The crystal structure, equilibrium and spectroscopic studies of bis(dialkyldithiocarbamate) copper(II) complexes [Cu2(R2dtc)4] (dtc=dithiocarbamate)

Article abstract of DOI:10.1016/S0277-5387(99)00242-9

Four copper(II) complexes of bis(dialkyldithiocarbamate) [Cd(R₂dtc)₂] (R=Me, Et, Pr, i-Pr; dtc=dithiocarbamate) have been prepared and characterized by elemental analysis, IR and ESR spectra studies. Their equilibrium constants (K), determined by a UV-vis spectrometry in EtOH, were influenced by the alkyl groups in the following order: i-Pr>n-Pr≈Et>Me. The single crystal structures of complex [Cu₂(R₂dtc)₄] have been determined using X-ray diffraction methods. The compounds [Cu₂(Et₂dtc)₄] and [Cu₂(Pr₂dtc)₄] are built of centrosymmetric neutral dimeric [Cu₂(R₂dtc)₄] entities. The copper atom lies in a distorted square-pyramidal environment. The four equatorial donors are two bidentate chelate sulfur atoms from two dtc ligands. One of the sulfur atoms from the third dtc ligand acts as a bridging ligand occupying the apical position of the symmetry-related copper atom in the dimer structure, which is viewed as two edge-sharing distorted square-pyramids. The structure of [Cu₂(i-Pr₂dtc)₄] is square planar with an exactly planar CuS₄ unit and nearly planar NCS₂ moieties. The Cu-S distances shows small decreases along the series n-Pr>Et>i-Pr, the biggest change being for the diisopropyl complex. The alkyl substituents at the nitrogen atom affect their coordination number and CuCu distance. In the solid, [Cu₂(n-Pr₂dtc)₄] has the shortest CuCu distance and [Cu(i-Pr₂dtc)₂] has the longest one. Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00242-9

Polyhedron 18 (1999) 3401–3406
www.elsevier.nl/locate/poly
The crystal structure, equilibrium and spectroscopic studies of
bis(dialkyldithiocarbamate) copper(II) complexes [Cu2(R2dtc)4]
(dtc5dithiocarbamate)
a
a
a
b
Fangfang Jiana , , Zuoxiang Wang , Zhiping Bai , Xiaozeng You , Hoong-Kun Fun ,
*
Kandasamy Chinnakalib, Ibrahim Abdul Razakb
aCoordination Chemistry Institute and the State Key Laboratory of Coordination Chemistry, Nanjing University, Center for Advanced Studies in
Science and Technology of Microstructure, Nanjing 210093, PR China
bX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
Received 15 December 1998; accepted 20 August 1999
Abstract
Four copper(II) complexes of bis(dialkyldithiocarbamate) [Cd(R2dtc)2] (R5Me, Et, Pr, i-Pr; dtc5dithiocarbamate) have been prepared
and characterized by elemental analysis, IR and ESR spectra studies. Their equilibrium constants (K), determined by a UV–vis
spectrometry in EtOH, were influenced by the alkyl groups in the following order: i-Pr.n-Pr¯Et.Me. The single crystal structures of
complex [Cu2(R2dtc)4] have been determined using X-ray diffraction methods. The compounds [Cu2(Et2dtc)4] and [Cu2(Pr2dtc)4] are
built of centrosymmetric neutral dimeric [Cu2(R2dtc)4] entities. The copper atom lies in a distorted square–pyramidal environment. The
four equatorial donors are two bidentate chelate sulfur atoms from two dtc ligands. One of the sulfur atoms from the third dtc ligand acts
as a bridging ligand occupying the apical position of the symmetry-related copper atom in the dimer structure, which is viewed as two
edge-sharing distorted square–pyramids. The structure of [Cu2(i-Pr2dtc)4] is square planar with an exactly planar CuS4 unit and nearly
planar NCS2 moieties. The Cu–S distances shows small decreases along the series n-Pr.Et.i-Pr, the biggest change being for the
diisopropyl complex. The alkyl substituents at the nitrogen atom affect their coordination number and Cu???Cu distance. In the solid,
[Cu2(n-Pr2dtc)4] has the shortest Cu???Cu distance and [Cu(i-Pr2dtc)2] has the longest one.
1999 Elsevier Science Ltd. All rights
reserved.
