An experimental and theoretical approach to phosphonodithioato complexes: Molecular orbital analysis by hybrid-DFT and EHT calculations on trans-bis[O-alkyl-phenylphosphonodithioato]NiII, and vibrational assignments

Article abstract of DOI:10.1139/v01-163

The synthesis and the full spectroscopic characterization (FT-IR, FT-Raman, ³¹P CP MAS NMR) of trans-bis[O-ethyl-phenylphosphonodithioato]Ni^II (3) are reported. On the basis of hybrid-Density Functional Theory (DFT) calculations and

Full text of DOI:10.1139/v01-163

1483
An experimental and theoretical approach to
phosphonodithioato complexes: molecular orbital
analysis by hybrid-DFT and EHT calculations on
trans-bis[O-alkyl-phenylphosphonodithioato]NiII,
and vibrational assignments
M. Carla Aragoni, Massimiliano Arca, Francesco A. Devillanova, John R. Ferraro,
Francesco Isaia, Francesco Lelj, Vito Lippolis, and Gaetano Verani
Abstract: The synthesis and the full spectroscopic characterization (FT-IR, FT-Raman, 31P CP MAS NMR) of trans-
bis[O-ethyl-phenylphosphonodithioato]NiII (3) are reported. On the basis of hybrid-Density Functional Theory (DFT)
calculations and Extended Hückel Theory (EHT) calculations, performed on the simpler trans-bis[O-methyl-phenylphos-
phonodithioato]NiII (2) model complex, the electronic structures of phosphonodithioato complexes in their ground states
are fully described, and in particular the vibrational features are deeply analyzed, allowing an unprecedented insight
into the vibrational features of trans-bis-O-methyl-phenylphosphono- and trans-bis(isopropylamidophosphono)-dithioato
complexes of nickel(II), palladium(II), and platinum(II).
Key words: phosphonodithioato complexes, IR, Raman, DFT calculation.
Résumé : On rapporte la synthèse et la caractérisation spectroscopique complète (FT-IR, FT-Raman, 31P CP MAS
RMN) du trans-bis[O-éthyl-phénylphosphonodithioato]NiII (3). Sur la base de calculs de densité fonctionnelle hybride
(DFT) et de calculs de Hückel étendus effectués (EHT) sur le complexe modèle plus simple, trans-bis-[O-méthyl-
phénylphosphonodithioato]NiII (2), on présente une description complète des structures électroniques des complexes
phosphonodithioato dans leurs états fondamentaux et, en particulier, on a analysé en détail les caractéristiques vibra-
tionnelles ce qui permet d’obtenir une vue sans précédent des caractéristiques vibrationnelles des complexes trans-bis-
O-méthyl-phénylphosphono- et trans-bis(isopropylamidophosphono)-dithioato du nickel(II), du palladium(II) et du pla-
tine(II).
Mots clés : complexes phosphonodithioato, IR, Raman, calculs DFT.
[Traduit par la Rédaction] Aragoni et al. 1491
Introduction
Reagent (LR, 2,4-bis(4-methoxy-phenyl)-[1,3,2,4]-dithiadi-
phosphetane-2,4-disulfide (1), see Scheme 1) (9), mainly known
as thionation agent. First, these complexes were obtained as
by-products in the synthesis of a new class [M(R,R¢timdt)2]
(M = Ni, Pd, Pt; R,R¢timdt = monoreduced form of
disubstituted imidazolidine-2,4,5-trithione) of neutral metal-
dithiolenes (10, 11), obtained by sulphuring disubstituted
imidazolidine-2-thione-4,5-diones with 1 in the presence of
a suitable metal powder or salt and of an alcohol, used be-
cause of solubility reasons (8). More generally, 1 was
proved to react with nucleophilic alcohols (ROH) and
amines (R¢NH2) to induce the opening of the P2S2 tetra-
atomic ring, yielding phosphonodithioato and amidophos-
phonodithioato anions. Furthermore, their reaction with NiII,
Many reports have been published on the syntheses, charac-
terizations, and reactivities of phosphoro- (1) and phosphino-
dithioates (2), since these compounds and their metal com-
plexes have found wide application in industrial and agricul-
tural fields (3), e.g., additives to lubricant oils (4), extraction
reagents for metals (3, 5), flotation agents for mineral ores,
and insecticides, rodenticides, and pesticides (3, 6). How-
ever, the chemistry of phosphono- and amido-phosphono-
dithioates has been only scarcely investigated, probably
because of synthetic difficulties (7). Recently (8), we have
reported a novel synthetic route to the preparation of square-
planar metal-phosphonodithioates deriving from Lawesson’s
Received March 7, 2001. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on October 12, 2001.
M.C. Aragoni, M. Arca, F.A. Devillanova, F. Isaia, V. Lippolis, and G. Verani.1 Dipartimento di Chimica Inorganica ed
Analitica, S.S. 554 bivio per Sestu, I-09042 Monserrato – Cagliari, Italy.
J.R. Ferraro. Chemistry and Materials Science Division, Argonne National Laboratory, 9700 Soth Cass Avenue, Argonne, Illinois 60439.
F. Lelj. Dipartimento di Chimica, Via N. Sauro 85, I-85100, Potenza, Italy.
1Corresponding author (telephone: +39-070-675-4474; fax: +39-070-675-4456; e-mail: verani@unica.it).
