The Silver(I) Mercury(II) Oxide Nitrate with the Empirical Formula
AgHg2N 0 5
Thomas J. Mormann and Wolfgang Jeitschko
Anorganisch-Chemisches Institut, Universität Münster,
Wilhelm-Klemm-Str. 8, D-48149 Münster, Germany
Reprint requests to Prof. Dr. W. Jeitschko.
Z. Naturforsch. 54 b, 1489-1494 (1999); received September 7, 1999
Crystal Structure, Silver Mercury(II) Oxide Nitrate
The title compound was prepared by solid state reaction of Ag2Ü with Hg(N03)2-H20 in air
at 350 °C. Its crystal structure was determined from single-crystal diffractometer data: Pnma,
a = 620.1(1) pm, b = 670.1(1) pm, c = 1267.5(2) pm, Z = 4, R = 0.026 for 586 structure factors
and 33 variable parameters. The compound may be represented by the formula Ag(Hg0 )2N0 3 .
The mercury(II) together with the oxygen atoms form zig-zag chains with linear coordination
of the mercury atoms. The oxygen atoms of these chains are linked via silver(I) atoms, thus
forming two-dimensionally infinite nets, which contain the trigonal planar nitrate groups in
interstices. Thus, only secondary Hg-O and Ag-O bonds are formed between the nets.
Introduction
Two well-characterized modifications of mercury(II) oxide HgO are known: a trigonal and an
orthorhombic modification. The trigonal form crystallizing in the enantiomorphous space groups
P3i21 and P3221 - can be prepared by precipita­
tion at relatively low temperature (50 °C) [1, 2]. It
irreversibly transforms on heating above 220 °C to
the orthorhombic (Pnma) modification [3], which is
identical with the mineral montroydite. Both forms
seem to occur with yellow and/or red color, de­
pending on the particle size; with red for wellcrystallized samples and yellow for microcrystalline
precipitates [4], To our knowledge the structure of
metastable mercury (I) oxide Hg 2Ü has not been es­
tablished. It may even exist only in the form of the
hydroxide Hg 2(OH )2 [4],
Numerous silver oxides have been reported. Well
characterized is the silver(I) oxide Ag 2Ü [5, 6]
which is isotypic with cuprite CU2O. An appar­
ently second modification with this formula turned
out to be a suboxide with the composition Ag 3 Ü
[7], The crystal structures of two modifications
of the silver(I, III) oxide AgO have been deter­
mined. One with monoclinic symmetry, crystalliz­
ing in the space group P2\/c [8-11], and a tetrago­
nal form with space group I4\/a [12]. Furthermore,
two higher silver oxides are well characterized: the
silver(II, III) oxide Ag30 4 [13] and the silver(III)
oxide A g203 [14].
Ternary compounds containing only silver, mer­
cury, and oxygen have not yet been reported. Simi­
larly, pure mercury nitrates (without water) seem to
be unknown. However, there are two well charac­
terized mercury oxide nitrates. One, with the empir­
ical formula Hg3N 2 0 g, contains a somewhat folded
Hg 30 2 honeycomb net; it may be represented by the
formula (H g 0 )2H g(N 03)2 [15]. The other, with the
empirical formula Hg 2N 0 4, contains puckered nets
formed by HgO zig-zag chains which are linked
via Hg2 bridges. This structure also contains dis­
crete nitrate groups, thus resulting in a formula
(H g 0)2H g2(N 03)2 [16, 17].
Three modifications of silver nitrate A g N 0 3 are
known: an orthorhombic room temperature form
[18-20], a rhombohedral high temperature modifi­
cation, which ist stable above 159 °C [21], and a
rhombohedral metastable modification, which can
be obtained by rapid cooling of melted A gN 03 [22].
Furthermore, the crystal structure of a cubic sil­
ver oxide nitrate AgyNOn has been determined. In
this compound most silver atoms - all at the same
Wyckoff position - together with most oxygen atoms
form a clathrate which contains the remaining silver
atoms and the nitrate groups in different cages. It
thus was proposed to describe this compound by the
formula (AgII’III)60 8 AgIN 0 3 [23],
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Th. J. Mormann and W. Jeitschko • A Silver(I) Mercury(II) Oxide Nitrate
1490
Table I. Atomic parameters of AgHg2NOs.
