [7]. The short NiꢀS distances in 3
compare with those in neutral ‘typical’ dithiolene com-
plexes, for example, [Ni(S2C2Ph2)2] (210.1(2) pm) [17] or
[Ni(S2C2(H)C6H4C8H17)2] (213.0(1) and 211.6(1) pm)
[18].
In summary, the X-ray structural results do not show
a ligand centered oxidation of 1 via 2 to 3 in the sense
that the ‘buS2’ ligands behave as ‘non-innocent’ ligands.
The (slight) decrease of the [NiS4] core distances rather
suggests that the relevant electrons being removed
reside in molecular orbitals having mainly [NiS4]
character.
Fig. 2. Cyclic voltammogram of (AsPh4)[Ni(‘buS2’)2] (2) in THF
(20°C, 6=20 mV s−1, potentials vs. NHE).

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D. Sellmann et al. / Inorganica Chimica Acta 300–302 (2000) 829–836
hyperfine coupling of the unpaired electron with the
Complexes 1 and 3 are diamagnetic as evidenced by
magnetically active 61Ni nucleus (I=3/2) was observed,
with A1(61Ni)=131(15), A2(61Ni)B17 and A1(61Ni)=
57(10) mT. This observation clearly demonstrates a
considerable metal contribution to the SOMO. Similar
their high-resolution NMR spectra. The 1H NMR spec-
trum of [Ni(‘buS2’)2] (3) in C6D6 at 25°C shows two
sharp tertiary butyl singlets at 1.15 and 1.77 ppm and
two broadened signals at 7.6 and 8.2 ppm for the
1
g
and
A
values were observed for (NR4)[Ni-
aromatic protons. Interestingly, in the H NMR spec-
trum of (AsPh4)2[Ni(‘buS2’)2] (1) in (CD3)2CO only one
broad tertiary-butyl signal at 1.95 ppm and one singlet
for the four ‘buS2’ benzene ring protons appeared. These
signals could not be resolved by lowering the tempera-
ture, because (AsPh4)2[Ni(‘buS2’)2] precipitated from the
(S2C2(CN)2)2] [20] and (NR4)[Ni(xylene-1,2-dithiolate)2]
[21]. They were controversially interpreted to indicate
an unpaired electron occupying a nickel centered
SOMO with some sulfur character [20] or a ligand
centered SOMO with some nickel character [21]. Thus
the question remained open, whether such complexes
must be described as NiIII complexes or rather radical
ligand complexes stabilized by NiII centers. The same
would hold true for (AsPh4)[Ni(‘buS2’)2] (2). It is noted,
however, that the Bg\ =2.0103(1) value of the 2,4,6-
tritertiarybutyl-benzene-thiyl radical [22] is clearly dif-
ferent from the g values observed for the nickel
complexes.
Fig. 4 shows the UV–Vis spectra of 1, 2 and 3 in
THF (10−4 M). They exhibit the typical p–p* benzene
ring absorptions at 300 nm, 2 and 3 show additional
intensive absorptions at 860 and 880 nm, respectively.
For nickel dithiolene complexes, bands in this region
have been assigned to ligand“metal CT bands of the
type p(L)“p1 [23], which are impossible in complexes
having formally d8 NiII centers, because p1 is fully
occupied.
1
solution. The number of H NMR signals could indi-
cate that the anion of 1 forms isomers in solution, and
upon crystallization only the centrosymmetric species
characterized by X-ray structure analysis precipitates.
Complex 2 is paramagnetic and did not yield a 1H
NMR spectrum, but an EPR spectrum (Fig. 3).
At 293 K, (AsPh4)[Ni(‘buS2’)2] (2) dissolved in
CHCl3–DMF shows an isotropic signal with Bg\ =
2.08. This signal is split into a rhombic signal at 120 K
with g1=2.18, g2=2.04 and g3=2.01. In order to get
more detailed information on the metal character of the
SOMO, a 61Ni enriched sample of (AsPh4)[Ni(‘buS2’)2]
(2) was prepared with an enrichment of 86.2%. This
sample showed at 120 K an EPR spectrum with identi-
cal g values as unlabelled 2. In addition, a strong
3.4. XPS in6estigations
In order to determine the influence of the overall
charge upon the nickel oxidation state in 1, 2 and 3,
XPS measurements were also carried out for the Ni
2p3/2 and S 2p binding energies (Eb). It is to be noted
that the calibration of XPS data can be difficult because
the reported Eb values tend to be severely influenced by
secondary effects, the preparation of the probes, and
the standards. For example, three different Eb (Ni 2p3/2
)
Fig. 3. EPR spectra of (AsPh4)[Ni(‘buS2’)2] (2) in CHCl3/DMF at 120
K (9.43 GHz, 100 kHz sweep width, 20.1 W (a) and 63.5 mW (b): (a)
with natural abundance of 61Ni; (b) with 86.2% of 61Ni).
values have been reported for (NBu4)[Ni(S2C2(CN)2)2]
(853.1 eV [24], 856.6 eV [25] and 857.3 eV [26]). Eb (Ni
2p3/2) values for square planar Ni complexes with [NiS4]
cores range from 852.8 eV [24] to 859.7 eV [27]. The
latter value is close to that of K2NiF6 (861.2 eV) [28]
which is considered to contain ‘genuine’ NiIV centers.