Can. J. Chem. 79: 1483–1491 (2001)
© 2001 NRC Canada
DOI: 10.1139/cjc-79-10-1483

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Can. J. Chem. Vol. 79, 2001
PdII, and PtII salts (MCl2 for M = Ni, Pd; K2MCl4 for M =
Pd, Pt) gives the corresponding phosphono- and amido-
phosphono-dithioato complexes (12). These can also be ob-
tained in a one-pot reaction by reacting 1 and the metal salts
in the presence of the nucleophilic reagent. This route
proved to be particularly well-suited for the synthesis of
phosphonodithioato complexes deriving from light alcohols,
since these can be used directly as solvents (8). Pursuant to
our interest in this field, to understand the electronic and
spectroscopic features of dithiophosphonato complexes, cal-
culations based on Density Functional Theory (DFT) (13)
have been performed on trans-bis[O-methyl-phenylphosphono-
dithioato]NiII (2, Scheme 1), chosen as a model compound.
In fact, DFT calculations are assuming a primary role, since
they give reliable results with inorganic compounds contain-
ing transition metals (11, 14) in their ground or excited
states (15). With this in view, we have prepared trans-bis[O-
ethyl-phenylphosphonodithioato]NiII (3), starting from 2,4-
diphenyl-[1,3,2,4]-dithiadiphosphetane-2,4-disulfide (4). Com-
plex 3 was previously reported, but only the cell constants
and an average Ni—S distance were published (16, 17).
logues of 2, where values similar to those reported for 5 and
9 were found (17). To qualitatively understand the interac-
tions between the atomic orbitals (AO’s) of the NiII ion and
the molecular orbitals (MO’s) of the two ligand units, the
Fragment Molecular Orbitals (FMO) approach within the
Extended Hückel Theory (EHT) is employed. In Fig. 2 an
interaction diagram showing the orbitals, whose eigenvalues
lay in the range between –8 and –15 eV, is reported. In the
idealized molecular C2h point group the five 3d AO’s span
in three ag (dx2 - y2 , dz2 , dxy) and two bg (dxz, dyz) representa-
tions. The 21ag highest occupied molecular orbital (HOMO)
of 2 is a p-MO largely made up of the dx2 AO (25%),
y2
which is raised in energy by the antibonding interaction with
the 21ag FMO of the ligands, constituted by a combination
of sulphur 3p AO’s perpendicular to the coordination plane.
The interaction between the 14bg FMO of the ligands (in
plane sulphur p AO’s) with the dxz and dyz AO’s (25 and
16%, respectively) origins the s-type 14bg lowest unoccu-
pied molecular orbital (LUMO). This MO happens to lay at
roughly the same energy as three other nonbonding MO’s
(22ag, 19bu, 13au), which are p-orbitals arising from the
combinations of the 2p AO’s of the aromatic carbon atoms.
The HOMO-1 13bg also contains a contribution of Ni AO’s
(namely dyz and dxz, 25 and 15%, respectively) p-interacting
with a bg combination of 3p sulphur AO’s. Metal d AO’s
also contribute to other MO’s at lower energies, such as
20ag (49% dz2 , 16% dxy), 18 ag (32% dx2 - y2 , 2% dz2 , 11%
dxy), 15ag (2% dz2 , 8% dxy), and 7bg (28% dxz).
Results and discussion
Molecular orbital analysis
The geometry optimization of trans-bis[O-methylphenyl-
phosphonodithioato]NiII (2, see Scheme 1) has been per-
formed starting from the structural data determined by X-ray
diffraction on trans-bis[O-ethyl-(4-methoxyphenyl)phosphono-
dithioato]NiII (5) and has correctly led to a square-planar co-
ordination of the central metal ion, with the phosphorus
atoms slightly out of the coordination plane (Fig. 1). In Ta-
ble 1, a comparison between the calculated bond distances
and angles and those measured by X-ray diffraction on 5
(8) (where the phenyl is replaced by a p-methoxyphenyl)
and trans-bis[isopropylamido-(4-methoxyphenyl)phosphono-
dithioato]NiII (9) (where the ethoxy group is replaced by an
isopropylamine moiety) (12) is reported. As found in the
case of other hybrid-DFT calculations on Ni complexes de-
riving from sulphured ligands (8, 11), the metal—sulphur
distances are slightly overestimated (a distance of 2.278 Å
has been calculated, compared to an average value of 2.20 Å
found experimentally) (17), while the remaining calculated
bond distances and angles are similar to the experimental
ones reported for 5 (8) and trans-bis[(4-methoxyphenyl)-O-
methyl-phosphonodithioato]MII (M = Pd (7) and Pt (8))
complexes (12).2 Some differences regarding the orientation
of the substituents to the phosphorus atom [S(1,2)-P(2)-
O(1), S(1,2)-P(1)-C(2), P(1)-O(1)-C(1)] should be attributed
to crystal packing effects. The comparison can also be ex-
tended to the reported structures of the PdII and PtII ana-
Vibrational assignments
The calculation of harmonic frequencies from DFT energy
gradients allows one to verify the optimized geometry and to
obtain an unprecedented insight into the vibrational spectra
of such complexes (Tables 2–4). Only two negative fre-
quency values have been calculated (–84 and –82 cm–1),
which are related to rotations of methyl and phenyl groups,
