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DOI: 10.1039/B007796H (Paper) J. Chem. Soc., Dalton Trans., 2001, 20-28

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. Rickard a and Andrew S. Lipton c
aDepartment of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand
bANSTO NMR Facility, Materials Division, Private Mail Bag 1, Menai, N.S.W. 2234, Australia
cEnvironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington, 99352, USA

Received (in Cambridge, UK) 26th September 2000 , Accepted 8th November 2000

First published on 7th December 2000

Abstract

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 EO42− 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)]·2EtOH 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 EO42− is much greater than that of an attached metal atom.

Introduction

The many structural studies of silver(I) complexes that have been reported demonstrate the structural versatility of silver(I) 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(I) complexes involving simple multiply charged inorganic anions such as sulfate or phosphate. Such ions can be coordinated in the fully deprotonated form (e.g. SO42−) or in a protonated form (e.g. HSO4−). A structure reported some years ago for the silver(III) complex [Ag(C6H16N10)]SO4[HSO4]·H2O (C6H16N10 = ethylenebis(biguanide)) reveals the presence of monodentate SO42− and HSO4− in the same molecule, with the somewhat unexpected result that the Ag–O distance for SO42− is greater than that for HSO4−.7 A few silver complexes involving SO42− 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 3via the isoelectronic relationship between HSO4− and H2PO4−. We have also studied these and some related compounds by vibrational and solid-state CPMAS NMR spectroscopy.

Vibrational spectroscopy is expected to provide information about the nature of the coordinated anion (e.g. whether protonated or not) and on its mode of coordination. This method is potentially useful in situations where such information cannot be obtained directly from single-crystal structure determinations. 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 characterizing the surface species. It is true that the coordination compounds studied here involve the metal in the formal +1 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 +1,13,15 so these two types of system may be more closely related than is apparent at first sight.

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 homonuclear 1J(31P,31P) scalar (or spin–spin) couplings when chemical inequivalence between each phosphorus site exists.5,6,19,20 In particular, the 1J(Ag,P) coupling constant is a useful probe of the coordination of the metal centres within these complexes. In this case, each phosphorus position is represented as a doublet 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 separations exist between inequivalent 2J(P,P) coupled sites (and A2X or ABX spin systems are described), the use of 2-D techniques alone is not sufficient and data acquisition at significantly 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 measurement 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.

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 temperature, 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%.
[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%.
[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 C36H33AgO5P2Se: C 54.4, H 4.2%.
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 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 °C 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%).
[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 F[hair space]2 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[hair space]| and Rw on |F[hair space]|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).
Crystal/refinement data. [Ag2(PPh3)4(SO4)]·2H2O 1 ≡ C72H64Ag2O6P4S, M = 1396.91, triclinic, space group P[1 with combining macron] (C[hair space]1 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)°, V = 3173.6 Å3, Z = 2, μ = 8.04 cm−1, N = 13366, No (I > 2σ(I[hair space])) = 11528, R = 0.048, Rw = 0.120.

[Ag2(PPh3)4(SeO4)]·2H2O 2 ≡ C72H64Ag2O6P4Se, M = 1443.81, triclinic, space group P[1 with combining macron] (C[hair space]23i, 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)°, V = 3169.1 Å3, Z = 2, μ = 13.35 cm−1, N = 13379, No (I > 2σ(I[hair space])) = 11996, R = 0.022, Rw = 0.053.

[Ag(PPh3)2(HSO4)]·H2O 3 ≡ C36H33AgO5P2S, M = 747.49, triclinic, space group P[1 with combining macron] (C[hair space]23i, 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)°, V = 3420.9 Å3, Z = 4, μ = 7.84 cm−1, N = 14368, No (I > 2σ(I[hair space])) = 12199, R = 0.034, Rw = 0.083.

[Ag(PPh3)2(HSeO4)]·H2O 4 ≡ C36H33AgO5P2Se, M = 794.39, triclinic, space group P[1 with combining macron] (C[hair space]23i, 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)°, V = 3460.4 Å3, Z = 4, μ = 17.67 cm−1, N = 14255, No (I > 2σ(I[hair space])) = 11986, R = 0.057, Rw = 0.117.

[Ag(PPh3)2(H2PO4)]·2EtOH 5 ≡ C40H44AgO6P3, M = 821.53, triclinic, space group P[1 with combining macron] (C[hair space]23i, 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)°, V = 1963.2 Å3, Z = 2, μ = 6.80 cm−1, N = 8235, No (I > 2σ(I[hair space])) = 7452, R = 0.024, Rw = 0.063.

CCDC reference number 186/2267.

See http://www.rsc.org/suppdata/dt/b0/b007796h/ for crystallographic files in .cif format.

Spectroscopy

Infrared spectra were recorded at 4 cm−1 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 cm−1 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 mm−1 wire mesh beam splitter, a mercury lamp source and a pyroelectric triglycine sulfate detector. Raman spectra were recorded at 4.5 cm−1 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-polarization[hair space]22 and magic-angle-spinning[hair space]23 techniques, coupled with spin temperature alternation[hair space]24 to eliminate spectral artifacts, 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 transformation. The 2-D 31P CPCOSY experiment was implemented with the TPPI (time proportional phase incrementation) method[hair space]25 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 period, 1H 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 transformation. All 31P chemical shifts were externally referenced to triphenylphosphine which has a shift of δ −9.9 with respect to 85% H3PO4. Solid-state 13C CPMAS NMR spectra were obtained at ambient temperature on a Bruker CXP-90 spectrometer 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 and initial 1H π/2 pulse width of 3.5 μs were common to all 13C 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.

