Synthesis and characterization of monomeric palladium(II) pyridine-2-chalcogenolate complexes

Article abstract of DOI:10.1016/S0277-5387(98)00426-4

Mononuclear palladium(II) complexes of the type [Pd(Epy)(S^∩S)(PPh₃)] [E=S or Se; S^∩S=S₂CNEt₂, S₂P(OR)₂ (R=Et, Prⁿ, Prⁱ)] have been prepared. All the complex

Full text of DOI:10.1016/S0277-5387(98)00426-4

Polyhedron 18 (1999) 1253–1258
Synthesis and characterization of monomeric palladium(II)
pyridine-2-chalcogenolate complexes
Sanjay Narayana, Vimal K. Jaina, , K. Panneerselvam , Tian Huey Lu , Shu-Fang Tung
b
b
c
*
aChemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India
bDepartment of Physics, National Tsing Hua University, Hsinchu, Taiwan-300
cSouthern Instrument Center, National Cheng Kung University, Tainan, Taiwan-701
Received 14 August 1998; accepted 24 November 1998
Abstract
Mononuclear palladium(II) complexes of the type [Pd(Epy)(S>S)(PPh3)] [E5S or Se; S>S5S2CNEt2, S2P(OR)2 (R5Et, Prn, Pri )]
have been prepared. All the complexes have been characterized by elemental analysis and NMR (1H, 31Ph1Hj, 77Seh1Hj) spectral data. The
NMR data indicate that there are two species in solution, i.e. one with chelating S>S ligand predominates (|95%) while the other with
chelating Epy and monodentate S>S existing in |5% concentration. The X-ray crystal structure of [Pd(Spy)hS2P(OPri )2j(PPh3)] has been
determined. The square planar palladium atom is coordinated to asymmetrically chelated (PriO)2PS22 ligand, PPh3 and pyS2 groups.
1999 Elsevier Science Ltd. All rights reserved.
Keywords: Palladium; Pyridine-2-thiolate; Pyridine-2-selenolate; Dialkyldithiophosphates; NMR; X-ray
1. Introduction
[S>S5S2CNEt2, S2P(OR)2 (R5Et, Prn, Pri )] [16] and
py2Se2 [17] were prepared according to literature methods.
Triphenylphosphine (Strem Chemicals Inc) and pySH
(Aldrich) were obtained from commercial sources. All
reactions were carried out in anhydrous solvents under a
nitrogen atmosphere. 1H, 31Ph1Hj and 77Seh1Hj NMR were
recorded on a Bruker DPX-300 NMR spectrometer in
CDCl3 solvent. Spectra were referenced with internal
The chemistry of transition metal chalcogenolates (RE2)
has received considerable attention in recent years. The
high propensity of organochalcogenide ligands (RE2, E5
S, Se, Te) to bridge two metal atoms normally makes them
unsuitable for isolation of monomeric complexes [1,2].
The latter have been attractive owing to their utility as
mononuclear precursors for semiconductor materials [3–
5], and in many catalytic organic transformations [6,7] as
well as in desulfurization of organosulfur compounds [8–
10]. Several strategies have been adopted to stabilize the
monomeric framework. These strategies include use of
chelating phosphine ligands [11] and 2S(CH2)nS2 [12].
Recently we have isolated a number of palladium(II) and
platinum(II) organochalcogenolates employing chelating
phosphine ligands [13,14]. Herein we report the synthesis
of some monomeric palladium(II) complexes stabilized
with internally functionalized chalcogenolates, i.e. pyS/
pySe.
1
chloroform peak (d 7.26 ppm for H), external H3PO4 for
31Ph1Hj and Me2Se [secondary reference Ph2Se2 in C6D6
(d1463 ppm)]. Microanalyses were performed by the
Analytical Chemistry Division of this research centre.
2.1. Preparation of [Pd(Spy)hS2CNEt2 j(PPh3 )]
To a methanolic solution of NaSpy [prepared from
pySH (51 mg, 0.46 mmol) in methanol (5 cm3) and
sodium hydroxide solution (0.9 cm3, 0.55 mmol, 0.61 M)]
was added a dichloromethane solution (5 cm3) of
[PdClhS2CNEt2j(PPh3)] (255 mg, 0.46 mmol) with vigor-
ous stirring, which was continued for 5 h. The solvents
were evaporated in vacuo and the residue was extracted
with dichloromethane (20 cm3) and filtered. The filtrate
was concentrated to 3 cm3 and hexane (5 cm3) was added,
2. Experimental
[Pd2Cl2(m-Cl)2(PPh3)2]
[15],
[PdCl(S>S)(PPh3)]
*
Corresponding author. Fax: 191- 022-556-0750.
