Crystal structures and vibrational and solid-state (CPMAS) NMR spectroscopy of some bis(triphenylphosphine)silver(I) sulfate, selenate and phosphate systems

Article abstract of DOI:10.1039/b007796h

The crystal structures of [Ag₂(PPh₃)₄(EO₄)]· 2H₂O (E = S 1 or Se 2) showed that these contain [Ag₂(PPh₃)₄(EO₄)] units with three-coordinate silver and EO₄^2- bridging the two silver atoms via two oxygen atoms. The complexes [Ag(PPh₃)₂(HEO₄)]·H₂O (E = S 3 or Se 4) contain [Ag(PPh₃)₂(HEO₄)] molecules in which HEO₄^- is terminally bound to the silver atoms by a single oxygen atom. The complex [Ag(PPh₃)₂(H₂PO₄)]·2E tOH 5 contains [Ag(PPh₃)₂(H₂PO₄)] molecules in which H₂PO₄^- is terminally bound to the silver atom, which is essentially three-coordinate. Heteronuclear ¹J(^107/109Ag,³¹P) and homonuclear ²J(³¹P,³¹P) spin-spin coupling constants for these compounds were determined by analysis of their high- (9.40 T) and very high-field (17.62 T) ³¹P CPMAS NMR spectra, with the aid of the 2-D ³¹P CPCOSY technique, and a strong inverse correlation was found between ¹J(^107/109Ag,³¹P) and the Ag-P bond length. IR and Raman studies show that the effect of a bound proton on the vibrational frequencies of EO₄^2- is much greater than that of an attached metal atom. The Royal Society of Chemistry 2001.

Full text of DOI:10.1039/b007796h

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Crystal structures and vibrational and solid-state (CPMAS) NMR
spectroscopy of some bis(triphenylphosphine)silver(I) sulfate,
selenate and phosphate systems
Graham A. Bowmaker,*a John V. Hanna,*b Clifton E. F. Rickarda and Andrew S. Liptonc
a Department of Chemistry, University of Auckland, Private Bag 92019, Auckland,
New Zealand
b ANSTO NMR Facility, Materials Division, Private Mail Bag 1, Menai, N.S.W. 2234,
Australia
c Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,
Richland, Washington 99352, USA
Received 26th September 2000, Accepted 8th November 2000
First published as an Advance Article on the web 7th December 2000
The crystal structures of [Ag2(PPh3)4(EO4)]ؒ2H2O (E = S 1 or Se 2) showed that these contain [Ag2(PPh3)4(EO4)]
units with three-coordinate silver and EO42Ϫ bridging the two silver atoms via two oxygen atoms. The complexes
[Ag(PPh3)2(HEO4)]ؒH2O (E = S 3 or Se 4) contain [Ag(PPh3)2(HEO4)] molecules in which HEO4Ϫ is terminally bound
to the silver atoms by a single oxygen atom. The complex [Ag(PPh3)2(H2PO4)]ؒ2EtOH 5 contains [Ag(PPh3)2(H2PO4)]
molecules in which H2PO4Ϫ is terminally bound to the silver atom, which is essentially three-coordinate. Hetero-
nuclear 1J(107/109Ag,31P) and homonuclear 2J(31P,31P) spin–spin coupling constants for these compounds were
determined by analysis of their high- (9.40 T) and very high-field (17.62 T) 31P CPMAS NMR spectra, with the
aid of the 2-D 31P CPCOSY technique, and a strong inverse correlation was found between 1J(107/109Ag,31P) and
the Ag–P bond length. IR and Raman studies show that the effect of a bound proton on the vibrational frequencies
of EO42Ϫ is much greater than that of an attached metal atom.
ated or not) and on its mode of coordination. This method is
potentially useful in situations where such information cannot
Introduction
The many structural studies of silver() complexes that have
been reported demonstrate the structural versatility of silver()
in its coordination chemistry.1,2 Many of these complexes
involve anionic ligands such as halides, pseudohalides and
oxyanions. Some types of oxyanion complex, such as those
involving carboxylate ligands, have been known for a long
time,3,4 and we have recently characterized a number of such
complexes by structural and spectroscopic methods.5,6 In the
course of this work our attention was drawn to the fact that
relatively little is known about the structures and properties of
silver() complexes involving simple multiply charged inorganic
anions such as sulfate or phosphate. Such ions can be coordin-
ated in the fully deprotonated form (e.g. SO42Ϫ) or in a pro-
tonated form (e.g. HSO4Ϫ). A structure reported some years
ago for the silver() complex [Ag(C6H16N10)]SO4[HSO4]ؒH2O
(C6H16N10 = ethylenebis(biguanide)) reveals the presence of
monodentate SO42Ϫ and HSO4Ϫ in the same molecule, with the
somewhat unexpected result that the Ag–O distance for SO42Ϫ is
greater than that for HSO4Ϫ.7 A few silver complexes involving
be obtained directly from single-crystal structure determin-
ations. A situation of this kind occurs in the study of oxyanions
adsorbed from aqueous solution onto a metal surface. A
number of studies have indicated the existence of adsorbed
sulfate, hydrogensulfate, phosphate, etc. at the surface of Group
11 metal electrodes.12–18 In most of these studies the evidence for
the state of protonation and the mode of bonding of the anion
to the surface is indirect, and it is possible that the correlation
between the vibrational spectra and the structures of well
characterized coordination compounds involving the same
anions and metal might provide a further means of charac-
terizing the surface species. It is true that the coordination
compounds studied here involve the metal in the formal ϩ1
oxidation state, whereas the atoms in a bulk metal electrode
formally have an oxidation state of zero. However, recent
studies have shown that the oxidation states of surface metal
atoms bound to the anionic adsorbate are close to ϩ1,13,15 so
these two types of system may be more closely related than
is apparent at first sight.
SO4 or HSO4 ligands have been reported,8–11 most of which
also contain N-donor ligands.9–11 There have been no reports of
such complexes with P-donor ligands. In the present work we
report the preparation and structures of two such compounds
and their selenate analogues, [Ag2(PPh3)4(EO4)]ؒ2H2O (E = S 1
or Se 2) and [Ag(PPh3)2(HEO4)]ؒH2O (E = S 3 or Se 4). Also
reported are similar results for [Ag(PPh3)2(H2PO4)]ؒ2EtOH 5,
which is related to 3 via the isoelectronic relationship between
For the NMR investigations of these systems, low field (2.11
T) 13C CPMAS studies and 31P CPMAS studies at high field
(9.40 T) and very high field (17.62 T) have been undertaken.
The 100% natural abundance of the I = 1/2 31P isotope makes
31P CPMAS NMR a particularly useful and sensitive technique.