and therefore, are due to the presence of a constrained inver-
sion center. A first examination of the simulated spectra
(Fig. 3) shows that the use of transferable scaling factors
(18), depending on the nature of the single vibrations, gives
better results compared to uniform scaling factors. In this
case, the slight differences found between the calculated and
the experimental bond lengths and angles force the use of
scaling factors that are quite different from unity, because of
the unharmonic nature of the experimental vibrations. In
general, scaling factors have been obtained by comparing
the calculated harmonic frequency values to those mea-
sured for 3 and trans-bis[(4-methoxyphenyl)-O-methyl-
phosphonodithioato]NiII (6), depending on the group in-
volved in the vibrations. Because of the presence of an in-
version centre, the IR spectra are expected to show only the
2 To ascertain if the differences between the calculated and the experimental bond lengths and angles were to be attributed to the hybrid na-
ture of the functional B3LYP, geometry optimization followed by frequency calculations have been performed with the same basis set on
the simplified trans-bis[O-methyl-methylphosphonodithioato]NiII model complex, with three different functional, namely BLYP, BPW91,
and B3LYP. Both the calculated geometries and frequencies are similar in the three cases. As regards the (PS2)2Ni local moiety, the bond
distances and angles calculated for the model complex are very similar to those calculated for 2 (Table 1). Although the BPW91 functional
slightly reduces the overestimation of the Ni—S bond distances (2.282, 2.249, 2.278 Å for BLYP, BPW91, and B3LYP), it tends to calcu-
late elongated P—S (2.044, 2.025, 2.020 Å for BLYP, BPW91, and B3LYP), P—O (1.656, 1.652, 1.628 Å for BLYP, BPW91, and
B3LYP), and P—C (1.849, 1.836, 1.829 Å for BLYP, BPW91, and B3LYP) distances, compared to the experimental average values deter-
mined for 5 (Table 1). As regards frequency calculations, the results are also pretty similar, since the three functionals calculated the same
number of bands in the same order, BLYP functional leading to smaller absolute wavenumber values.
© 2001 NRC Canada

Aragoni et al.
1485
Scheme 1.
S
S
S
R
S
P
P
OCH3
R =
(1)
(4)
R
R'
S
S
S
S
R
R' = -OCH3
R' = -OC2H5
M = Ni(II)
M = Ni(II)
(2)
(3)
R =
R =
P
M
P
R
R'
R' = -OC2H5
R' = -OCH3
M = Ni(II)
M = Ni(II)
Pd (II)
(5)
(6)
(7)
(8)
OCH3
Pt (II)
R' = NH-i-C H
M = Ni(II)
Pd (II)
(9)
3
7
(10)
(11)
Pt (II)
Fig. 1. Atom numbering scheme for model complex 2.
Table 1. Selected optimized bond lengths (Å), angles (°), and di-
hedral angles (°) for model complex trans-bis[O-methylphenyl-
phosphonodithioato]NiII (2) compared with average values
measured on analogue Ni complexes 5 and 9.
2
5a
9a
Bond lengths (Å)
Ni(1)—S(1,2)
P(1)—S(1,2)
P(1)—O(1)
P(1)—C(2)
2.278
2.020
1.631
1.823
1.437
2.221
1.994
1.580
1.782
1.456
2.223
2.015
—
—
1.795
O(1)—C(1)
Bond angles (°)
S(1)-Ni(1)-S(2)
S(1)-Ni(1)-S(1¢)b
Ni(1)-S(1,2)-P(1)
S(1)-P(1)-S(2)
S(1,2)-P(1)-C(2)
S(1,2)-P(1)-O(1)
O(1)-P(1)-C(2)
P(1)-O(1)-C(1)
88.086
91.914
80.498
103.264
112.140
111.642
106.160
125.218
88.14
91.86
83.78
101.57
113.94
114.09
99.85
87.64
92.81
84.96
99.67
112.67
—
—
—
120.9
aAverage values. Supplementary materials of refs. 8 and 12.
antisymmetric modes, and the Raman spectra only the sym-
metric ones. At any rate, as far as the ligand units are con-
cerned, for each mode, two combinations (symmetric and
antisymmetric) are possible, among which the symmetric
combinations appear in the Raman spectra and the
antisymmetric ones in the IR spectra. Since the two ligand
groups are independent, the two combinations are almost de-
generate in energy. Therefore, symmetric vibrational modes
related to the two ligand units appear in the IR spectra
through their antisymmetric combinations, as well as symmet-
ric coupling of antisymmetric vibrations are Raman active.
An examination of the vibrational frequencies calculated
for 2 shows that the bands having the highest energies can
bSymmetry transformation used to generate (¢): –x, –y, –z.
be found in the region 3100–2900 cm–1 (Table 2). In this
region the symmetric and antisymmetric C—H stretching
normal modes are calculated, and can be found in all the
dithiophosphonato complexes, with minor differences due to
the nature of the substituents.
Below 1600 cm–1, the first band found in the FT-IR and
Raman spectra of 6 is a strong peak at 1597 and 1590 cm–1,
respectively, sided by a less intense peak falling at 1567 cm–1
in both spectra. These peaks correspond to the two possible
© 2001 NRC Canada

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Can. J. Chem. Vol. 79, 2001
Fig. 2. Qualitative EHT–FMO interaction diagram between the NiII 3d atomic orbitals (right side) and the ligand molecular orbitals
(left side) in model complex 2. FMO labelling refers to the C2h point group. Molecular orbitals are connected to FMO’s contributing
for at least 20%.