Keywords: Crystal structure; Dialkyldithiocarbamate; Equilibrium constant
1. Introduction
[8,9]. The ability of dtc to bind to metal has been known
for many years. It forms a chelate with virtually all
transition elements [10]. The bidentate anion is also well
known as a bridge between two transition metal centers
[11]. Water-soluble dialkyldithiocarbamate complexes are
known to have been tested in various medical applications
[12]. Dialkyldithiocarbamate sodium salts are also good
extracting agents for many transition metals [13], and are
good floating agents of flotation concentrate for novel
metal ores and copper ores [14].
In recent years the study of binuclear copper(II) com-
plexes has been a subject of considerable interest because
of their structural, magnetic, EPR and electrochemical
properties [15]. In this paper, we report infrared (IR) and
EPR spectra of four copper(II) complexes of bis-
(dialkyldithiocarbamate), and a study of their equilibrium
Dialkyl-substituted dithiocarbamate anions have proved
to be highly versatile chelating agents for the separation of
metals as metal chelates using gas chromatography [1].
Their good performance in liquid–liquid extraction and
other analytical procedures [2] makes them more attractive
for the determination of metals using gas chromatography
[3]. Some of dialkyl-substituted dithiocarbamate salts have
also shown interesting biological effects which include
anti-alkylation [4,5] or anti-HIV properties [6,7]. They are
also used as effective antidotes for cadmium intoxication
*Corresponding author. Tel.: 186-25-431-7944; fax: 186-25-443-
2747.
E-mail address: jianff@public1.ptt.js.cn (F. Jian)
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00242-9
1999 Elsevier Science Ltd. All rights reserved.

3402
F. Jian et al. / Polyhedron 18 (1999) 3401–3406
Table 1
constants. Also the crystal structure of copper(II) bis-
(dialkyldithiocarbamate) complex has been determined.
Elemental analyses of the copper(II) complexes
Complex
% C
% H
% N
Cu(Me2dtc)2
Calc.
Cu(Et2dtc)2
Calc.
Cu(n-Pr2dtc)2
Calc.
Cu(i-Pr2dtc)2
Calc.
23.68
(23.71)
33.43
(33.36)
40.34
(40.41)
40.45
(40.41)
4.01
(3.98)
5.24
(5.60)
7.10
(6.78)
6.90
(6.78)
9.31
(9.22)
7.73
(7.78)
7.23
(6.73)
7.05
(6.73)
2. Experimental
2.1. Physical measurements
Elemental analyses were obtained at the Analysis and
Measurement Centre of Nanjing University. The IR spectra
were recorded in the 4000–400 cm21 region using KBr
pellets on a Nicolet 170 SX FTIR spectrometer. Magnetic
measurements on powder samples were performed in the
75–300 K temperature range by means of a Faraday-type
magnetometer. The powder EPR spectra were recorded on
a Bruker 2000-SRC spectrometer.
SADABS [17] program. The structure was solved by direct
methods and refined using the full-matrix least-squares
method on F2obs by using the SHELXTL [18] software
package. All non-H atoms were anisotropically refined.
The hydrogen atoms were located by difference synthesis
and refined isotropically. The molecular graphics were
created by using SHELXTL. Atomic scattering factors and
anomalous dispersion corrections were taken from the
International Table for X-Ray Crystallography [19].
2.2. Synthesis
All chemicals were obtained from a commercial source
and used without further purification.
2.3. Preparation of the N,N9-dialkyldithiocarbamate
sodium [Na(R2dtc)]
2.6. Equilibrium studies
UV–vis solution spectra were recorded on a Shimadzu
UV-240 spectrophotometer at room temperature. Addition
of colorless dialkyldithiocarbamate sodium EtOH solution
to the EtOH solution of [CuCl2] promptly forms brown
dialkyldithiocarbamate copper(II) complexes, which show
the intensity characteristic absorption at a wavelength of
ca. 435 nm. The stoichiometry and stability constants were
determined using the method described by Job [20], and
calculated using the method of Meng and Jiang [21].
To a stirred solution of dialkyl-secondary amine (0.05
mol, Me2NH is 40% aqueous solution) in ethanol (5 ml)
was added, at less than 48C, carbon disulfide (3.1 ml, 0.052
mol) and sodium hydroxide (50% aqueous solution, 4 ml).