Previous 31P CPMAS NMR studies of bis-silver phosphine
systems have shown that each unique phosphorus site is
characterized by both heteronuclear 1J(107/109Ag,31P) and homo-
nuclear 1J(31P,31P) scalar (or spin–spin) couplings when chemical
inequivalence between each phosphorus site exists.5,6,19,20 In
2Ϫ
Ϫ
Ϫ
HSO4 and H2PO4Ϫ. We have also studied these and some
related compounds by vibrational and solid-state CPMAS
NMR spectroscopy.
1
particular, the J(Ag,P) coupling constant is a useful probe of
Vibrational spectroscopy is expected to provide information
about the nature of the coordinated anion (e.g. whether proton-
the coordination of the metal centres within these complexes. In
this case, each phosphorus position is represented as a doublet
20
J. Chem. Soc., Dalton Trans., 2001, 20–28
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b007796h

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of doublets with an intensity distribution governed by the
chemical shift difference between the 2J(P,P) coupled pairs and
a A2X, ABX or AMX spin system description is subsequently
invoked. In the event that more than 2 inequivalent phosphorus
sites exist in the unit cell, techniques such as 2-D 31P CPCOSY
at high field (9.40 T) have become an important tool for the
disentanglement and correct assignment of the complex spectra
that ensue.6,19,20 However, when very small chemical shift sep-
AgNO3 (3.57 g, 21 mmol) in water (10 ml). The resulting bright
yellow precipitate was collected and washed with water, ethanol
and diethyl ether. Yield 2.58 g (88%).
Ag2HPO4. A mixture of 90% H3PO4 (0.39 g, 3.6 mmol) and
Ag3PO4 (0.84 g, 2.0 mmol) was placed in an oven at 90 ЊC for
1 h, whereupon the yellow Ag3PO4 turned creamy white. The
mixture was treated with a little acetone and allowed to stand
for 30 min at room temperature. The product was collected and
washed with acetone. Yield 0.92 g (98%).
2
arations exist between inequivalent J(P,P) coupled sites (and
A2X or ABX spin systems are described), the use of 2-D tech-
niques alone is not sufficient and data acquisition at signifi-
cantly higher fields becomes necessary. The advent of very
high field instrumentation operating at 17.62 and 18.80 T (i.e.
1H frequencies of 750 and 800 MHz, respectively) partially
relaxes the strongly coupled A2X and ABX spin conditions that
can dominate these 31P multiplets at lower field strengths, thus
facilitating more confident assignment and accurate measure-
ment of spectral parameters. The results presented in this paper
describe the first very high field 31P CPMAS NMR study of any
metal phosphine system and demonstrate the complementary
nature of the emerging data to those acquired on more
routinely accessed high field instrumentation.
[Ag(PPh3)2(H2PO4)]ؒ2EtOH 5. A mixture of Ag3PO4 (0.1 g,
0.24 mmol) and PPh3 (0.38 g, 1.43 mmol) was dissolved in
ethanol (5 ml) to which 90% H3PO4 (0.06 g, 0.55 mmol) had
been added. The clear solution was allowed to stand in the air.
The white crystalline product, which formed as the volume
of the solution reduced to about 3.5 ml by evaporation, was
collected and washed with ice-cold ethanol. Yield 0.47 g (79%).
Found: C 58.4, H 4.9%. Calc. for C40H44AgO6P3: C 58.5, H
5.4%.
Structure determinations
Spheres of X-ray data were collected using a Siemens SMART
diffractometer with a CCD area detector system at 200 K.
Monochromatic Mo-Kα radiation (λ = 0.71073 Å) was
employed in all cases. Absorption corrections were applied
semi-empirically by analysis of equivalents and equivalent
reflections averaged to give the unique data sets. The structures
were solved by direct methods and refined by full matrix least
squares on F2 after absorption corrections. Anisotropic thermal
parameters were refined for the non-hydrogen atoms; (x, y, z,
Uiso)H were included constrained at estimated values. Neutral
atom complex scattering factors were used. Conventional
residuals R on |F| and Rw on |F|2 are quoted at convergence.
Computation used SHELX 97.21 The carbon atoms of the PPh3
ligands are labelled C(lmn) where l is the ligand number, m the
ring number 1, 2 or 3 and n the atom number 1–6 with the
carbon bound to the phosphorus labelled C(lm1).
Experimental
Preparation of compounds
[Ag2(PPh3)4(SO4)]ؒ2H2O 1. A mixture of Ag2SO4 (0.31 g,
1.0 mmol) and PPh3 (1.05 g, 4.0 mmol) was dissolved in boiling
ethanol (10 ml). Water (5 ml) was added to the hot solution,
which was allowed to stand in the air. The white crystalline
product, which formed as the volume of the solution reduced
to about 10 ml by evaporation, was collected and washed with
ethanol. Yield 1.14 g (82%). Found: C 62.1, H 4.8%. Calc. for
C72H64Ag2O6P4S: C 61.9, H 4.6%.
[Ag2(PPh3)4(SeO4)]ؒ2H2O 2. A mixture of Ag2SeO4 (0.36 g,
1.0 mmol) and PPh3 (1.05 g, 4.0 mmol) was dissolved in boiling
ethanol (10 ml). The resulting cloudy greyish green solution was
filtered while hot to yield a colourless filtrate, and water (5 ml)
added to the hot filtrate. The white crystalline product, which
formed as the warm solution slowly cooled to room temper-
ature, was collected and washed with ice-cold ethanol–water
(2:1). Yield 1.12 g (78%). Found: C 60.0, H 4.5%. Calc. for
C72H64Ag2O6P4Se: C 59.9, H 4.5%.
Crystal/refinement data. [Ag2(PPh3)4(SO4)]ؒ2H2O 1 ≡ C72H64-
1
¯
Ag2O6P4S, M = 1396.91, triclinic, space group P1 (Ci no. 2),
a = 12.4953(4), b = 13.3274(4), c = 21.7824(6) Å, α =
91.1910(10), β = 95.3060(10), γ = 118.2300(10)Њ, V = 3173.6 Å3,
Z = 2, µ = 8.04 cmϪ1
R = 0.048, Rw = 0.120.
[Ag2(PPh3)4(SeO4)]ؒ2H2O
,
N = 13366, No (I > 2σ(I)) = 11528,
2 ≡ C72H64Ag2O6P4Se, M =
[Ag(PPh3)2(HSO4)]ؒH2O 3. A mixture of Ag2SO4 (0.31 g,
1.0 mmol) and PPh3 (1.05 g, 4.0 mmol) was dissolved in boiling
ethanol (10 ml) to which 98% H2SO4 (0.11 g, 1.1 mmol) had
been added. The clear solution was allowed to stand in the air.
The white crystalline product, which formed as the volume
of the solution reduced to about 5 ml by evaporation, was
collected and washed with ice-cold ethanol. Yield 1.10 g (73%).