0277-5387/99/$ – see front matter
PII: S0277-5387(98)00426-4
1999 Elsevier Science Ltd. All rights reserved.

1254
S. Narayan et al. / Polyhedron 18 (1999) 1253–1258
which on slow evaporation gave red crystals (204 mg,
71%).
Spy containing complexes were prepared and pertinent
data are given in Table 1.
2.3. Preparation of [Pd(Sepy)hS2CNEt2 j(PPh3 )]
2.2. Preparation of [Pd(Spy)hS2P(OPri)2 j(PPh3 )]
To a methanolic solution of py2Se2 (42 mg, 0.13
mmol), a dilute methanolic solution of NaBH4 (10 mg,
0.26 mmol) was added under a nitrogen atmosphere. After
To
a
dichloromethane solution (15 cm3) of
[PdClhS2P(OPri )2j(PPh3)] (193 mg, 0.31 mmol) was
added solid NaSpy (41 mg, 0.31 mmol) with vigorous
stirring and the whole mixture was stirred for 3 h. This was
filtered and the filtrate was concentrated in vacuo. The
residue was recrystallized from CH2Cl2 –hexane mixture to
give a red crystalline solid (172 mg, 80%). Similarly other
5
min
a
dichloromethane solution (5 cm3) of
[PdClhS2CNEt2j(PPh3)] (145 mg, 0.26 mmol) was added
and the whole mixture was stirred for 5 h. The solvents
were evaporated in vacuo and the residue was extracted
with dichloromethane and filtered. The filtrate was concen-
Table 1
Physical, analytical and NMR spectroscopic data for palladium complexes containing Spy or Sepy ligand
Complex
Recrystallization
solvent (% yield)
m.p.
% Analysis found (calcd.)
NMR spectroscopic data in CDCl3
8C
C
H
N
31Ph1Hj NMR
1H NMR
[Pd(Spy)(S2CNEt2)(PPh3)]
CH2Cl2–hexane
(71)
184–185
89–90
54.0
4.8
5.0
30.2 (s, PPh3)
33.2c (s, PPh3)
1.16 (t), 1.19 (t) (7.3 Hz each, NCH2Me); 1.27 (t,
7.2 Hz, NCH2Me)c; 3.65 (m, NCH2); 6.70 (t)c,
6.75 (t, 5.5 Hz, 1 H, py); 7.15–7.49 (m), 7.65–7.71
(m) (Ph12 H, py); 8.26 (d, 3.7 Hz 1 H, py).
1.33 (t, 7.0 Hz, OCH2Me); 4.15 (m, OCH2); 6.70
(br)c; 6.72 (t, 5.8 Hz, 1H, py); 7.13–7.39 (m),
7.52–7.72 (m) (Ph11 H, py); 8.19 (br, 1 H, py).
(53.6)
(4.7)
(4.5)
[Pd(Spy)hS2P(OEt)2j(PPh3)]
[Pd(Spy)hS2P(OPrn)2j(PPh3)]
CH2Cl2–hexane
(67)
48.8
48.8
4.1
2.5
32.6 (s, PPh3)
100.7 (s, dithio)
33.0c (s, PPh3)
104.1c (s, dithio)
32.6 (s, PPh3)
100.8 (s, dithio)
33.2c (s, PPh3)
104.5c (s, dithio)
(4.4)
(2.1)
acetone–diethyl
ether (85)