After stirring for 4–5 h, evaporation of the volatile was
performed without heating. The pure [Na(R2dtc)] was
obtained by recrystallisation from the solution of ethanol.
Yield: 80–90%.
2.4. Preparation of copper(II) complexes [Cu(R2dtc)2 ]
3. Results and discussion
To a heated aqueous solution of Na(R2dtc) (0.01 mol)
was added an EtOH solution of Cu perchlorate
[Cu(ClO4)2, 0.054 mol] with stirring. A brown precipitate
was deposited. Upon collection by filtration, the deposit
was washed with water and dried over P4O10. It was
collected and submitted for elemental analysis. The ana-
lytical data on the copper(II) complexes are presented in
Table 1.
3.1. Description of the structure
The structure of complex [Cu2(Et2dtc)4] and [Cu2(n-
Pr2dtc)4] is built up of centro-symmetric dimeric entities.
Fig. 1 shows a perspective view of the dimeric unit with
the atomic numbering scheme of [Cu2(n-Pr2dtc)4], and
Fig. 2 a perspective view of the crystal packing in the unit
cell.
2.5. X-ray structure determination
The coordination sphere of the copper(II) ions is best
described as a distorted square–pyramid. The basal coordi-
nation positions are occupied by four sulfur atoms belong-
ing to two dtc ligands. The five coordination is achieved by
linking to a sulfur atom from the third ligand occupying
the equatorial site in the coordination polyhedron of the
centrosymmetrically related copper(II) ion. Therefore, each
bridging sulfur simultaneous occupies an equatorial coordi-
nation site on one copper(II) ion and an apical site on the
other Cu(II). The topology of the informed dimers is then
viewed as edge-sharing distorted square–pyramids. As
A summary of the key crystallographic information is
given in Table 2. Selected bond distances and angles are
listed in Table 3. The selected crystal of [Cu2(R2dtc)4] was
mounted on a SMART CCD diffractometer. Reflection
data were measured at 208C using graphite monochro-
˚
mated MoKa (l50.71073 A) radiation with a detector
distance of 4 cm and swing angle of 2358. The collected
data were reduced by using the program SAINT [16] and
empirical absorption correction was carried out using the

F. Jian et al. / Polyhedron 18 (1999) 3401–3406
3403
Table 2
Summary of crystallographic result for [Cu(R2dtc)2]
Complex
[Cu2(n-Pr2dtc)4]
C28H56Cu2N4S8
832.32
0.4230.1030.08
P2(1)/c
[Cu2(Et2dtc)4]
[Cu(i-Pr2dtc)2]
Empirical formula
Formula weight
Crystal size (mm)
Space group
C20H40Cu2N4S8
720.12
0.2430.2030.20
P21 /c
C14H28CuN2S4
416.17
2030.3030.35
]
P1
Unit cell dimensions
˚
a (A)
8.3023(2)
8.5756(4)
13.2515(2)
90
100.035(1)
90
2012.38(7)
1.374
1.496
9.9043(8)
10.6127(8)
15.496(1)
90
102.002(7)
90
1593.2(2)
1.501
1.877
11.582(2)
11.720(2)
7.636(3)
96.28(2)
96.29(2)
88.73(2)
1024.1(5)
1.350
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
3
˚
Vol. (A )
Dcalc (Mg m23
)
Abs. coeff. (mm21
F(000)
)
876
748
438
˚
˚
˚
Radiation
Temp. (K)
MoKa (l50.71073 A)
293(2)
MoKa (l50.71073 A)
293(2)
MoKa (l50.71073 A)
293(2)
u range (8)
Limiting indices
2.91–27.50
210#h#10, 0#k#24,
0#l#17
2.25–26.46
0#h#11, 0#k#12
218#l#18
3129
29.67–30.02
210#h#10, 0#k#24
0#l#17