Found: C 57.9, H 4.4%. Calc. for C36H33AgO5P2S: C 57.8, H
4.5%.
1
¯
1443.81, triclinic, space group P1 (Ci, No. 2), a = 12.46450(10),
b = 13.2640(1), c = 21.85890(10) Å, α = 91.5060(10), β =
95.2500(10), γ = 117.950(1)Њ, V = 3169.1 Å3, Z = 2, µ = 13.35
cmϪ1, N = 13379, No (I > 2σ(I)) = 11996, R = 0.022, Rw = 0.053.
[Ag(PPh3)2(HSO4)]ؒH2O 3 ≡ C36H33AgO5P2S, M = 747.49,
triclinic, space group P1 (C1i, no. 2), a = 12.58420(10),
¯
b = 13.26680(10), c = 24.3057(3) Å, α = 92.9180(10), β =
103.5360(10), γ = 118.0230(10)Њ, V = 3420.9 Å3, Z = 4,
µ = 7.84 cmϪ1, N = 14368, No (I > 2σ(I)) = 12199, R = 0.034,
Rw = 0.083.
[Ag(PPh3)2(HSeO4)]ؒH2O 4. A mixture of Ag2SeO4 (0.36 g,
1.0 mmol) and PPh3 (1.05 g, 4.0 mmol) was dissolved in warm
ethanol (10 ml) until nearly all of the solids dissolved. On
cooling, a white solid (probably [Ag2(PPh3)4(SeO4)]) separated.
To this a solution of H2SeO4 (0.22 g, 1.5 mmol) in water (5 ml)
was added and the mixture heated with stirring until the
white solid just dissolved. The white crystalline product, which
formed as the solution slowly cooled to room temperature, was
collected and washed with ice-cold ethanol–water (1:1). Yield
1.33 g (84%). Found: C 54.7, H 4.2%. Calc. for C36H33AgO5-
P2Se: C 54.4, H 4.2%.
[Ag(PPh3)2(HSeO4)]ؒH2O 4 ≡ C36H33AgO5P2Se, M = 794.39,
triclinic, space group P1 (C1i, no. 2), a = 12.5358(2),
¯
b = 13.2580(2), c = 24.5566(2) Å, α = 104.3870(10), β =
91.6760(10), γ = 117.4040(10)Њ, V = 3460.4 Å3, Z = 4, µ = 17.67
cmϪ1, N = 14255, No (I > 2σ(I)) = 11986, R = 0.057, Rw = 0.117.
[Ag(PPh3)2(H2PO4)]ؒ2EtOH 5 ≡ C40H44AgO6P3, M = 821.53,
triclinic, space group P1 (C1i, no. 2), a = 10.10970(10),
¯
b = 13.11210(10), c = 16.00730(10) Å, α = 73.100(1), β =
77.5550(10), γ = 78.8890(10)Њ, V = 1963.2 Å3, Z = 2, µ = 6.80
cmϪ1, N = 8235, No (I > 2σ(I)) = 7452, R = 0.024, Rw = 0.063.
CCDC reference number 186/2267.
Ag3PO4. A solution of KH2PO4 (0.95 g, 7 mmol) and KOH
(0.79g, 14 mmol) in water (20 ml) was added to a solution of
See http://www.rsc.org/suppdata/dt/b0/b007796h/ for crystal-
lographic files in .cif format.
J. Chem. Soc., Dalton Trans., 2001, 20–28
21

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Spectroscopy
Infrared spectra were recorded at 4 cmϪ1 resolution at room
temperature as Nujol mulls between KBr plates on a Perkin-
Elmer Spectrum 1000 Fourier-transform infrared spectrometer.
Far-infrared spectra were obtained with 2 cmϪ1 resolution
at room temperature as pressed Polythene discs on a Digilab
FTS-60 Fourier-transform infrared spectrometer employing an
FTS-60V vacuum optical bench with a 5 lines mmϪ1 wire mesh
beam splitter, a mercury lamp source and a pyroelectric trigly-
cine sulfate detector. Raman spectra were recorded at 4.5 cmϪ1
resolution using a Jobin-Yvon U1000 spectrometer equipped
with a cooled photomultiplier (RCA C31034A) detector. The
514.5 nm exciting line from a Spectra-Physics Model 2016
argon-ion laser was used.
Solid-state 31P CPMAS NMR spectra were obtained at
ambient temperature on Bruker MSL-400 (9.40 T) and Varian
Inova-750 (17.62 T) spectrometers operating at 31P frequencies
of 161.92 and 303.53 MHz, respectively. Conventional cross-
polarization22 and magic-angle-spinning23 techniques, coupled
with spin temperature alternation24 to eliminate spectral arti-
facts, were implemented using Bruker 4 mm (9.40 T) and
Jakobsen 5 mm (17.62 T) double-air-bearing probes in which
MAS frequencies of ≥10 kHz were achieved. A recycle delay of
15–30 s, Hartmann–Hahn contact period of 2–5 ms and a
1H decoupling field of 80–85 kHz (during acquisition) were
common to all 31P spectra. For the 9.40 T experiments, the
initial 1H π/2 pulse width (prior to the Hartmann–Hahn match)
was 3 µs, and at 17.62 T the initial 1H π/2 pulse width was 6 µs.
No spectral smoothing was employed prior to Fourier trans-
formation. The 2-D 31P CPCOSY experiment was implemented
with the TPPI (time proportional phase incrementation)
method25 for acquisition of phase-sensitive data in both the F1
and F2 dimensions. The application of this technique has been
discussed in detail elsewhere.26 The recycle delay, contact
Fig. 1 Molecular structure of [Ag2(PPh3)4(SO4)]ؒ2H2O 1 showing all
non-hydrogen atoms and the atom numbering scheme. 50% Probability
amplitude displacement ellipsoids are shown.
Table 1 Core geometries for [Ag2(PPh3)4(EO4)]ؒ2H2O (E = S 1 or Se 2)
1 (E = S)
2 (E = Se)
Ag(1)–P(1)
Ag(1)–P(2)
Ag(1)–O(1)
Ag(2)–P(3)
Ag(2)–P(4)
Ag(2) ؒ ؒ ؒ O(3)
Ag(2)–O(4)
E–O(1)
2.5018(11)
2.4190(11)
2.235(4)
2.4625(11)
2.4380(11)
2.643(4)
2.394(3)
1.472(4)
1.464(4)
1.466(4)
2.4994(4)
2.4105(5)
2.2342(15)
2.4590(4)
2.4289(4)
2.6507(16)
2.4025(13)
1.6403(15)
1.6271(13)
1.6394(14)
1.6489(13)
E–O(2)
E–O(3)
E–O(4)
1.489(3)
P(1)–Ag(1)–P(2)
P(1)–Ag(1)–O(1)
P(2)–Ag(1)–O(1)
Ag(1)–O(1)–E
P(3)–Ag(2)–P(4)
P(3)–Ag(2)–O(3)
P(4)–Ag(2)–O(3)
P(3)–Ag(2)–O(4)
P(4)–Ag(2)–O(4)
O(3)–Ag(2)–O(4)
Ag(2)–O(3)–E
128.04(4)
92.81(11)
136.65(12)
129.6(2)
128.60(4)
113.73(10)
112.04(10)
102.97(9)
121.36(9)
56.42(11)
92.79(18)
102.63(17)
128.353(16)
90.38(4)
1
138.40(5)
127.94(8)
128.755(16)
112.73(4)
111.74(4)
100.36(4)
122.21(4)
62.46(4)
period, H π/2 pulse width and MAS rate were the same as
those implemented in the above 1-D 31P CPMAS experiments.