114–115
49.7
4.6
1.4
0.86 (t, 7.4 Hz, OCH2CH2Me); 0.92 (t, 7.4 Hz,
OCH2CH2Me)c, 1.62 (m, OCH2CH2); 3.95 (m,
OCH2); 4.15 (br, OCH2)c; 6.60 (br, py)c; 6.65 (t,
5.7 Hz, 1 H, py); 7.08 (d, 7.6 Hz, 1 H, py); 7.29–
7.37 (m), 7.57–7.63 (m) (Ph11 H, py); 8.20 (br,
1 H, py).
(50.3)
(4.8)
(2.0)
[Pd(Spy)hS2P(OPri )2j(PPh3)]
CH2Cl2–hexane
(80)
125–127
185–186
50.8
4.8
2.2
32.6 (s, PPh3)
96.7 (s, dithio)
33.2c (s, PPh3)
100.6c (s, dithio)
1.25 (d, 6.2 Hz, OCHMe2); 1.31 (d, 5.8 Hz,
OCHMe2)c; 4.73 (m, OCH); 6.60 (br, py)c; 6.65
(t, 6.0 Hz, 1 H, py); 7.08 (t, 7.0 Hz, 1 H, py); 7.28–
7.35 (m), 7.58–7.64 (m) (Ph11 H, py); 7.75 (br,
py)c; 8.10 (d, 4 Hz, 1 H, py).
(50.3)
(4.8)
(2.0)
[Pd(Sepy)(S2CNEt2)(PPh3)]a
CH2Cl2–hexane
(60)
48.4
4.1
4.5
30.5 (s, PPh3)
33.5c (s, PPh3)
1.14 (t), 1.19 (t) (each 7.0 Hz, OCH2Me); 1.26 (t,
7.2 Hz, OCH2Me)c; 3.55–3.68 (m, OCH2); 3.72 (q,
OCH2)c; 6.80 (t, py)c; 6.86 (t, 5.0 Hz 1 H, py);
7.18–7.41 (m), 7.64–7.71 (m) (Ph12 H, py); 8.32
(d, 3.1 Hz, 1 H, py).
(49.9)
(4.3)
(4.2)
[Pd(Sepy)hS2P(OEt)2j(PPh3)]b
[Pd(Sepy)hS2P(OPrn)2j(PPh3)]
ether–hexane
(84)
99–101
45.9
3.9
2.4
32.0 (s, PPh3)
101.1 (s, dithio)
33.6c (s, PPh3)
104.2c (s, dithio)
32.1 (s, PPh3)
101.5 (s, dithio)
33.6c (s, PPh3)
104.7c (s, dithio)
1.30 (t, 7.0 Hz, OCH2Me); 1.39 (t, 7.0 Hz,
OCH2Me)c; 4.05–4.15 (m, OCH2); 4.20 (m,
OCH2)c; 6.90 (br, 1 H, py); 7.19–7.72 (m, Ph11 H,
py); 8.29 (br, 1 H, py).
(45.6)
(4.1)
(2.0)
ether–hexane
(52)
110–115
46.8
4.6
1.8
0.85 (t, 7.4 Hz, OCH2CH2Me); 0.90 (t, 7.5 Hz,
OCH2CH2Me)c; 1.64 (m, OCH2CH2); 3.90 (td, 7
Hz (t); 9 Hz (d), OCH2); 4.10 (br, OCH2); 6.65
(m)c; 6.80 (m, 1 H, py); 7.12 (m, 1 H, py); 7.29–7.40
(m), 7.51–7.65 (m) (Ph11 H, py); 8.00 (m)c, 8.22
(m, 1 H, py).
(47.1)
(4.5)
(1.9)
[Pd(Sepy)hS2P(OPri )2j(PPh3)]
ether–hexane
(78)
115–118
46.7
4.7
1.7
32.1 (s, PPh3)
1.21 (d, 6.2 Hz, OCHMe2); 4.67 (m, OCH); 6.80
(m, 1 H, py); 7.12 (m, 1 H, py); 7.28–7.36 (m), 7.51–
7.62 (m) (Ph11 H, py); 8.22 (m, 1 H, py).
(47.1)
(4.5)
(1.9)
97.5 (s, dithio)
a
77Seh1Hj NMR in CDCl3: 286 ppm relative to Me2Se.
77Seh1Hj NMR in CDCl3: 316 ppm relative to Me2Se.
b
c Due to chelated Epy complex containing monodentate S>S ligand (|5% by integration of H NMR spectra). In H NMR spectra all the resonances due to
1
1
the species (c) were not resolved because of the overlapping with the signals of the major species.