X
>’
7
Ba
0.2528(3)
0.19802(6)
0.200(2)
0.1331(14)
0.1069(16)
0.1995(17)
0.320(2)
1/4
0.00003(9)
1/4
0.0920(14)
1/4
1/4
1/4
0.84564(10)
0.09692(2)
0.3561(11)
0.3949(6)
0.0194(8)
0.6704(8)
0.2772(10)
2.76(2)
1.337(9)
1.7(3)
2.4(2)
1.2(2)
1.9(2)
2.9(3)
Atom
Pnma
Ag
Hg
N
01
02
03
04
4c
8d
Atom
U ii
U2 2
U3 3
U 12
U 13
f/23
Ag
Hg
551(8)
209(2)
306(6)
140(1)
193(5)
159(1)
0
-4(4)
18(8)
-16(1)
0
4(2)
4c
8d
4c
4c
4c
“The last column in the upper part of the table lists the isotropic displacement parameters of the light atoms and the
equivalent isotropic displacement parameters of the metal atoms in units of 104 pm2. The anisotropic displacement
parameters of the metal atoms (pm- ) are defined by exp(-27r2[/z2(a*)2£/n + ... + 2kIb*c*U2?,]).
The up to now only known silver mercury ox­
ide nitrate was reported more recently [24], It has
the empirical formula AgöHgNOn and it is iso­
typic with cubic AgyNOn [23]. Both compounds
have the same AgöOg clathrate cages, and thus
the mercury atoms of Ag6HgNOn substitute for
those silver atoms of AgyNOn, which are located
in the cages. Interestingly, the near-neighbor envi­
ronments of both compounds suggest that the metal
positions in the cages have different oxidation num­
bers, corresponding to Ag(I) and Hg(II), respec­
tively. In contrast, the silver mercury oxide nitrate
AgHg 2NC>5 , described here, consists of well defined
two-dimensionally infinite silver mercury oxygen
nets which enclose only nitrate groups. It thus may
be represented by the formula Ag(Hg 0 )2N 0 3 . A
preliminary account of the presently reported re­
sults has been given at a conference [25].
Sample Preparation
The samples of AgHg2NO.s were prepared in open sil­
ica tubes by annealing for two weeks at 350 °C. The single
crystals used for the structure determination were iso­
lated from a sample where the starting compounds Ag2Ü
(Riedel de Haen, > 99 %) and Hg(N0 3)2-H20 (Fluka, >
99 %) had been mixed in the molar ratio 1:2. This sample
contained impurities, which were not further identified.
A pure sample of AgHg2NOs was obtained by reaction
under the same conditions with the ideal molar starting ra­
tio of 14, established during the structure determination.
The sample was placed in the cold furnace and heated to
350 °C within one hour. At first the silver oxide dissolves
in the melted compound Hg(N0 3)2-H20. At higher tem­
peratures the nitrogen oxides and the water evaporate.
Thus the reaction might be formulated by the equation
2Ag20 + 8Hg(N03)2-H20 =
4AgHg2N 0 5 + 12N02 + 3 0 2 + 8H20
ignoring the NO/NO2 equilibrium. After the annealing,
the samples were cooled in the furnace to room tempera­
ture within 5 h.
The reaction product at the bottom of the silica tube
consisted of yellow needles with easy cleavage along the
needle axis. This makes it difficult to isolate compact sin­
gle crystals. Energy-dispersive X-ray fluorescence analy­
ses of the samples resulted in atomic ratios Ag:Hg varying
between 0.957:2.043 and 1.011:1.989, in good agreement
with the ideal ratio of 1:2. No impurity elements heavier
than sodium could be detected.