A total of 256 F1 increments were acquired into 256 word
blocks, with both dimensions zero-filled to 1 K words and
weighted with Gaussian multiplication prior to Fourier trans-
formation. All 31P chemical shifts were externally referenced to
triphenylphosphine which has a shift of δ Ϫ9.9 with respect
to 85% H3PO4. Solid-state 13C CPMAS NMR spectra were
obtained at ambient temperature on a Bruker CXP-90 spec-
trometer operating at a 13C frequency of 22.63 MHz. The
cross-polarization methods outlined above were implemented
on a Doty 7 mm probe in which MAS frequencies of 4 kHz
were achieved. A recycle delay of 10 s, contact period of 5 ms
91.17(6)
100.18(6)
Ag(2)–O(4)–E
The 1:2 silver dihydrogenphosphate complex with triphenyl-
phosphine was obtained by a reaction analogous to (1); eqn.
(2). In this case the stable ethanol disolvate 5 was obtained from
ethanol solution.
and initial 1H π/2 pulse width of 3.5 µs were common to all 13
C
spectra. No spectral smoothing was employed prior to Fourier
transformation, and 13C chemical shifts were referenced to
TMS via an external sample of solid hexamethylbenzene.
Ag3PO4 ϩ 6PPh3 ϩ 2H3PO4 → 3[Ag(PPh3)2(H2PO4)] (2)
Crystal structure determinations
The structure of [Ag2(PPh3)4(SO4)]ؒ2H2O 1 is shown in Fig. 1.
Its core geometries and those for the isomorphous selenium
analogue 2 are given in Table 1. These compounds contain
Ag(PPh3)2 units that are linked by a bridging EO4 group, which
coordinates to the two silver atoms in the dimer via the two
oxygen atoms O(1) and O(4). The two silver atoms are essen-
tially three-coordinate with almost planar environments,
although there is a weak secondary Ag(2) ؒ ؒ ؒ O(3) interaction
that results in Ag(2)–O(4) being slightly longer than Ag(1)–
O(1). The E–O bonds involving the oxygen atoms O(1) and O(4)
in the primary coordination spheres of the two silver atoms are
slightly longer than those involving O(2) and O(3). The Ag(1)–
O(1) and Ag(2)–O(4) distances in the sulfate complex are
shorter than those in most previously reported silver sulfate
complexes, where the coordination number of silver ranges
from three to six.7,9–11 A shorter Ag–O of 2.18(4) Å has been
reported in the 1:3 complex [Ag2L3(SO4)(H2O)] (L = [1,2,4]-
Results and discussion
The 1:2 silver() sulfate or selenate complexes with triphenyl-
phosphine are readily prepared by reaction of stoichiometric
quantities of the reagents in aqueous ethanol. The correspond-
ing hydrogensulfate or hydrogenselenate complexes are obtained
under similar conditions by addition of slightly more than the
stoichiometrically required amount of the appropriate acid
H2EO4 (E = S or Se) to the reaction mixture before crystalliz-
ation, eqn. (1). If insufficient water is present in these reactions,
Ag2EO4 ϩ 4PPh3 ϩ H2EO4 → 2[Ag(PPh3)2(HEO4)] (1)
products containing ethanol of solvation are sometimes
obtained, but these are unstable with respect to loss of ethanol.
When sufficient water is present the products are obtained as
the stable hydrates 1–4.
22
J. Chem. Soc., Dalton Trans., 2001, 20–28

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Fig. 2 Molecular structure of [Ag(PPh3)2(HSO4)]ؒH2O 3. Details as
for Fig. 1.
Fig. 3 Molecular structure of [Ag(PPh3)2(H2PO4)]ؒ2EtOH 5. Details
as for Fig. 1.
Table 2 Core geometries for [Ag(PPh3)2(HEO4)]ؒH2O (E = S 3 or Se 4)
3 (E = S)
4 (E = Se)
Table 3 Core geometries for [Ag(PPh3)2(H2PO4)]ؒ2EtOH 5
Ag(1)–P(1)
Ag(1)–P(2)
Ag(1)–O(1)
Ag(2)–P(3)
Ag(2)–P(4)
Ag(2)–O(5)
Ag(2)–O(9)
E(1)–O(1)
E(1)–O(2)
E(1)–O(3)
E(1)–O(4)
E(2)–O(5)
E(2)–O(6)
E(2)–O(7)
E(2)–O(8)
2.4316(7)
2.4394(7)
2.410(2)
2.4300(7)
2.4265(8)
2.412(2)
2.446(3)
1.431(2)
1.436(2)
1.538(3)
1.472(3)
1.441(2)
1.532(2)
1.470(3)
1.450(3)
2.4405(13)
2.4349(13)
2.421(4)
2.4278(15)
2.4275(14)
2.402(4)
2.439(5)
1.589(4)
1.596(4)
1.626(6)
1.687(6)
1.611(4)
1.618(5)
1.689(4)
1.624(5)
Ag–P(1)
Ag–P(2)
Ag–O(1)
P(3)–O(1)
P(3)–O(2)
P(3)–O(3)
P(3)–O(4)
2.4309(4)
2.4595(4)
2.3280(12)
1.5051(12)
1.5169(13)
1.5759(14)
1.5759(14)
P(1)–Ag–O(1)
P(2)–Ag–O(1)
P(1)–Ag–P(2)
P(3)–O(1)–Ag
123.96(3)
107.27(3)
127.671(14)
103.77(6)
of these two molecules are unexpectedly different. In one the
silver atom Ag(1) is essentially three-coordinate with an almost
planar environment, whereas in the other the silver atom Ag(2)
is four-coordinate because this molecule also contains a bound
water molecule, with Ag(2)–O(9) ≈ 2.45 Å. The non-bonded
water molecule forms hydrogen bonds to the bonded water
molecule and an oxygen of the HEO4 group of the three-
coordinate silver atom and thus links the two parts together.