3528
No. unique reflections
4602
R
0.0548
0.0538
0.031
Rw
0.0391
0.0953
0.046
Goodness-of-fit on F2
Largest diff. peak and hole
1.059
0.935
1.26
0.621 and 20.437 e A23
0.517 and 20.574 e A23
0.19 and 20.33 e A23
Table 3
˚
expected, the axial Cu–S bond distance h2.740(1) A in
˚
Selected bond distances (A) and bond angles (8) of [Cu(R2dtc)2]
˚
[Cu2(n-Pr2dtc)4], and 2.844(1) A in [Cu2(Et2dtc)4]j is
Atoms
[Cu2(n-Pr2dtc)4]
[Cu2(Et2dtc)4]
[Cu(i-Pr2dtc)2]
longer than the equatorial Cu–S ones. The four basal sulfur
atoms are coplanar but the Cu atom deviates from that
plane showing a slight tetrahedral distortion. The bond
Cu–S(1)
Cu–S(3)
Cu–S(4)[1
Cu–S(2)
Cu–S(4)
S(1)–C(1)
S(3)–C(8)
S(1)–Cu–S(2)
S(3)–Cu–S(4)
2.3161(11)
2.3184(12)
2.7401(12)
2.3166(11)
2.3221(11)
1.718(4)
1.707(4)
76.68(4)
76.25(4)
2.3071(9)
2.299(1)
2.844(1)
2.314(1)
2.3270(9)
1.718(3)
1.710(3)
77.00(3)
76.59(3)
102.20(3)
2.2912(7)
2.2912(7)
˚
length of Cu–S(4), 2.322(1) A in [Cu2(n-Pr2dtc)4] and
2.3270(9) A in [Cu2(Et2dtc)4], is the longest in Cu–S
2.2856(6)
2.2856(6)
1.726(2)
1.715(2)
77.27(2)
˚
equatorial bond lengths because S(4) is a bridging atom.
The bond angles of S(1)–Cu–S(2) and S(3)–Cu–S(4) are
nearly the same h76.68(4)8, 76.25(4)8 in [Cu2(n-Pr2dtc)4],
and 77.00(3)8, 76.59(3)8 in [Cu2(Et2dtc)4]j.
The bridging network Cu–S(4)–Cu[1–S(4)[1 is
strictly planar owing to the inversion centre, with a Cu???
76.73(2)
102.73(2)
S(1)–Cu–S(4) 161.80(5)
˚
Cu separation of 3.4179(6) A in [Cu2(n-Pr2dtc)4], and
3.572(1) A in [Cu2(Et2dtc)4]. The Cu–S(4)–Cu[1–
˚
S(4)[1 and S(1)–S(2)–S(3)–S(4) planes are nearly per-
pendicular with a dihedral angle of 88.91(3)8 in [Cu2(n-
Pr2dtc)4]. Our present results are in general accord with
this earlier determination, although at a higher level of
precision [22,23].
The structure of complex [Cu(i-Pr2dtc)2] is built up of
monomeric molecules. The two molecules, A and B, are
crystallographically independent. Each Cu atom occupies a
center of inversion, and, accordingly, the Cu and four S
atoms are coplanar. The mean molecular planes of A and B
are nearly perpendicular to each other, the dihedral angle
being 89.3(2)8. The average of the four independent
˚
copper–sulfur distances is 2.2886(7) A. The shortest
Fig. 1. Molecular structure for Cu2(Pr2dtc)4 with the atomic numbering
scheme.
˚
copper–copper intermolecular distance is 7.629(1) A. This

3404
F. Jian et al. / Polyhedron 18 (1999) 3401–3406
Fig. 2. A view of the crystal packing down the a axis for Cu2(Pr2dtc)4.
redetermination confirms the main features of the earlier
study [24,25].
tive of dithiocarbamate acting as a bidentate ligand [29].
As can be seen from Table 5, the effect of the substituents
bonded at the nitrogen atom on the C–N and C–S bonds
can be investigated. The lengthening of the alkyl chain is
To compare with the other copper(II) bis-
(dialkyldithiocarbamate) complexes [22–26], we found
that [Cu2(n-Pr2dtc)4] has the shortest Cu???Cu distance and
[Cu(i-Pr2dtc)2] has the longest one (Table 4). Also, the
alkyl substituents at the nitrogen atom affect their coordi-
nation number and Cu–S distance (Table 4).