Structure Determination
A single-crystal of AgHg^NOj with dimensions
20x 20x 100 /./nr was selected for the structure deter­
mination on the basis of Laue patterns. It was investi­
gated with an Enraf-Nonius CAD4 diffractometer using
graphite-monochromated MoKa radiation and a scintil­
lation counter with pulse-hight discrimination. The soft­
ware assigned orthorhombic symmetry with the cell di­
mensions a = 619.63(6) pm, b = 669.59(10) pm, c =
1265.48(8) pm, V = 0.5251 nm3. These agree reason­
ably well with the lattice constants obtained from a leastsquares fit of CuKo 1 Guinier powder data using a-quartz
(a = 491.30 pm, c = 540.46 pm) as an internal standard:
a = 620.1(1) pm. b = 670.1(1) pm. c = 1267.5(2) pm,
V = 0.5267 nm3. We consider the latter to be more ac­
curate, because the lattice constants from the four-circle
Th. J. Mormann and W. Jeitschko • A Silver(I) Mercury(II) Oxide Nitrate
Ag: 03
02
201
03
Hg: 03
02
01
01
02
04
04
224.6(10) N:
238.1(10)
247.8(9)
277.7(11)
201.9(6) O l:
202.3(6)
277.0(9)
283.5(8)
292.5(8)
293.3(11)
329.3(11)
201
04
04
02
N
Ag
Hg
Hg
124.0(11) 0 2 :
124.5(17)
290.1(19)
297.4(16)
124.0(11) 03:
247.8(9)
277.0(9)
283.5(8) 04:
2Hg
Ag
2Hg
N
2Hg
Ag
Ag
N
N
2Hg
2Hg
202.3(6)
238.1(10)
292.5(8)
297.4(16)
201.9(6)
224.6(10)
277.7(11)
124.5(17)
290.1(19)
293.3(11)
329.3(11)
1491
Table II. Interatomic distances of AgHg2N05,
calculated with the lattice constants obtained
from the Guinier powder data. All distances
shorter than 333 pm are listed.
whole reciprocal sphere. An absorption correction was
made on the basis of psi scan data. After data averaging
for the Laue symmetry nimm 1228 unique reflections were
obtained (internal residual for F2 values = 0.080).
The structure was determined and refined with the pro­
gram package SHELX-97 [26]. The positions of the heavy
atoms were obtained from a Patterson synthesis, the other
positions were located by difference Fourier syntheses.
For the structure refinement a full-matrix least-squares
program was used with atomic scattering factors, cor­
rected for anomalous dispersion, as provided by the pro­
gram. The structure was refined in the space group Pnma
(No. 62), the one with the highest symmetry compatible
with the space group extinctions. There are Z = 4 formula
units in the cell. The calculated density is 7.61 g em-3 . In
the final refinement cycles anisotropic displacement pa­
rameters were used for the metal atoms; the light atoms
were refined with isotropic displacement parameters. The
final conventional residual is R = 0.026 for 586 structure
factors greater than 2 standard deviations. A weighted
residual of Rw = 0.064 was obtained for all 1228 unique
F2 values. The highest and lowest residual electron den­
sities are 1.8 and -2.2 e-A-3 , respectively. The atomic
parameters and interatomic distances are listed in the Ta­
bles I and II.
Discussion
Fig. 1. Near-neighbor coordinations in AgHgaNOs. The
thermal ellipsoids of the metal atoms are drawn at the 95
% probability limit. The sizes of the other atoms are not
representative of their displacement parameters. For the
atoms with labels all neighbors as listed in Table II are
shown.
diffractometer are affected be systematic errors due to
absorption.
The intensity data were collected with 0/20 scans and
background counts on both ends of each scan. A total
of 6136 reflections was collected up to 20 = 70° in the
The crystal structure of AgHg 2NC>5 may be rep­
resented by the formula AgN 0 3 -2 Hg 0 , thus in­
dicating the oxidation number +1 and +2 for the
silver and mercury atoms, respectively. The sil­
ver atoms have coordination number (CN) 5 with
four close oxygen neighbors (at distances between
224.6 and 247.8 pm) forming the transition be­
tween tetrahedral and square-planar coordination.
The O-Ag-O angles of 2x82.9°, 2 x 106.9°, 135.3°,
and 149.2° are in between 6x109.5° (tetrahedral)
and 4x 9 0 ° and 2x180° (square-planar). The fifth
1492
Th. J. Mormann and W. Jeitschko • A Silver(I) Mercury(II) Oxide Nitrate
Fig.