Although the protons in the HEO4 groups are not located in
the crystal structure determination, the fact that the E–O bonds
involving the oxygen atoms O(3) and O(6) are significantly
longer than the others suggests that the protons reside on these
atoms. Despite the differences in the coordination environments
in the two molecules, the Ag(1)–O(1) and Ag(2)–O(5) distances
are not very different, with values of about 2.41 Å. This is
slightly longer than the Ag–O distances involving the bridging
sulfate in 1 and 2. This is as expected, since the strongly bound
proton in HEO4Ϫ should reduce the coordinating ability of this
species relative to EO42Ϫ. For comparison, Ag–O distances in
the recently determined crystal structure of AgHSO4 lie in the
range 2.41–2.69 Å.8 The P–Ag–P angles in 3 and 4 (ca. 131Њ) are
slightly greater than those in 1 and 2 (ca. 128Њ), in agreement
Ϫ
P(1)–Ag(1)–P(2)
P(1)–Ag(1)–O(1)
P(2)–Ag(1)–O(1)
Ag(1)–O(1)–E(1)
P(3)–Ag(2)–P(4)
P(3)–Ag(2)–O(5)
P(4)–Ag(2)–O(5)
P(3)–Ag(2)–O(9)
P(4)–Ag(2)–O(9)
O(5)–Ag(2)–O(9)
Ag(2)–O(5)–E(2)
131.38(2)
113.72(8)
103.15(8)
133.14(14)
130.48(3)
108.28(6)
110.46(6)
107.32(11)
105.70(13)
84.92(10)
136.00(14)
131.13(5)
100.99(15)
113.66(15)
129.03(24)
131.20(5)
108.07(12)
108.19(11)
106.9(2)
Ϫ
105.66(19)
88.45(18)
129.76(21)
triazolo[1,5-a]pyrimidine) in which a four-coordinate silver
atom is bound to a monodentate sulfate ligand and to three N
atoms of three triazolopyrimidine ligands.10 However, in the
1:2 complex [Ag2L2(SO4)(H2O)], in which the sulfate bridges
two four-coordinate silver atoms, Ag–O increases to 2.393(7),
2.556(7) Å.10 The shorter Ag–O in the present sulfate (at Ag(1)
in particular) is no doubt due at least in part to the smaller
coordination number (three) in this compound.
All four PPh3 ligands in complexes 1 and 2 are crystal-
lographically inequivalent, and the degree of inequivalence as
measured by the Ag–P bond lengths is almost identical for the
E = S and Se compounds. This contrasts with the results of the
31P CPMAS NMR study of these compounds (see below). The
Ag–P bond lengths range from 2.41 to 2.50 Å and the P–Ag–P
angles from 128.0 to 128.8Њ. The water molecules are hydrogen
bonded to the EO4 oxygen atoms, which are not bonded to Ag.
The structure of [Ag(PPh3)2(HSO4)]ؒH2O 3 is shown in
Fig. 2. Its core geometries and those for the isomorphous
selenium analogue 4 are given in Table 2. These compounds
contain two distinct Ag(PPh3)2(HEO4) molecules in which the
with the above conclusion that the Ag–O bonding involving
HEO4Ϫ is weaker than that involving EO4
.
2Ϫ
The structure of [Ag(PPh3)2(H2PO4)]ؒ2EtOH 5 is shown in
Fig. 3. The core geometry parameters are given in Table 3. This
compound contains unique Ag(PPh3)2(H2PO4) molecules in
which the H2PO4 group is terminally bound to the silver atom
via O(1). The silver atom is essentially three-coordinate with a
Ϫ
very nearly planar environment. The protons in the H2PO4
group are located on O(3) and O(4), consistent with the fact
that the P–O bonds involving these oxygen atoms are signifi-
cantly longer than the others. The Ag–O(1) distance of
2.3280(12) Å is significantly shorter than the shortest Ag–O
Ϫ
distances (ca. 2.41 Å) in the HEO4 complexes 3 and 4, con-
sistent with the fact that H2PO4Ϫ is a stronger base than HSO4
Ϫ
or HSeO4Ϫ. This is also reflected in fact that the P–Ag–P angle
in 5 (ca. 128Њ) is smaller than those in 3 and 4 (ca. 131Њ). The
solvate ethanol molecules in 5 are hydrogen bonded to the
Ϫ
HEO4 groups are terminally bound to the two silver atoms
Ag(1) and Ag(2) via O(1) and O(5) respectively. The structures
J. Chem. Soc., Dalton Trans., 2001, 20–28
23

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phosphate oxygen atom O(2), with O(2) ؒ ؒ ؒ O(5) 2.680(2) and
O(2) ؒ ؒ ؒ O(6) 2.688(2) Å. These distances are typical for weak
hydrogen bonding.27 There is also a possible weak interaction
between Ag and O(2) [Ag ؒ ؒ ؒ O(2) 2.838(1) Å].
Solid-state NMR spectroscopy
The solid-state 13C CPMAS NMR spectra of complexes 1–4
exhibit only one broad unresolved resonance at δ ca. 130 due to
the PPh3 ligands. In addition, the spectrum of 5 shows further
resonances at δ 18.0, 56.3 and 58.9 due to ethanol molecules
of solvation. The last two signals are attributed to the CH2
carbons on the two crystallographically inequivalent ethanol
molecules.
The solid-state 31P CPMAS NMR spectra and 2-D 31P
CPCOSY spectra of complexes 1–5 are shown in Figs. 4 and 5,
respectively. In general, the spectra consist of partially resolved
multiplets due to the individual 31P chemical shifts emanating
from the chemically inequivalent phosphorus nuclei bound
to each Ag. Each resonance displays 1J(107,109Ag,31P) scalar
coupling between the 31P (I = 1/2) and 107,109Ag nuclei (I = 1/2),
and further correlation between these doublets is established
via strong 2J(31P,31P) coupling in which each 31P doublet is
also scalar coupled to the other 31P doublet, thus yielding
complicated fine structure. A more detailed and complete
analysis of the coupling is obtained from the 31P 2-D CPCOSY
spectra of Fig. 5, and values of all 1J(Ag,P) and most
2J(P,P) coupling constants can be determined. The 31P
NMR parameters measured from these spectra are listed in
Table 4.