S. Narayan et al. / Polyhedron 18 (1999) 1253–1258
1255
for
trated to 3 cm3 and hexane 5 cm3 was added, which on
slow evaporation gave a red crystalline solid (105 mg,
60%).
Table 2
Crystal
data
and
structure
refinement
details
[Pd(Spy)hS2P(OPri )2j(PPh3)]
Empirical formula
MW
C29H33NO2P2S3Pd
692.1
2.4. Preparation of [Pd(Sepy)hS2P(OEt)2 j(PPh3 )]
Crystal system
Space group
Monoclinic
P21 /n
˚
a (A)
14.867(4)
10.160(1)
21.136(4)
93.18(1)
3187.7(11)
4
To a methanolic solution of NaSepy [prepared from
py2Se2 (41 mg, 0.13 mmol) and NaBH4 (11 mg, 0.27
mmol) in (10 cm3) methanol] was added a dichlorome-
thane solution (10 cm3) of [PdClhS2P(OEt)2j(PPh3)] (133
mg, 0.23 mmol) with stirring which was continued for 3 h.
The solvents were evaporated in vacuo and the residue was
extracted with dichloromethane and passed through a
Florisil column. The solution was dried in vacuo and the
residue recrystallized from a diethyl ether–hexane mixture
as a red crystalline solid (134 mg, 84%). Similarly other
Sepy containing complexes were prepared. Pertinent data
are given in Table 1.
˚
b (A)
˚
c (A)
b (8)
V (A )
3
˚
Z
T (K)
293(2)
˚
l (A)
0.71073
0.20330.21930.252
1.442
5591
3643
343
0.1067
0.0464
Crystal size (mm)
Density (mg/m3)
No. reflections collected
No. unique reflections .2s(I)
No. parameters
Rw
R1
2.5. Attempted preparation of [Pd(Spy)hS2P(OR)2 j]
ow(uFou2uFcu)2, including 3643 reflections with I.2s (I),
adjusting 343 parameters converged at R and Rw indices of
0.0464 and 0.1067, respectively. All calculations were
carried out using the NRCVAX [20] and SHELXL-93 [21].
Neutral atom scattering factors were used [22] and anomal-
ous scattering terms were included in Fc [23].
To a dichloromethane suspension of [hPdCl(Spy)jn] (94
mg, 0.37 mmol), solid NH4[S2P(OEt)2] (76 mg, 0.37
mmol) was added and the whole mixture was stirred for 3
h. Reactants were filtered and the filtrate was concentrated
in vacuo. The residue was first extracted with ether,
filtered, dried in vacuo and recrystallized from hexane as
yellow crystals (yield 65 mg, 73%) identified as
[PdhS2P(OEt)2j2] (m.p. 114–1168C) from analysis [C, 20.1
(20.2); H, 4.2 (3.4)] and NMR data. After diethyl ether
extraction, the residue was dissolved in CH2Cl2 and
filtered, dried in vacuo (yield 25 mg, 41%) identified as
[hPd(Spy)2j2] m.p. 2108 (d) [C, 30.8 (30.6); H, 2.1 (2.5);
N, 7.2 (8.6) %].
3. Results and discussion
Reaction (Eq. (1)) of [PdCl(S>S)(PPh3)] with one mole
equivalent of sodium pyridine-2-thiolate or -selenolate
affords complexes of the type [Pd(Epy)(S>S)(PPh3)] in
fairly good yield. Attempts to prepare complexes free from
triphenylphosphine by the reaction of [hPdCl(Spy)jn] with
[NH4S2P(OR)2] (Eq. (2)) in dichloromethane leads to
disproportionation to [hPd(Spy)2j2] and [PdhS2P(OR)2j2]
as characterized by elemental analysis and NMR data.
The reaction of [hPdCl(Spy)jn] with NH4S2P(OPrn)2,
NH4S2P(OPri )2, or NaS2CNEt2.3H2O proceeds similarly.