-8
15 bg
14 bg
22 ag ; 19 bu; 13 au
LUMO
-9
-10
-11
-12
-13
-14
-15
ag
au
bg
bu
z
x
y
P
Ni
P
S
S
HOMO
21 ag
13 bg
14 bg
21 ag
20 ag
17-18 ag
10 bg
d a.o.'s
15 ag
6 bg
15 ag
7 bg
14 ag
7 bg
Table 2. Selected calculated scaled and unscaled harmonic frequencies (cm–1), scaling factors, reduced masses (amu), calculated IR in-
tensities (km mol–1), and normal mode descriptions for model complex 2, and corresponding FT-IR and FT-Raman (italics) experimen-
tal bands (cm–1) for complexes 3 and 6, in the region 3300–3000 cm–1.
Unscaled
Scaling Scaled freq. Reduc. mass IR int.
Exp. freq. 3
Exp. freq. 6
Normal mode
description
freq. (cm–1)
factor
(cm–1)
3088
(amu)
(km mol–1)
45.7
(cm–1)
(cm–1)
3216
3211
3205
3197
3188
3166
0.96
1.098
3084 (vw)
3070 (vw)
3053
3088 (vw)
Symmetric stretching
C(3–7)—H
Antisymmetric stretching
C(4–6)—H
Stretching
C(3,4,7)—H
Antisymmetric stretching
C(3–6)—H
Symmetric stretching
C(5–7)—H
Symmetric stretching
C(1)—H(1,2 in phase;
3 out of phase)
Antisymmetric stretching
C(1)—H(2,3)
0.96
0.96
0.96
0.96
0.96
3082
3076
3069
3060
3040
1.094
1.091
1.088
1.085
1.106
20.6
14.3
12.4
0.1
3075 (vw), 3074
40.0
3140
3064
0.96
0.96
3015
2941
1.108
1.030
34.5
2975 (mw)
2972 (w), 2973
122.8
2930 (w), 2930 2941 (mw), 2943
Symmetric stretching
C(1)—H
Note: In this region calculated symmetric and antisymmetric modes fall at the same frequencies.
combinations of ns and nas C—C stretching of the phenyl
rings, and are very weak in the FT-IR spectrum of 3 (IR:
1585 and 1571 cm–1; Raman: only 1586 cm–1). In the simu-
lated spectrum of 2 (Table 3) these two bands are also calcu-
lated to be very weak (2.2 and 0.5 km mol–1, respectively).
Therefore, this suggests that the presence of the methoxy
substituent in the para position of the phenyl ring causes an
enhancement in the intensity of the bands in the IR spectra,
which is presumably due to the negative charges on the oxy-
gen atoms, which enhance the variation in the dipole mo-
© 2001 NRC Canada

Aragoni et al.
1487
Table 3. Selected calculated scaled and unscaled harmonic frequencies (cm–1), scaling factors, reduced masses (amu), calculated IR in-
tensities (km mol–1), and normal mode descriptions for model complex 2, and corresponding FT-IR and FT-Raman (italics) experimen-
tal bands (cm–1) for complexes 3 and 6, in the region 1700–500 cm–1.
Unscaled
Scaling Scaled freq. Reduc. mass
IR int.
Exp. freq. 3
Exp. freq. 6
Normal mode
description
freq. (cm–1) factor
(cm–1)
(amu)
(km mol–1) (cm–1)
(cm–1)
1647
1632
1530
0.96
0.96
0.95
1585, 1585
5.250, 5.245
2.2
0.5
8.8
1585 (w) 1586
1597 (vs) 1590
Symmetric stretching
C(3,4)—C(5,6)
1571, 1571
1454, 1454
5.138, 5.135
1.927, 1.930
1571 (w)
1567 (mw), 1566 Antisymmetric stretching
C(2,6)—C(3,7)
Symmetric bending
C(3–6)—H
1466 (mw)
1449 (w), 1450
1465 (mw)
1525
1522
1501
1482
0.95
0.95
0.95
0.95
1449, 1449
1446, 1446
1426, 1426
1408, 1408
1.048
1.073, 1.072
1.129
20.6
37.8
10.7
32.1
1454 (mw), 1454 Rocking C(1)—H(1–3)
Scissoring C(1)—H(2,3)
1436 (ms)
1389 (m), 1389
1426 (w), 1424
1407 (mw)
Wagging C(1)—H(2,3)
Antisymmetric stretching
C(3,4)—C(5,6) + sym-
metric bending
2.117
C(5–7)—H
1365
0.95
1297, 1297
1.914, 1.913
10.2
1308 (mw)
1307 (ms), 1307
1294 (s), 1294
Antisymmetric stretching
C(2,6)—C(3,7) +
bending C(3,4,7)—H
Stretching C(2)—C(3,4)
+ bending C(3–6)—H
Symmetric bending
C(3,4)-C(5,6)-H +
1332
1219
0.95
0.97
1265, 1265
1182, 1182
2.718, 2.720
5.6
1284 (vw), 1282
1.151
17.7
1182 (mw), 1184 1186 (sh), 1175
bending C(5,6)-C(3,4)-H
Out of plane wagging
C(1)—H(2,3) +
1199
0.97
1165, 1165
1.303, 1.304
20.5
1157 (m), 1158
1165 (m)
bending
O(1)-C(1)-H(1)
1121
1112
0.99
0.99
1113, 1113
1101, 1101
2.959
1.630
105.6
2.9
1113 (vs), 1113
1101 (sh)
1118 (vs), 1118
1108 (sh)
Stretching C(2)—P(1)
Antisymmetric stretching
C(3,4)—C(5,6) +
bending CH aromatic
Stretching O(1)—C(1)
1074
1081
1054
1014
0.93
1003
1005
1054, 1054
1014, 1014
7.132
7.263
2.907, 2.898
5.494, 5.458