The 31P CPMAS spectra of [Ag2(PPh3)4(SO4)]ؒ2H2O at 9.40
and 17.62 T shown in Fig. 4(a) appear as a single AMX-type
multiplet. This is contrary to the result of the crystal structure
determination which suggests that 4 inequivalent phosphorus
sites should exist. There was no detectable change to the 31P
CPMAS spectrum of 1 upon reduction of the sample temper-
ature to 200 K (i.e. the temperature at which X-ray structural
analyses were performed), which confirmed that dynamic
processes were not responsible for averaging any part of the
spectrum at the higher ambient temperatures. The fact that
2J(P,P) coupling is evident means that the observed ‘equiv-
alence’ (or accidental overlap of chemical shifts) of the P atoms
involves those attached to different silver centres, so that the two
P2Ag units comprising the dimer ‘appear’ equivalent, although
this is not dictated by crystal symmetry. The 17.62 T spectrum
did not result in further resolution of the signals, but broader
lines and reduced resolution of the 2J(P,P) coupling in the
multiplet at δ 16.7 suggest a marginally greater chemical shift
separation for the two phosphorus sites giving rise to this
signal. The situation for [Ag2(PPh3)4(SeO4)]ؒ2H2O, which is
isostructural with 1, is a marked contrast where the complex
spectra at 9.40 and 17.72 T (see Fig. 4(b)) consist of two over-
lapping AMX multiplets (i.e. 4 doublets of doublets). Here the
multiplet structure from each site is sufficiently chemically
shifted to be almost completely resolved. It is interesting from
Fig. 4(a) and (in particular) 4(b) that although greater chem-
ical shift dispersion (in hertz) is introduced at 17.62 T, the
intrinsic resolution defining the J(Ag,P) and J(P,P) structure
is reduced, a phenomenon that has been attributed to bulk
susceptibility broadening.28 The 31P 2-D CPCOSY results at
9.40 T of complexes 1 and 2 shown in Fig. 5(a) and (b)
substantiate the above observations. The CPCOSY spectrum
for [Ag2(PPh3)4(SeO4)]ؒ2H2O clearly displays all 16 resonances
to be fully resolved as off-diagonal correlations, while that for
[Ag2(PPh3)4(SO4)]ؒ2H2O shows only a single multiplet exhibit-
ing off-diagonal correlations which are diffuse and poorly
resolved. Furthermore, the latter spectrum shows evidence of
additional correlations on the high-field side of the primary
multiplet structure, indicating the presence of further multiplet
structure. This is consistent with the presence of two P2Ag units
Fig. 4 Solid-state 31P CPMAS NMR spectra acquired at 9.40 and
17.62 T of (a) [Ag2(PPh3)4(SO4)]ؒ2H2O 1, (b) [Ag2(PPh3)4(SeO4)]ؒ2H2O
2, (c) [Ag(PPh3)2(HSO4)]ؒH2O 3, (d) [Ag(PPh3)2(HSeO4)]ؒH2O 4 and (e)
[Ag(PPh3)2(H2PO4)]ؒ2EtOH 5. All chemical shifts are relative to solid
PPh3.
Table 4 Solid-state CPMAS 31P NMR parameters
1J(Ag,P)/
Hz
2J(P,P)/
Hz
Complex
δ(31P)a
[Ag2(PPh3)4(SO4)]ؒ2H2O
[Ag2(PPh3)4(SeO4)]ؒ2H2O
16.7 0.2
19.6 0.2
15.4 0.1
21.5 0.1
15.5 0.1
20.1 0.1
15.6 0.2
17.4 0.2
19.2 0.3
16.7 0.3
19.5 0.3
14.1 0.1
21.6 0.1
14.8 0.1
453 10
492 10
148
148
148
148
148
148
148
148
—
—
—
148
148
—
5
5
3
3
3
3
5
5
352
531
414
484
5
5
5
5
[Ag(PPh3)2(HSO4)]ؒH2O
469 10
484 10
492 10
437 15
488 15
1
2
[Ag(PPh3)2(HSeO4)]ؒH2O
[Ag(PPh3)2(H2PO4)]ؒ2EtOH
406
492
—
5
5
3
3
a Relative to solid PPh3.
that are only very slightly chemically shifted with respect to
each other.
The 1- and 2-D spectra of [Ag(PPh3)2(HSO4)]ؒH2O (see
Figs. 4(c) and 5(c)) are assigned as a superposition of 2 ABX
24
J. Chem. Soc., Dalton Trans., 2001, 20–28

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Fig. 5 Solid-state 31P 2-D CPCOSY spectra acquired at 9.40 T of complexes 1–5. Details as in Fig. 4.
multiplets. The more strongly coupled ABX multiplet at lower
but in this case both multiplets closely resemble A2X-type mani-
field (δ ≈19.2) gives the appearance at 9.40 T of an A2X pattern,
but at very high field some partially resolved 2J(P,P) fine
structure emerges, confirming its ABX nature. The more weakly
coupled ABX multiplet at higher field (based around shifts of
δ 15.6 and 17.4) is responsible for the more resolved structure
evident at both fields This is consistent with the crystal struc-
ture of complex 3 as depicted in Fig. 2, which exhibits two quite
different monomeric P2Ag moieties in the unit cell. The close-
ness of the Ag–P bond distances given in Table 2 suggests that
both P2Ag moieties should exhibit near chemical equivalence in
the 31P multiplets, with the P2Ag(2) moiety expected to produce
the more nearly equal chemical shifts. Thus, the lower field
multiplet is assigned to this unit. From the 31P 2-D CPCOSY
data of Fig. 5(c) the ABX sub-spectrum of the P2Ag(1) moiety
produces 2J(P,P) coupling which is readily observed as off-
diagonal correlations located very close to the main diagonal.
The 1-D spectrum of [Ag(PPh3)2(HSeO4)]ؒH2O at 9.40 T (see
Fig. 4(d)) is similar in appearance to that of its HSO4 analogue,
folds. The CPCOSY spectrum of Fig. 5(d) at this field shows
that no off-diagonal correlations exist, confirming that both
P2Ag moieties may be described as strongly coupled A2X spin
systems. However, the 17.62 T spectrum (see Fig. 4(d)) shows
that only the low field multiplet at δ 19.5 is a true A2X spin
system, while the higher field multiplet at δ ≈16.7 is more
accurately described as an ABX system with partially resolved
2J(P,P) structure now being observed. The A2X multiplet
(doublet) at δ 19.5 is assigned to the P2Ag(2) moiety because of
the near equality of the two Ag–P bond lengths for this unit
(Table 2).
The spectra of [Ag(PPh3)2(H2PO4)]ؒ2EtOH shown in Fig.
4(e) consist of a single resonance due to the H2PO4 phosphorus
atom, and a superimposed AMX multiplet due to the P2Ag
unit. Coupling correlations in this latter system are clearly
evident in the corresponding CPCOSY spectrum of Fig. 5(e).
The 1J(Ag,P) coupling constants in Table 4 cover a wide
range, from 352 to 531 Hz. It has previously been shown that
J. Chem. Soc., Dalton Trans., 2001, 20–28
25

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Fig. 6 Plot of 1J(Ag,P) vs. the Ag–P bond length r(AgP) for
[Ag2(PPh3)4(SeO4)]ؒ2H2O (᭹) and [Ag(PPh3)n(NO3)] (n = 1–4) (᭿).