2.6. X-ray crystallography
X-ray diffraction data (Table 2) for the complex
[Pd(Spy)hS2P(OPri )2j(PPh3)] were collected at room tem-
perature with an Enraf–Nonius CAD-4 diffractometer
using graphite monochromated Mo-Ka radiation (l5
[PdCl(S>S)(PPh3)] 1 NaEpy → [Pd(Epy)(S>S)(PPh3)] 1
NaCl (1)
Na2PdCl4 1 NaSpyaqueous MeOH
→
[hPdCl(Spy)jn]
˚
0.71073 A). The unit cell parameters were determined
from least-squares refinement of the setting angle of 25
reflections with 2u in the range 6.05 to 13.768. The data
were corrected for Lorentz-polarization effects and for
linear decay. Experimental absorption corrections based on
psi-scans of three reflections were carried out for the
structure [18]. The structure was solved by heavy atom
methods using the SHELX-86 program [19] and successive
difference Fourier Synthesis. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were fixed
geometrically and refined with the riding model. The final
full matrix least squares refinements on I minimizing
↓NH4[S2P(OR)2]
1
1
]
]
2 [Pd(Spy)2] 1 2 [PdhS2P(OR)2j2] 1 NH4Cl (2)
[S>S5S2P(OR)2 (R5Et, Prn, Pri ), S2CNEt2; E5S or Se].
All the [Pd(Epy)(S>S)(PPh3)] complexes are wine red–
maroon in colour, soluble in common organic solvents
except petroleum ether or hexane. The IR spectra of
dialkyldithiophosphate complexes showed absorptions in
the regions 960–1155 and 720–855 cm21 which may be

1256
S. Narayan et al. / Polyhedron 18 (1999) 1253–1258
attributed to y(P)–O–C and yP–O–(C) stretching modes,
respectively.
The 31Ph1Hj NMR spectra (Table 1) exhibited two sharp
singlets attributable to PPh3 and [S2P(OR)2]2 ligands. The
resonance due to the former is deshielded (|2 ppm) on
substituting chloride in [PdCl(S>S)(PPh3)] by an Epy
ligand while the signal due to [S2P(OR)2]2 was little
affected. The resonance due to [S2P(OR)2]2 in palladium
complexes appears to be little influenced by X substitutents
1
in [PdX(S>S)(PR3)] (X5Cl, SR, SeR, Me) [24]. The H
NMR spectra exhibited characteristic peak multiplicity and
integration. As reported in other cases [25–27], the ethyl
groups on the diethyldithiocarbamate ligand are non-equiv-
alent. Unlike other organoplatinum–palladium complexes
containing the [S2P(OPri )2]2 ligand [24–26,28], the
methyl groups in the present case are isochronous as only
1
one doublet was observed in the H NMR spectra.
1
The H and 31Ph1Hj NMR spectra of these complexes
showed additional peaks integrating to |5% and were
deshielded from the main resonances. These signals may
be attributed to a species containing a chelated Epy ligand.
Structure I may tentatively be proposed. For II (PPh3 trans
to N) one would expect 31P resonances at |29 ppm, e.g. in
the case of [PdCl(Spy)(PPh3)] [32].
Fig. 1. Molecular structure with numbering scheme for
[Pd(Spy)hS2P(OPri )2j(PPh3)].
single bond values confirming partial double bond charac-
ter. The Pd(1)–S(3) bond distance in the present case can
be compared with the values reported for the thiolate
linkage in [Pd(SMe)hS2CNEt2j(PEt3)] [33]. The pyridine
plane of the Spy ligand is nearly perpendicular to the
coordination plane of palladium as reported in
[PdEt(SPh)(PMe3)2] [35].
In order to evaluate whether the complexes reported
here can serve as precursors for the synthesis of palladium
chalcogenides, thermolysis of two representative complex-
es was studied. Thus thermogravimetric analysis (TGA) of
The structure of [Pd(Spy)hS2P(OPri )2j(PPh3)] was estab-
lished unambiguously by X-ray diffraction analysis. An
ORTEP
[29]
view
of
the
molecule
[Pd(Spy)hS2P(OPri )2j(PPh3)] together with the atomic
numbering scheme is shown in Fig. 1. Selected bond
lengths and angles are listed in Table 3. The structure
consists of a discrete molecule with a square planar
configuration around palladium which is coordinated to an
asymmetrically chelated (PriO)2PS22 ligand, PPh3 and
pyS- groups. The palladium atom is slightly away
Table 3
˚
Selected
bond
lengths
(A)
and
angles
(8)
for
[Pd(Spy)hS2P(OPri )2j(PPh3)]
˚
[0.035(1) A] from its coordination plane defined by P(1)–
Bond lengths
S(1)–S(2)–S(3). Various angles around palladium are
deviated from their ideal values because of the strain in the
four-membered chelate ring and can be compared with the
values reported in [PdClhS2P(OPri )2j(PPh3)] [30].