The only investigation on three different oxidation
states of square planar NiS4 species was reported for
(NBu4)n[Ni(dmit)2], n=2, 1, 0 (dmit2− =dimercapto-
isotrithione(2−)) [29], in which no significant change
was found for the Eb (Ni 2p3/2) values. The Eb (S 2p)
value of the thiolate sulfur donors showed a slight trend
from 163.9 eV in (NBu4)2[Ni(dmit)2] to 162.8 eV in
neutral [Ni(dmit)2]. The oxidation was interpreted as
strictly ligand centered.
Fig. 4. UV–Vis–NIR spectra of (AsPh4)2[Ni(‘buS2’)2] (1),
(AsPh4)[Ni(‘buS2’)2] (2) and [Ni(‘buS2’)2] (3), (10−4 M in THF).
For these reasons, the XPS spectra of 1, 2, 3 were
recorded under strictly identical conditions. Table 3

D. Sellmann et al. / Inorganica Chimica Acta 300–302 (2000) 829–836
835
frontier orbitals in [Ni(‘buS2’)2] complexes may finally
be concluded from the reactivity of 1 and 2 and the
remarkable inertness of 3 towards CO.
Table 3
Binding energies Eb, FWHM a (eV) of 1·2Et2O, 2 and 3
Compound
Ni(II)
Ni(III)
Ni(IV)
Eb (Ni 2p3/2
Eb (S 2p)
)
858.1(1.3)
not measured
859.7(1.7)
162.4(2.2)
860.5(1.8)
163.4(2.5)
5. Supplementary material
a Full-width at half maximum.
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC-
137423 (1·2Et2O), 137424 (2), 137425 (3). Copies of the
data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44-1223-336033; e-mail: deposit@ccdc.cam.
ac.uk).
lists the results. The data show a clear trend for both
the Eb (Ni 2p3/2) and Eb (S 2p) values, which both
increase with formally increasing Ni oxidation states.
The Eb (Ni 2p3/2) increase of 2.4 eV lies in the same
range as the difference of 3.3 eV between NiF2 (857.5
eV) and K2NiF6 (860.8 eV) [28], and it contrasts the
[Ni(dmit)2]2−/1−/0 series, which shows no significant Eb
(Ni 2p3/2) changes. Thus, the XPS data also support the
conclusion that the oxidation of 1 to 3 is neither ligand
nor metal-centered but instead concerns electrons in
molecular orbitals of [NiS4] character.
Acknowledgements
Support of this work by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie
is gratefully acknowledged. We wish to thank Dr M.
Moll for the recording of NMR spectra and W.
Donaubauer for technical assistance in the X-ray dif-
fraction studies. We are also indebted to Dr A. Elvers
and Dr M. Meth for recording the EPR spectra.
4. Conclusions
Square-planar nickel thiolate complexes with [NiS4]
cores in three different oxidation states have been syn-
thesized and unambiguously characterized. The
[Ni(‘buS2’)2]n− species with n=2, 1, 0 can be converted
into each other by redox reactions, and the redox
potentials lie in the range observed also for alkane–
dithiolate complexes. The NiꢀS distances slightly, but
significantly, decrease when the nickel oxidation state
formally increases from +2 via +3 to +4 in 1, 2 and
3. The formal oxidation state increase corresponds with
the 2.4 eV increase of Ni 2p3/2 binding energies ob-
served by XPS. The Eb (Ni 2p3/2) difference between 1
and 3 is of the same order as the difference of 3.3 eV
between NiF2 and K2NiF6, which are taken as standard
NiII and NiIV complexes. The increasing Eb (S 2p)
values of 1, 2 and 3 demonstrate that the oxidation
1“3 also influences the sulfur electrons. Thus, the
X-ray structural and XPS data suggest that the oxida-
tion of 1–3 concerns electrons which reside in orbitals
having [NiS4] character. This is in accord with the
chemical properties of 1, 2 and 3, whose [Ni(‘buS2’)2]
species behave as typical covalent metal complexes.
No evidence could be obtained that the electronic
changes in 1, 2 and 3 extend beyond the nickel sulfur
core into the benzene rings. The observed hyperfine
interaction with the nickel center in the EPR spectrum
of 2 demonstrates considerable nickel contribution to
the SOMO. All results considered together suggest that
1,2-benzenedithiolate(2−) and its ‘buS2’2− derivatives
are normal 1,2-dithiolate ligands. A similar conclusion
had previously been drawn for iron 1,2-benzenedithio-
late complexes [30]. A significant nickel character of the
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