Inset: best fit curve for [Ag2(PPh3)4(SeO4)]ؒ2H2O (᭺), with data for
[Ag(PPh3)2(HSO4)]ؒH2O (᭝), [Ag(PPh3)2(HSeO4)]ؒH2O (᭞) and [Ag-
(PPh3)2(H2PO4)]ؒ2EtOH (᭛).
1
one of the factors that determines the magnitude of J(M,P)
coupling constants in metal–phosphine complexes is the
number of phosphine ligands coordinated.29–33 The averages of
the two 1J(Ag,P) coupling constants for the various P2Ag
groups in the compounds listed in Table 4 lie in the range
440–490 Hz, and these values are similar to those reported
previously for other silver complexes containing an oxyanion
and two coordinated PPh3.5,29 However, it is clear from the
results in Table 4 that the 1J(Ag,P) values of compounds with a
fixed number of coordinated phosphines still cover a consider-
able range, suggesting that other factors are also important
in determining the magnitude of this parameter. An obvious
factor is the strength of the Ag–P bond, which is reflected in
the Ag–P bond length; [Ag2(PPh3)4(SeO)4]ؒ2H2O is a unique
example which provides four completely resolved coupling
constants from four Ag–P bonds of different length in a single
compound. For this complex it was possible to resolve all four
31P chemical shifts, and an analysis of the multiplet positioning
in the 2-D CPCOSY spectrum facilitates assignment of pairs of
1J(Ag,P) values unambiguously to one of the two Ag atoms in
the structure. With the additional assumption that the larger
of the two coupling constants is associated with the shorter of
the two bonds, the plot of 1J(Ag,P) vs. the bond length r(AgP)
shown in Fig. 6 is obtained. It is noted that this assumption
gives the internally consistent result that Ag(1), which exhibits
the larger difference in r(AgP), also exhibits the larger differ-
ence in 1J(Ag,P) values in comparison to Ag(2). Also shown in
Fig. 7 Mid-range IR difference spectra showing bands due to the oxy-
anions in (a) [Ag2(PPh3)4(SO4)]ؒ2H2O/[Ag2(PPh3)4(SeO4)]ؒ2H2O, (b)
[Ag(PPh3)2(HSO4)]ؒH2O, (c) [Ag(PPh3)2(HSeO4)]ؒH2O and (d) [Ag-
(PPh3)2(H2PO4)]ؒ2EtOH. Spectrum (a) was obtained by subtraction
of the selenate spectrum from the sulfate spectrum; positive bands are
due to sulfate and negative bands to selenate. Spectra (b)–(d) were
obtained by subtracting the spectrum of coordinated PPh3 (see text).
The unlabelled features are residuals of the much stronger PPh3 bands
in the original spectra.
many bands due to the PPh3 ligand. The presence of the latter
bands interferes with the assignment of the oxyanion bands,
but it is possible to minimize this interference by means of
spectral subtraction. This was done in two ways, the results
of which are shown in Fig. 7. For the sulfate and selenate
compounds, direct subtraction of the spectrum of 2 from that
of 1 results in almost complete elimination of the bands due to
2Ϫ
2Ϫ
coordinated PPh3, leaving bands due to the SO4 and SeO4
ions as positive and negative features respectively (Fig. 7(a)).
The same method worked reasonably well for 3 and 4, but
because of partial overlap of the HSO4Ϫ and HSeO4Ϫ bands an
alternative method was adopted which was also applicable to
the H2PO4Ϫ complex 5. This method involves extraction of the
spectrum of coordinated PPh3 from the spectrum of the selen-
ate complex 2, where the overlap of the oxyanion and PPh3
bands is minimal, and subtracting this from the spectra of the
complexes 3–5 (Fig. 7(b)–(d)).
1
Fig. 6 is a plot of J(Ag,P) vs. r(AgP) for [Ag(PPh3)n(NO3)]
(n = 1–4) using previously reported data.29 From this it is clear
1
that the dependence of J(Ag,P) on bond length is similar for
the two different types of compound. Furthermore, the data
for [Ag(PPh3)2(HSO4)]ؒH2O, [Ag(PPh3)2(HSeO4)]ؒH2O and
[Ag(PPh3)2(H2PO4)]ؒ2EtOH all lie very close to the curve for
[Ag2(PPh3)4(SeO4)]ؒ2H2O in Fig. 6. It is noteworthy that [Ag2-
(PPh3)4(SO4)]ؒ2H2O, which shows almost identical Ag–P bond
lengths to those of the selenate analogue, does not show the
same range of 1J(Ag,P) values, due to the unexpected near
equivalence of the two P2Ag units as described above.
The wavenumbers of the bands assigned to vibrations of the
oxyanions are compared with those for the uncomplexed
oxyanions34–36 in Table 5. The symmetry types and activities
nϪ
of the normal modes of the free EO4 species (point group
symmetry Td) are A1(R) ϩ E(R) ϩ 2T2(IR, R). The binding
Vibrational spectroscopy
The mid-range IR spectra (4000–400 cmϪ1) of complexes 1–5
of a proton to these ions causes considerable band shifts and
Ϫ
splittings, as is evident in the spectra of HSO4 (Table 5). The
contain bands due to the coordinated oxyanions, together with
most significant effect of protonation is a lowering of the
26
J. Chem. Soc., Dalton Trans., 2001, 20–28

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Table 5 Wavenumbers (cmϪ1) of the vibrations of the EO4 (E = S, Se or P) groups in complexes 1–5,a and of the correspondng uncomplexed
oxyanions
Compound
ν1(A1)
ν3(T2)
ν2(E)
ν4(T2)
[Ag2(PPh3)4(SO4)]ؒ2H2O 1
[Ag2(PPh3)4(SeO4)]ؒ2H2O 2
[Ag(PPh3)2(HSO4)]ؒH2O 3
[Ag(PPh3)2(HSeO4)]ؒH2O 4
[Ag(PPh3)2(H2PO4)]ؒ2EtOH 5
942(R)
818(R)
1060
866, 860(R)
1050, 1045(R)
983
833
1040
866
1070
1134, 1103, 1076, 1055
882, 838
1221, 1186, 1167, 1153, 873, 847
932, 894, 723, 698
1132, 947, 883, 876
1105
875
1195, 895
921, 743
1150, 945, 880
614, 600
435
586
389
536
611
432
594
396
357
328
378
450
335
411
324
393, 360
2Ϫ b
SO4
2Ϫ b
SeO4
Ϫ c
HSO4
Ϫ d
HSeO4
Ϫ e
H2PO4
515
a IR bands are unlabelled; Raman bands are labelled (R). b Ref. 31. c Ref. 32. d This work, measured from the Raman spectrum of a ca. 4 mol LϪ1
aqueous solution of KHSeO4. e Ref. 33.
frequency of one of the T2 ν(E–O) modes, and these lower
frequency modes are described as ν(E–OH) modes, being
mainly due to a ν(E–O) vibration involving the protonated
oxygen atom. Thus, the ν(S–OH) band of HSO4Ϫ is assigned at
Ϫ
895 cmϪ1 35
.