1018.7
—
1014 (vs)
1027
1003 (vs)
1005
1.00
1.00
2.7
5.4
Phenyl ring breathing
Phenyl ring breathing
C(3,4,7)
P(1)—O(1) + deforming
out of plane CH
aromatic
998
1011 (sh)
789 (vs)
749
1.00
749
4.173
413.0
783 (s)
752
725
726
703
752
725
726
703, 703
3.094
7.403
7.342
2.510, 2.491
—
119.3
—
801
1.00
1.00
0.96
Breathing phenyl rings
38.8
Deforming out of plane
C(3,4,7)—H
Antisymmetric stretching
P(1)—S(1,2)
657
631
21.853
103.3
627 (s)
622 (ms)
645
627
625
582
619
627
625
559
15.258
6.893
7.974
—
11.9
—
623
618
1.00
0.96
Breathing phenyl rings
11.950
224.4
565 (vs)
550 (s)
Symmetric stretching
P(1)—S(1,2) + stretch-
ing P(1)—C(2)
583
560
11.804
—
566
553
ments. Many differences can be found in the vibrational
spectra of 3 and 6 in the region of 1500–1400 cm–1. In par-
ticular, four normal modes are calculated to be IR-active and
they fall at 1449, 1446, 1426, and 1408 cm–1: the first two
are due to rocking and scissoring modes of the methyl
groups, the third to an umbrella wagging of the methyl
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Can. J. Chem. Vol. 79, 2001
Table 4. Selected calculated scaled and unscaled harmonic frequencies (cm–1), scaling factors, reduced masses (amu), calculated IR in-
tensities (km mol–1) and normal mode descriptions for model complex 2, and corresponding FT-IR and FT-Raman (italics) experimen-
tal bands (cm–1) for complexes 3 and 6, in the region 500–50 cm–1.
Unscaled
Scaling
factor
Scaled freq.
(cm–1)
Reduc.
mass (amu)
IR int.
Exp. freq. 3
Exp. freq. 6
Normal mode
description
freq. (cm–1)
(km mol–1)
(cm–1)
(cm–1)
498
1.00
498
4.339
19.6
Out of plane phenyl ring
deforming
499
395
401
336
499
435
442
375
4.354
12.401
12.842
5.769
—
39.4
—
491
435 (m)
440
1.10
1.12
470 (ms)
470
375 (mw)
Bending S(1)-Ni-S(2)
8.9
375 (m)
Bending P(1)-O(1)-C(1) +
twisting phenyl ring
337
327
320
378
360
360
5.895
1.788
8.700
—
3.9
—
386
375
1.10
0.96
Bending S(1)-Ni(1)-S(1)¢
Symmetric wagging
P(1)—O(1),C(2)
319
313
294
1.10
1.10
1.10
351
344
323
28.800
27.979
11.266
79.5
1.9
—
354 (s)
308
351 (m)
343
Antisymmetric stretching
Ni—S(1,2)
Antisymmetric stretching
Ni—S(1,2)
Symmetric stretching
Ni—S(1,2) + S(1)-P(1)-
S(2) bending + P(1)-
O(1)-C(1) bending
Bending P(1)-O(1)-C(1) +
S(1)-P(1)-S(2)
314
259
248
244
211
1.07
1.10
1.10
1.10
277
273
268
232
5.341
15.680
5.410
29.6
—
277 (mw)
273
278
Antisymmetric stretching
Ni—S(1,2)
—
Symmetric Ni(1)—S(1,2) +
bending P(1)-O(1)-C(1)
Symmetric stretching
Ni(1)—S(1,2) + bending
S(1)-P(1)-S(2)
12.968
19.9
204 (mw)
202 (w)
206
136
133
1.10
1.10
1.10
226
150
147
8.324
9.157
2.1
—
Bending S(1)-Ni(1)-S(1)¢ +
twisting O(1)-P(1)-C(2)
Symmetric breathing
P(1)-S(1,2)-Ni(1)
142
137
11.898
1.8
NiS4 out of plane
deforming
118
113
1.00
1.00
118
113
1.417
4.150
—
—
116
102
Methyl group rotation
Methyl group rotation +
twisting NiS4
113
86
91
1.00
91
4.859
—
84
NiS4P2 ring deformation
group, and the fourth to a combination of C—C stretching
and C-C-H in plane bending of the phenyl rings. Since the
first two modes are almost degenerate in frequency, these
four normal modes result in three bands in the simulated
spectrum. The first and the third bands are found in the FT-
IR spectrum of 3 at 1436 (the shift due to the presence of an
ethyl instead on a methyl group) and 1389 cm–1, while the
umbrella deformation is not found in 3, since no OCH3
group is present. With regards to 6, the bands are more nu-
merous due to the presence of another OCH3 group, as a
substituent, at the phenyl ring, whose C–H deformations
should fall in the same region. Such deformations can be con-
sidered responsible for a very strong IR band at 1500 cm–1,
which is found almost identical in the corresponding com-
plexes 7 and 8 (12), and in trans-bis[isopropylamido-(4-
methoxyphenyl)-phosphonodithioato]MII (M = Ni (9), Pd (10)
and Pt (11)). This region of the vibrational spectrum shows
only very weak bands in the Raman spectra of all compounds.