2. Crystal
structure o f
A gH g 2 N 0 5 . The structure consists
o f two- dimensionally infinite lay­
ers, which are emphazised. At the
top only the mercury and their
strongly bonded oxygen neighbors
are shown. In the next two layers
below, the silver atoms and then
the nitrate groups are added. Weak
secondary metal-oxygen bonds (all
greater than 276 pm) are shown be­
tween the lowest two layers. At the
bottom one layer is view ed in a di­
rection perpendicular to the layer.
oxygen neighbor at the somewhat larger Ag-O dis­
tance of 277.7 pm is situated approximately on the
pseudo-fourfold axis (Fig. 1). The average Ag-O
distance of 247.2 pm is in excellent agreement with
that of 245 pm calculated from the radii given by
Shannon for five-coordinated Ag+1 [27], especially
if one considers the considerable spread of Ag-O
distances in the present compound.
The mercury atoms of AgHg2NOs are in approx­
imately linear oxygen coordination with an O-HgO angle of 177.5(4)°. This linear coordination is
most frequently found for Hg+2. The corresponding
two H g-0 distances of 201.9(6) and 202.3(6) pm
agree well with the H g(II)-0 distances of 203(2)
and the average of 205(3) pm for the trigonal [2]
and orthorhombic [3] modifications of HgO, re­
spectively, 202.6(4) pm in HgMoÜ 4 [28], 205(2)
pm in Hg5Re20io(I) [29], 201(2) and 203(2) pm in
Hg5Re20io(II) [30], as well as the Hg-O distances
of 201.0(8) and 202.9(8) pm for the Hg(II) atoms
in Hg2Re05 [31]. In all of these compounds the
Hg(II) atoms have additional oxygen neighbors sit­
uated close to a plane perpendicular to the short
H g-0 bonds. For the HG(II) atoms in AgHg 2NOs
there are five oxygen neighbors at distances ranging
between 277.0(9) and 329.3(11) pm.
The nitrate groups are practically planar with ON -0 angles of 117.3(1.4)° and 121.3(7)° (2 x ). The
N -0 distances of 2 x 124.0( 11) and 124.5( 17) pm are
nearly equal. Disregarding the practically nonbond-
Th. J. Mormann and W. Jeitschko • A Silver(I) Mercury(II) Oxide Nitrate
ing N-O distances of 290 and 297 pm, the oxygen
atoms have four or five near neighbors of which
some are only weakly bonding (Table II).
As already mentioned above, AgHg 2 NOs shows
some tendency for cleavage, and indeed the struc­
ture may be described as consisting of layers which
contain all of the strong bonds. This is shown in
Fig. 2, produced with the aid of the program DIA­
MOND [32]. In the top of that figure the mercury
atoms of one layer are shown together with their
strongly bonded 0 2 and 0 3 atoms, forming O-HgO-Hg zig-zag chains. In the next layer below these
chains are linked to each other via the silver atoms of
the same layer. Further below, in the third layer from
the top we have added the nitrate groups. And fi­
nally between the next two layers we show the weak
Ag-O and Hg-O bonds of 278, 277, and 293 pm, re­
spectively. A view of one layer perpendicular to its
plane is shown at the bottom of Fig. 2. It can be
seen that the oxygen atoms of the nitrate group do
not form very strong bonds to the silver or mercury
atoms, although each nitrate group clearly belongs
predominantly only to one Ag(HgO )2 layer. Hence,
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1493
the structure can also be represented by the formula
A g(H g0) 2 N03.
The structure of AgHgoNOs shows some sim­
ilarity to the structures of (Hg 0 )2 Hg(N 0 3 ) 2 [15],
(Hg 0 )2 Hg 2 (N 0 3 )2 [16, 17], and Hg 2 R e 0 5 [31]. In
all of these four structures the Hg(II) atoms form
zig-zag chains which are parallel to each other.
These chains are linked via Ag or Hg atoms or
Hg(I)-Hg(I) pairs, thus forming two-dimensionally
infinite nets of the compositions AgHg 2 0 2, Hg 3 0 2,
and two times Hg 4 0 2 in the four compounds, re­
spectively.
Acknowledgements
We thank Mrs. U. Ch. Rodewald and Mr. K. Wagner
for the competent collection of the intensity data at the
four-circle diffractometer and the work at the scanning
electron microscope. We are also indebted to the Heraeus
Quarzschmelze for a generous gift of silica tubes. This
work was further supported by the Fonds der Chemischen
Industrie and by the International Centre for Diffraction
Data.
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Th. J. Mormann and W. Jeitschko • A Silver(I) Mercury(II) Oxide Nitrate
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