Similarly, two ν(P–OH) bands occur for H2PO4
,
at 945, 880 cm .
Ϫ1 36 It is evident from the results in Table 5 that,
although coordination of these oxyanions to silver causes some
perturbation of the anion vibrations, the bands do not shift
very far from those of the uncomplexed parent anions, and can
be assigned on the same basis. For the sulfate and selenate
complexes 1 and 2, the T2 ν(E–O) band ν3 is split due to the low
symmetry environment of the anion in these complexes (Fig. 1),
and the average frequency of the band is slightly lower than that
of the free anion. For the hydrogen-sulfate and -selenate com-
pounds 3 and 4, the ν(E–O) bands derived from ν3 of the parent
2Ϫ
EO4 cover a much wider range than in 1 and 2, but these
bands can readily be assigned on the basis of the assignments
discussed above for HSO4Ϫ. Thus, the band at about 850 cmϪ1
for 3 correlates with ν(S–OH) at 895 cmϪ1 for HSO4Ϫ, the fre-
quency again being lowered by coordination to silver. The
group of bands around 1200 cmϪ1 correlates with ν(S–O) at
1195 cmϪ1 for HSO4Ϫ. This mode, which has E symmetry in the
free ion (point group C3v),35 is split due to the lower symmetry
environment and the presence of two inequivalent sites for the
ion in the complex (Fig. 2). The band at 1060 cmϪ1 correlates
with the band at 1040 cmϪ1 of HSO4Ϫ, which has been
described as the totally symmetric ν(S–O) stretch in the free
ion.35 Similar bands to those of 3 are observed for the hydro-
genselenate complex 4 (Table 5). The assignments of the PO4
group vibrations in the dihydrogenphosphate complex 5 follows
readily from those of free H2PO4Ϫ (Table 5).
The bands in the Raman spectra due to the oxyanions are
much weaker than those of triphenylphosphine, and only those
that derive from ν1(A1) of the EO4 groups were identified
(Table 5).
The far-IR spectra of complexes 1–5 are shown in Fig. 8. By
analogy with results for formate complexes [M(PPh3)n(O2CH)]
(M = Cu or Ag; n = 2 or 3), ν(Ag–O) modes are expected
Fig. 8 Far-IR spectra of (a) [Ag2(PPh3)4(SO4)]ؒ2H2O, (b) [Ag2(PPh3)4-
(SeO4)]ؒ2H2O, (c) [Ag(PPh3)2(HSO4)]ؒH2O, (d) [Ag(PPh3)2(HSeO4)]ؒ
H2O and (e) [Ag(PPh3)2(H2PO4)]ؒ2EtOH.
to occur below 300 cmϪ1 5,6
All of the spectra show a weak,
.
relatively sharp band at about 210 cmϪ1 which is attributed
to the PPh3 ligand, together with weak, broader bands (or
multiple bands) in the 150–300 cmϪ1 region which are possibly
due to ν(Ag–O). This is consistent with a shift in the position
of the band from approximately 200 cmϪ1 for the hydrogen-
sulfate complex 3 to about 260 cmϪ1 for the dihydrogen-
phosphate complex 5 (Fig. 8), since a shorter (and therefore
stronger) Ag–O bond is observed in this complex (2.328 Å,
Table 3) relative to 3 (2.410 Å, Table 2). We therefore assign
these bands as ν(Ag–O) modes, although some uncertainty
remains because of their low intensity. The far-IR spectra of
Ag3PO4 and Ag2HPO4 were also recorded, and these show
strong ν(Ag–O) bands at 202 and 234 cmϪ1 respectively. Given
that the bonding modes for the anions are different (quadruple
bridging in Ag3PO4,37 terminal in 5), and that bridging bond
vibrations normally occur at lower frequencies than terminal
bond vibrations, these assignments are quite consistent with
ν(Ag–O) ≈ 260 cmϪ1 in 5.
The present vibrational results for the coordinated oxyanions
nϪ
EO4 in their unprotonated and partially protonated forms
provide information that may assist in identification of these
anions in related bonding situations. Thus, a surface-enhanced
3Ϫ
Raman spectroscopy (SERS) study of the adsorption of PO4
and HPO42Ϫ ion on silver resulted in the assignment of ν(Ag–O)
at 230 and 240 cmϪ1 respectively,18 quite close to the value of ca.
260 cmϪ1 observed in the present study for the H2PO4Ϫ complex
5. In many of the studies of sulfate on metal surfaces, there is
2Ϫ
some uncertainty about whether the adsorbed species is SO4
J. Chem. Soc., Dalton Trans., 2001, 20–28
27

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Ϫ 12,14,16,17,38–43
17 H. Uchida, M. Hiei and M. Watanabe, J. Electroanal. Chem., 1998,
or HSO4
.
This is rather surprising in view of the
452, 97.
results described herein, which show that the effect of a bound
proton on the SO4 vibrations is much greater than that of an
18 G. Niaura, A. K. Gaigalas and V. L. Vilker, J. Phys. Chem. B, 1997,
101, 9250.
2Ϫ
attached metal atom, so that a distinction between SO4 and
19 G. A. Bowmaker, Effendy, J. V. Hanna, P. C. Healy, B. W. Skelton
and A. H. White, J. Chem. Soc., Dalton Trans., 1993, 1387.
20 J. V. Hanna and S. W. Ng, Acta Crystallogr., Sect. C, 2000, 56, 24.
21 G. M. Sheldrick, SHELX 97, Program for Refinement of Crystal
Structures, University of Göttingen, 1997.
HSO4Ϫ on the basis of vibrational spectroscopy should be pos-
sible. In particular, the strong activation of the ν1(A1) band in
the IR, and the presence of a low frequency ν(S–OH) IR band
should be diagnostic of HSO4
Ϫ
.
22 A. Pines, M. G. Gibby and J. S. Waugh, J. Chem. Phys., 1973, 59,
569.
23 E. R. Andrew, A. Bradbury and R. Eades, Nature (London), 1958,
182, 1659.
Acknowledgements
24 E. O. Stejskal and J. Schaefer, J. Magn. Reson., 1975, 18, 560.
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G. A. B. and J. V. H. would like to thank the Australian Institute
of Nuclear Science and Engineering (AINSE) for funding of
AINSE Project No. 00/011. The very high field (17.6 T) NMR
data presented were collected in the Environmental Molecular
Sciences Laboratory, a national scientific user facility spon-
sored by the Department of Energy’s Office of Biological and
Environmental Research and located at the Pacific Northwest
National Laboratory.
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