The bands between 1300 and 1100 cm–1 are attributed to
vibrations mainly regarding bending modes of the bonds in
the aromatic rings. Among them, the bands falling at 1182
and 1177 cm–1 in the FT-IR spectra of 3 and 6, respectively
(calculated at 1182 cm–1 for 2), are also found with remark-
able intensities in their symmetric combination in the Raman
spectra (1184 and 1175 cm–1 in 3 and 6, respectively). It is
interesting to note that in this case the presence of the sub-
stitution in the phenyl ring also causes an increase in the in-
tensities of the IR bands. Only the IR bands falling at 1113,
1113, and 1118 cm–1 (exactly the same values of the Raman
peaks) in the simulated spectrum of 2 and in those of 3 and
© 2001 NRC Canada

Aragoni et al.
1489
Fig. 3. Simulated FT-IR spectrum based on calculated scaled harmonic frequencies for model complex 2, experimental middle-IR
(1700–450 cm–1) and far-IR (450–50 cm–1) spectra for complexes 3 and 6. The bands corresponding to same normal modes are con-
nected by dotted lines. Vertical arrows are related to bands involving the methoxy group bound to the phenyl ring in complex 6.
2
3
6
1650
1450
1250
1050
850
650
450
250
50
ν(cm−1)
6, respectively, are strong, possibly because of the P—C
stretching contribution to the vibrational mode. The most in-
tense band in the whole simulated spectrum of 2 is found at
1003 cm–1 and is due to a pure (calculated reduced mass 7.1
amu) C—O stretching normal mode. This band in fact is
found as a very strong IR peak both in 3 (1014 cm–1) and in
6 (1003 cm–1). The side band found in FT-IR spectrum of 5
at 1030 cm–1 is absent in 3 and can, therefore, be tentatively
attributed to the C—O stretching vibration of the methoxy
substituent at the phenyl ring. In fact, while the peak around
1000 cm–1 is absent in amidophosphonodithioato complexes
in the region between 750 and 500 cm–1 (fingerprint region),
are mainly due to out of plane ring deforming of the phenyl
moieties and to the P—S and P—O stretching modes. In
particular, the band calculated at 631 cm–1 is attributed to
the antisymmetric stretching of the P—S bonds and appears
as an intense peak in all the vibrational spectra of all the
synthesized complexes (3: 627 (IR), 623 (Raman) cm–1; 5:
623 (IR), 616 (Raman) cm–1; 6: 622 (IR), 618
(Raman) cm–1; 7: 611 (IR), 611 (Raman) cm–1; 8: 612
(IR), 612 (Raman) cm–1; 9: 608 (IR), 600 (Raman) cm–1;
10: 599 (IR), 596 (Raman) cm–1; 11: 598 (IR), 599
(Raman) cm–1).
In the far-IR region (Table 4) the antisymmetric S-Ni-S
bending appears at 470 and 435 cm–1 for 3 and 6, respec-
tively. This band, calculated at 435 cm–1 for 2, depends
strongly on the groups bound to the phosphorus atoms, and
in fact varies by about 60 cm–1 on passing from
phosphonodithioato (435, 431, 429 cm–1 for 6, 7, and 8, re-
spectively) to amidophosphonodithioato complexes (496,
497, 495 cm–1 for 9, 10, and 11, respectively), while its posi-
tion is virtually independent of the metal, indicating similar
force constants for this bending mode. Therefore it is not
surprising that the change of the alkoxy group determines a
clear variation in the position of this band. Following the
same trend found for the antisymmetric stretching, the sym-
metric one falls in the Raman spectra at 470 cm–1 for 3,
440 cm–1 for 6, 432 cm–1 for both 7 and 8, and 463 cm–1 for
9, 10, and 11. The antisymmetric metal—sulphur stretching
is calculated at 351 cm–1 to be the most intense band in this
region and is found at roughly the same frequency in the FT-
IR spectra of both 3 and 6. This band falls at 356, 314, and
308 cm–1 for compounds 9, 10, and 11, respectively, and is
9–11, the nO-CH vibration is found at 1020, 1019, and
3
1019 cm–1 for 9, 10, and 11, respectively. In the Raman
spectra of complexes 3 and 6, the peak around 1000 cm–1 is
strong only for 3, while for 6 the band is weak.
In the FT-IR spectrum of 3 the strong peak found at
959 cm–1 (very weak in the Raman spectrum at 961 cm–1) is
absent in the calculated spectrum of 2. Therefore, this band
should be due to a vibrational mode of the ethoxy
substituent, since it is also absent in 6 but present in 5 at 941
and 943 cm–1 in the FT-IR and Raman spectra, respectively.