accompanied by
a decrease of the v(C–N) band
wavenumber; the lowest value is found for Cu(i-Pr2dtc)2,
further lengthening of the chain does not lead to appreci-
able changes. The C–S bond lengths in derivatives involv-
ing different alkyl substituents are nearly equal; it can,
therefore, be assumed that the electronic effects practically
do not influence the v(C–S) vibrations. It is thus reason-
able to attribute the lowering of the v(C–S) frequencies
with the lengthening alkyl chain to the growing mass of
the alkyl group, the irregularities in the wavenumbers
being associated with their spatial arrangement. Neverthe-
less, it is necessary to note that the spectra of the Cu
3.2. IR spectrum
Table 5 shows the most significant IR bands of the
prepared compounds. The band in the range 1480–1510
cm21 is attributed to the v(C–N) stretching vibration [27],
the band in the range 940–990 cm21, belonging to the
v(C–S) stretching vibration [27,28], is considered indica-
Table 4
Effect of different alkyl substituents
a
a
Compound
Cu(Me2dtc)2
Cu2(Et2dtc)4
Cu2(n-Pr2dtc)4
Cu(i-Pr2dtc)2
˚
Cu???Cu distance (A)
4.043
6
2.3105
3.572
5
2.3117
3.418
5
2.3183
7.629
4
2.2886
Coordination number
˚
Cu–S distance (A)
a Means data from crystal structure redetermined.
Table 5
Most significant IR bands (cm21) of Cu(R2dtc)2 complexes
Compound
Cu(Me2dtc)2
Cu2(Et2dtc)4
Cu2(n-Pr2dtc)4
Cu(i-Pr2dtc)2
v(C–N)
v(C–S)
1511s
975s
1504s
913s, 997s
1498s
985s
1491s
940s

F. Jian et al. / Polyhedron 18 (1999) 3401–3406
3405
complexes, [Cu2(n-Pr2dtc)4] and [Cu(i-Pr2dtc)2], for which
the structure have been determined here using X-ray
diffraction and which have a bidentate chelate and a
bidentate bridged ligand, do not show any complexity in
the zones of v(C–N) or v(C–S). For this reason it does not
appear easy to decide from such IR data whether chelate or
bridging dithiocarbamates are present.
between [Cu(Et2dtc)2] and [Cu(n-Pr2dtc)2]. In addition,
the half-field signal is not observed for (b) and (c) at room
temperature, which suggests no magnetic interaction be-
tween the two Cu ions through the sulfur bridge. The
values of gi $ g' . 2.0 of (a) and (b) suggested that the
ground state was dx
and the coordination geometry was
2
2
–y
octahedral or square–pyramidal [30,31]. The EPR spec-
trum of [Cu(i-Pr2dtc)2] exhibited the expected four-line
patterns, narrow linewidths characteristic of CuS4 com-
pounds [32]. The observation of gi and g' indicates
tetragonal distortion and supports the square–planar struc-
ture [15,23,33]. These results were consistent with the
crystal structures.
The temperature dependence of the magnetic suscep-
tibility per two copper atoms for the [Cu(n-Pr2dtc)2]
compound is shown in Fig. 4. The smooth decrease of xm
with T in the range 300–75 K indicates that an anti-
ferromagnetic interaction operates between the two copper
atoms. With application of the Hamiltonian H 5 2 JS1S2,
the J values can be calculated from the modified Bleaney–
Bower equation:
3.3. EPR spectra and magnetic properties
The polycrystalline powder room-temperature EPR
spectra for [Cu(R2dtc)2] complexes are shown in Fig. 3. It
may be seen from Fig. 3 that the shape and the parameters
of the EPR spectra of (b) and (c) are very similar, which
shows that there are the same coordination environments
xm 5 2Nb2g2 /KT[3 1 exp(2J/KT)] 1 2Na
where J is the single–triplet (S–T) energy gap. The other
symbols have their usual meaning. Assuming a constant
value for Na of 6031026, the parameters J and g were
determined by minimizing R described as o(xmobs
2
xcmalc)2 /o(xobs)2, with J544.93 cm21, g52.032 and R5
1.8831024.
.
3.4. Equilibrium studies
The equilibrium studies are taken in the EtOH solution
by adding the ligands to the copper(II) compound. The
intensity measurements were taken at the wavelength 435
nm, using the Meng–Jiang two-points formula [21]. The
Fig. 4. Experimental (m) and calculated (——) temperature dependence
of xm for [Cu(n-Pr2dtc)2].
Fig. 3. Room-temperature EPR spectra of the [Cu(R2dtc)2] complexes.