Pd(1)–P(1)
Pd(1)–S(1)
Pd(1)–S(2)
Pd(1)–S(3)
2.2582(13)
S(1)–P(2)
S(2)–P(2)
S(3)–C(19)
N(1)–C(19)
N(1)–C(20)
1.997(2)
1.986(2)
1.773(6)
1.331(7)
1.336(8)
2.3523(14)
2.3946(14)
2.3111(15)
Although all the Pd–S bond distances are within the
normal Pd–S bonding values [30–34], the Pd–S bonds
trans to PPh3 and Spy ligands are different. Both thiolate
and tertiary phosphines are known to have a strong trans
influence [35]. Slight lengthening of the Pd(1)–S(2) bond
Bond angles (8)
P(1)–Pd(1)–S(1)
P(1)–Pd(1)–S(2)
P(1)–Pd(1)–S(3)
S(1)–Pd(1)–S(3)
S(2)–Pd(1)–S(3)
S(1)–Pd(1)–S(2)
P(2)–S(1)–Pd(1)
P(2)–S(2)–Pd(1)
94.40(5)
174.32(6)
88.69(5)
176.59(5)
93.97(5)
83.09(5)
86.21(7)
85.31(6)
C(19)–S(3)–Pd(1)
S(1)–P(2)–S(2)
O(1)–P(2)–O(2)
O(1)–P(2)–S(2)
O(1)–P(2)–S(1)
O(2)–P(2)–S(1)
O(2)–P(2)–S(2)
103.3(2)
104.44(8)
96.6(2)
116.4(2)
112.4(2)
113.9(2)
113.6(2)
˚
(0.04 A) than that of the Pd(1)–S(1) indicates that PPh3
has a stronger trans influence than Spy ligand. The Pd–P
bond length is, as usual, less than the radius sum. The P–S
bond lengths are intermediate between double bond and

S. Narayan et al. / Polyhedron 18 (1999) 1253–1258
1257
Fig. 2. Thermogravimetric curve for [Pd(Spy)hS2P(OPri )2j(PPh3)] (initial weight 5.1 mg).
[Pd(Spy)hS2P(OPri )2j(PPh3)]
and
with the CCDC, No 102639. They are available from
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK on
request.
[Pd(Sepy)hS2P(OPri )2j(PPh3)] were carried out. Both
complexes underwent three stage decomposition paths.
The first two stages showed elimination of organic res-
idues, with one sulfur from the dithiophosphate. In the
third stage it appears from the weight loss and the residual
weight of the material that PdS2 in the case of
[Pd(Spy)hS2P(OPri )2j(PPh3)] and PdSeS in the case of
[Pd(Sepy)hS2P(OPri )2j(PPh3)] are formed (Figs. 2 and 3).
Acknowledgements
One of the authors (SN) is grateful to DAE for the
award of a Senior Research Fellowship to him. The authors
thank Drs. J.P. Mittal and C. Gopinathan for their encour-
agement in this work. The facilities provided by the
Analytical Chemistry Division, BARC for microanalyses
are gratefully acknowledged. The authors (KP and THL)
Supplementary data
Crystallographic supplementary data have been deposited
Fig. 3. Thermogravimetric curve for [Pd(Sepy)hS2P(OPri )2j(PPh3)] (initial weight 4.9 mg).

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S. Narayan et al. / Polyhedron 18 (1999) 1253–1258
[18] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Cryst. A24 (1968)
351.
[19] G.M. Sheldrick, Acta Cryst. 40A (1990) 467.
[20] E.J. Gabe, Y. Le Page, J.P. Charland, F.L. Lee, P.S. White, J. Appl.
Cryst. 22 (1989) 384.
thank the National Science Council, ROC, for support
under grants NSC 87-2811-M007-0048 and NSC87-2112-
M007-009.
[21] G.M. Sheldrick, SHELXL-93, Program for the refinement of crystal
structures, University of Gottingen, Germany.
References
[22] D.T. Cromer, J.T. Waber, International tables for X-ray crystal-
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