Conversely, in the FT-IR spectrum of 6, the band at 836 cm–1
(Raman 839 cm–1), absent in 3 and in the simulated spec-
trum of 2, appears in all the vibrational spectra of com-
pounds derived from 1 and, consequently, it might be due to
another mode of the methoxy substituent at the phenyl.
Moving to lower frequencies, a very intense IR band is
found at 783 and 789 cm–1 (786 and 801 cm–1 in the Raman
spectra) for compounds 3 and 6, respectively; this band can
be correlated to the vibration calculated for 2 at 749 cm–1,
which is due to a combination of P—O stretching and C—H
out of plane deforming of the phenyl ring. These vibrations,
© 2001 NRC Canada

1490
Can. J. Chem. Vol. 79, 2001
found at 346, 316, and 303 cm–1 for 5, 7, and 8, respec-
tively. The symmetric Ni—S stretching, calculated at
323 cm–1, is found in the FT-Raman spectra of all com-
plexes: 308 (3), 311 (5), 314 (6), 326 (7), 316 (8), 313 (9),
327 (10), and 316 cm–1 (11), with the bands being shifted
according to the ratio between the different metal–sulphur
bond strength constants and the respective reduced masses.
According to the previous assignments the symmetric
stretching frequency is found at higher frequencies com-
pared to the unsymmetric ones for nickel-complexes 3, 5, 6,
and 9, as well as in the calculated spectrum of 2. Surpris-
ingly, in the case of Pd and Pt complexes 7, 8, 10, and 11
this trend is inverted. Another band, calculated at 358 cm–1
as a symmetric combination of torsions around the P atoms,
can be found in the Raman spectra at 343 (6), 342 (7), 342
(8), 342 (9), 344 (10), and 345 cm–1 (11). This band be-
comes weaker in the case of 3 and 5 (340 and 344 cm–1, re-
spectively). Finally, in the lowest frequency region of the
far-IR and Raman spectra the bands are originated by de-
forming of the metallacycle PS2Ni ring and by alkoxy group
rotations.
Quantum-chemical calculations were performed using the
commercially available suite of programs Gaussian 98 pack-
age (19). Density Functional Calculations (DFT) (13) were
performed on 2 using the hybrid B3LYP functional (which
uses a mixture (20) of Hartree-Fock and DFT exchange
along with DFT correlation: the Lee–Yang–Parr correlation
functional (21) together with the Becke’s gradient correc-
tion) (22).3 The 6–31G(d) basis sets (23) were employed
throughout. Integration was performed numerically using a
total of 7500 points for each atom (Ultrafinegrid option).
Starting from geometrical parameters determined for 5 (8),
and avoiding introduction of any geometrical constraints but
the inversion centre, a geometry optimization was performed
and the results of the calculations were examined with the
Molden 3.6 program (24). The optimized structure was veri-
fied by normal-mode harmonic frequency analysis. Har-
monic frequencies were obtained using the second
derivatives of the DFT energy, computed by numerical dif-
ferentiation of the DFT energy gradients. Calculated har-
monic frequencies were extracted from the output file and
converted in a simulated spectrum with the Tabfreq program
after application of the scaling factors, using DFT-calculated
intensities and a halfbandwidth of 10 cm–1. According to a
referee suggestion, the computational results2, both the ge-
ometry optimization and the frequency calculations, have
been repeated on the simpler trans-bis[O-methyl-methylphos-
phonodithioato]NiII model complex, using BLYP, BPW91,
and B3LYP functionals and the same 6–31G(d) (23) basis set
for all atoms. The first two functionals are pure-DFT
functionals, combining the Becke’s (25) 1988 exchange
functional with the Lee et al.’s (including both local and
nonlocal terms) (26), and Perdew and Wang’s (27) 1991
gradient-corrected correlation functionals, respectively. The
qualitative MO analyses have been obtained by means of
Extended Hückel Theory (EHT) (28) using the CACAO
(Computer Aided Composition of Atomic Orbitals) program
package (29) on the hybrid-DFT optimized geometry, regu-
larized to obtain an idealized C2h symmetry. The interaction
diagram was generated using the fragment molecular orbital
(FMO) approach (30). Parameters used in EHT calculations
were taken from the standard database of CACAO.
Conclusions
The synthesis of trans-bis[O-ethyl-phenylphosphono-
dithioato]NiII (3) confirms the reactivity of substituted
[1,3,2,4]-dithiadiphosphetane-2,4-disulfides towards nucleo-
philic reagents, previously explored by us on commercially
available Lawesson’s Reagent (1). The complex has been
fully characterized by spectroscopic techniques. At any rate,
the most interesting results come from an examination of the
DFT-calculated vibrational frequencies, which allows us to
deeply investigate not only the FT-IR and FT-Raman spectra
of 3, but also those of other phosphono- (6–8) and amido-
phosphono-dithioato (9–11) complexes previously reported
by us.
Experimental
All the reagents and solvents were purchased from Aldrich
or Merck and were used without further purification. Ele-
mental analyses were performed with an EA1108 CHNS-O
Fisons instrument. Fourier transform infrared spectra (FT-
IR) were recorded on a Bruker IFS55 spectrometer at room
temperature using a flow of dried air. Far-IR (500–50 cm–1)
spectra (resolution 2 cm–1) were recorded as polythene pel-
lets with a Mylar beam-splitter and polythene windows.