3406
F. Jian et al. / Polyhedron 18 (1999) 3401–3406
Table 6
Stability constants of [Cu(R2dtc)2] complexes in EtOH solution
Compound
Cu(Me2dtc)2
Cu2(Et2dtc)4
Cu2(n-Pr2dtc)4
6.293107
Cu(i-Pr2dtc)2
1.403108
Keq
3.033107
6.853107
[7] E.M. Hersh, G. Brewton, D. Abrams, J. Bartlett, P. Gill, R. Gorter,
M. Gottlieb, J.J. Jonikas, S. Landesman, A. Levine, A. Marcel, E.A.
Petersen, M. Whiteside, J. Zahradnik, C. Negron, F. Boutitie, J.
Caraux, J.M. Dupuy, R. Saimi, J. Am. Med. Assoc. 265 (1991)
1538.
equilibrium constants obtained are listed in Table 6. As can
be seen in Table 6, the order of the stability of the
difference alkyl groups in [Cu(R2dtc)2] decreased as
follows:
[8] G.R. Gale, L.M. Atkins, E.M. Walker Jr., A.B. Smith, M.M. Jones,
Ann. Clin. Lab. Sci. 14 (1984) 137.
i-Pr . n-Pr ¯ Et . Me
This order due to the variety of alkyl groups may be partly
attributed to changes in the residual positive charge and
also partly to the steric hindrance of branched alkyl groups,
but the electronic effects are rather more important than
[9] L.A. Shinobu, S.G. Jones, M.M. Jones, Acta Pharmacol. Toxicol. 54
(1984) 189.
[10] F.H. Allen, O. Kennard, Chem. Des. Autom. News 8 (1993) 31.
[11] M. Bardaji, N.G. Connelly, M.C. Gimeno, J. Jimenez, P.G. Jones, A.
Laguna, M. Laguna, J. Chem. Soc., Dalton Trans. (1994) 1163.
[12] J. Xie, T. Funakoshi, H. Shimada, S. Kojima, Res. Commun. Mol.
Pathol. Pharmacol. 86 (1994) 245.
[13] Jiri Stary, in: The Solvent Extraction of Metal Chelate, Pergamon,
1964, p. 180.
[14] B.X. Jian, in: The Floating Agent, Metallurgy Industries Press,
Beijing, 1981, p. 215, In Chinese.
[15] C.A. Sureshan, P.K. Bhattacharya, Polyhedron 16 (1997) 489.
[16] G.M. Sheldrick, SAINT v4 Software Reference Manual, Siemens
Analytical X-ray Systems, Madison, WI, 1996.
[17] G.M. Sheldrick, SADABS, Program for Empirical Absorption
Correction of Area Detector Data, University of Gottingen, Ger-
many, 1996.
steric effects in this case. Also we found, when the [Cu21
]
is surplus, there is a breakpoint in the absorption curve
which suggests the formation of M:L51:1 instability
coordination complexes.
To compare with the solid state, the [Cu(R2dtc)2]
complexes in the solution have a similar change for a
variety of alkyl groups. [Cu(i-Pr2dtc)2] has the shortest
Cu–S distance in the solid and the highest stability in the
solution.
[18] G.M. Sheldrick, SHELXTL, v5 Reference Manual, Siemens Ana-
lytical X-ray Systems, Madison, WI, 1996.
Acknowledgements
[19] A.J. Wilson, International Table for X-ray Crystallography, Vol. C,
Kluwer Academic Publishers, Dordrecht, 1992, Tables 6.1.1.4 (p.
500) and 4.2.6.8 (p. 219), respectively.
[20] P. Job, Ann. Chim. Phys. 9 (1928) 113.
[21] F.S. Meng, M. Jiang, in: Ionic Equilibrium in Analytical Chemistry,
Science Press, Beijing, 1997, p. 170, In Chinese.
This work was supported by the NSFC project
29501001 and the NSF of Jiangsu Province Project BK
95024401. The authors would like to thank the Malaysian
Government and the Universiti Sains Malaysia for research
grant R&D No. 190/9609/2801.
[22] M. Bonamico, G. Dessy, A. Mugnoli, A. Vaciago, L. Zambonelli,
Acta Crystallogr. 19 (1965) 886.
[23] G. Peyronel, A. Pignedoli, L. Antolini, Acta Crystallogr., Sect. B 28
(1972) 3596.
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