Middle IR spectra (resolution 2 cm–1) were recorded as KBr
pellets, with a KBr beam-splitter and KBr windows. FT-
Raman spectra were recorded with a resolution of 2 cm–1
on a Bruker RFS100 FT-Raman spectrometer, fitted with an
In-Ga-As detector (room temperature) operating with a Nd-
YAG laser (excitation wavelength 1064 nm) with a 180°
scattering geometry. Solid state 31P CP MAS NMR spectra
were recorded on a Varian Unity Inova 400 MHz instrument
operating at 161.9 MHz with samples packed into a zirco-
nium oxide rotor. The 31P NMR chemical shifts were cali-
brated indirectly through the H3PO4 85% peak (d 0.0).
Synthesis of 2,4-diphenyl-[1,3,2,4]-dithiadiphosphetane-2,4-
disulfide (4)
Compound 4 was synthesized according to literature
methods (31), by reacting (PhP)3S3 with an excess of ele-
mental sulphur in carbon disulfide.
Synthesis of C16H20O2P2S4Ni (3)
An excess of NiCl2·6H2O was reacted with 4 (2.7 g,
8 mmol) in absolute ethyl alcohol solution (100 mL). The
filtered solid was recrystallized from CH2Cl2–EtOH (1:1
v/v) solution. Yield: 3.56 g, 7.2 mmol, 90%; mp 179°C. FT-
IR (3500–450 cm–1 KBr; 450–50 cm–1 polythene) (cm–1):
3050 (vvw), 3020 (vvw), 2975 (mw), 2930 (w), 2896 (w),
2857 (vvw), 1585 (w), 1571 (vw), 1481 (vw), 1468 (mw),
1449 (w), 1436 (ms), 1389 (m), 1307 (w), 1309 (w), 1282
(vw), 1182 (mw), 1157 (m), 1113 (vs), 1069 (w), 1014 (vs),
3 Gaussian 98 z-matrix input and output file, optimized cartesian coordinates of atoms in complex 2, list of unscaled calculated harmonic fre-
quencies, IR activity, reduced mass, force constants, and atom displacements for each calculated vibration are available from author on request.
© 2001 NRC Canada

Aragoni et al.
1491
7. (a) L. Malatesta and R. Pizzotti. Chim. e Ind. 27, 6 (1945);
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997 (ms), 959 (vs), 783 (s), 748 (s), 721 (s), 690 (ms), 627
(s), 614 (m), 565 (vs), 468 (m), 469 (m), 435 (vw), 416
(vw), 379 (w), 354 (vs), 302 (w), 279 (w), 254 (w), 227 (w),
204 (m), 179 (mw), 151 (w), 122 (mw), 99 (w), 73 (w), 55
(w). FT-Raman (solid state; in parethesis intensities scaled
on the highest peak taken equal to 10.0 are reported): 3053
(3.6), 2966 (0.9), 2930 (0.9), 2919 (0.8), 2989 (0.4), 1586
(2.7), 1440 (0.5), 1158 (0.9), 1113 (2.2), 1027 (0.9), 998
(5.4), 623 (1.7), 566 (6.0), 470 (0.9), 386 (0.8), 308 (4.5),
273 (2.5), 2.42 (4.0), 142 (7.8), 114 (10.0), 86 (9.1) cm–1.
31P CP MAS NMR d: 108.2, 106.9. Anal. calcd. for
C16H20O2P2S4Ni: C 39.0, H 4.1, S 26.0; found C 39.1, H 4.4,
S 25.6.
11. M.C. Aragoni, M. Arca, F. Demartin, F.A. Devillanova, A.
Garau, F. Isaia, F. Lelj, V. Lippolis, and G. Verani. J. Am.
Chem. Soc. 121, 7098 (1999).
Synthesis of complexes 5–11
The syntheses of compounds 5–11 were accomplished as
previously reported (8, 12).
12. M.C. Aragoni, M. Arca, F. Demartin, F.A. Devillanova, C.
Graiff, F. Isaia, V. Lippolis, A. Tirpicchio, and G. Verani. Eur.
J. Chem. 10, 2239 (2000).
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chemistry. Springer–Verlag, New York. 1991.
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15. C. Daul. Int. J. Quantum Chem. 52, 867 (1994).
16. J.P. Fackler, Jr. and L.D. Thompson, Jr. Inorg. Chim. Acta, 48,
45 (1981).
Acknowledgments
We thank the Consorzio interuniversitario per le Applicazioni
di Supercalcolo per Università e Ricerca (CASPUR) and Dr.
Nico Sanna for hosting the calculations on 2, the Consorzio
Interuniversitario del Nord Est Italiano per il Calcolo
Automatico (CINECA) for hosting calculations on trans-
bis[O-methyl-methylphosphonodithioato]NiII model complex,
and the Regione Autonoma della Sardegna and MURST
(Ministero dell’Università e della Ricerca Scientifica) for fi-
nancial support.
17. H. Hartung. Z. Chem. 6, 241 (1967).
18. G. Rauhut and P. Pulay. J. Phys. Chem. 99, 3093 (1995).
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© 2001 NRC Canada