Results and discussion

The 1∶2 silver(I) sulfate or selenate complexes with triphenylphosphine are readily prepared by reaction of stoichiometric quantities of the reagents in aqueous ethanol. The corresponding 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 crystallization, eqn. (1). If insufficient water is present in these reactions, 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.
 
Ag2EO4 + 4PPh3 + H2EO4 → 2[Ag(PPh3)2(HEO4)] (1)

The 1∶2 silver dihydrogenphosphate complex with triphenylphosphine was obtained by a reaction analogous to (1); eqn. (2). In this case the stable ethanol disolvate 5 was obtained from ethanol solution.

 
 Ag3PO4 + 6PPh3 + 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 essentially three-coordinate with almost planar environments, although there is a weak secondary Ag(2)[thin space (1/6-em)]⋯[thin space (1/6-em)]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]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.
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) 2.5018(11) 2.4994(4)
Ag(1)–P(2) 2.4190(11) 2.4105(5)
Ag(1)–O(1) 2.235(4) 2.2342(15)
Ag(2)–P(3) 2.4625(11) 2.4590(4)
Ag(2)–P(4) 2.4380(11) 2.4289(4)
Ag(2)[thin space (1/6-em)]⋯[thin space (1/6-em)]O(3) 2.643(4) 2.6507(16)
Ag(2)–O(4) 2.394(3) 2.4025(13)
E–O(1) 1.472(4) 1.6403(15)
E–O(2) 1.464(4) 1.6271(13)
E–O(3) 1.466(4) 1.6394(14)
E–O(4) 1.489(3) 1.6489(13)
     
P(1)–Ag(1)–P(2) 128.04(4) 128.353(16)
P(1)–Ag(1)–O(1) 92.81(11) 90.38(4)
P(2)–Ag(1)–O(1) 136.65(12) 138.40(5)
Ag(1)–O(1)–E 129.6(2) 127.94(8)
P(3)–Ag(2)–P(4) 128.60(4) 128.755(16)
P(3)–Ag(2)–O(3) 113.73(10) 112.73(4)
P(4)–Ag(2)–O(3) 112.04(10) 111.74(4)
P(3)–Ag(2)–O(4) 102.97(9) 100.36(4)
P(4)–Ag(2)–O(4) 121.36(9) 122.21(4)
O(3)–Ag(2)–O(4) 56.42(11) 62.46(4)
Ag(2)–O(3)–E 92.79(18) 91.17(6)
Ag(2)–O(4)–E 102.63(17) 100.18(6)


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.
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.

All four PPh3 ligands in complexes 1 and 2 are crystallographically 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 128.8°. 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 HEO4− groups are terminally bound to the two silver atoms Ag(1) and Ag(2) via O(1) and O(5) respectively. The structures 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 HEO4− should reduce the coordinating ability of this species relative to EO42−. 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. 131°) are slightly greater than those in 1 and 2 (ca. 128°), in agreement with the above conclusion that the Ag–O bonding involving HEO4− is weaker than that involving EO42−.

Table 2 Core geometries for [Ag(PPh3)2(HEO4)]·H2O (E = S 3 or Se 4)
  3 (E = S) 4 (E = Se)
Ag(1)–P(1) 2.4316(7) 2.4405(13)
Ag(1)–P(2) 2.4394(7) 2.4349(13)
Ag(1)–O(1) 2.410(2) 2.421(4)
Ag(2)–P(3) 2.4300(7) 2.4278(15)
Ag(2)–P(4) 2.4265(8) 2.4275(14)
Ag(2)–O(5) 2.412(2) 2.402(4)
Ag(2)–O(9) 2.446(3) 2.439(5)
E(1)–O(1) 1.431(2) 1.589(4)
E(1)–O(2) 1.436(2) 1.596(4)
E(1)–O(3) 1.538(3) 1.626(6)
E(1)–O(4) 1.472(3) 1.687(6)
E(2)–O(5) 1.441(2) 1.611(4)
E(2)–O(6) 1.532(2) 1.618(5)
E(2)–O(7) 1.470(3) 1.689(4)
E(2)–O(8) 1.450(3) 1.624(5)
     
P(1)–Ag(1)–P(2) 131.38(2) 131.13(5)
P(1)–Ag(1)–O(1) 113.72(8) 100.99(15)
P(2)–Ag(1)–O(1) 103.15(8) 113.66(15)
Ag(1)–O(1)–E(1) 133.14(14) 129.03(24)
P(3)–Ag(2)–P(4) 130.48(3) 131.20(5)
P(3)–Ag(2)–O(5) 108.28(6) 108.07(12)
P(4)–Ag(2)–O(5) 110.46(6) 108.19(11)
P(3)–Ag(2)–O(9) 107.32(11) 106.9(2)
P(4)–Ag(2)–O(9) 105.70(13) 105.66(19)
O(5)–Ag(2)–O(9) 84.92(10) 88.45(18)
Ag(2)–O(5)–E(2) 136.00(14) 129.76(21)


Molecular structure of [Ag(PPh3)2(HSO4)]·H2O 3. Details as for Fig. 1.
Fig. 2 Molecular structure of [Ag(PPh3)2(HSO4)]·H2O 3. Details as for Fig. 1.

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 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 significantly longer than the others. The Ag–O(1) distance of 2.3280(12) Å is significantly shorter than the shortest Ag–O distances (ca. 2.41 Å) in the HEO4− complexes 3 and 4, consistent with the fact that H2PO4− is a stronger base than HSO4− or HSeO4−. This is also reflected in fact that the P–Ag–P angle in 5 (ca. 128°) is smaller than those in 3 and 4 (ca. 131°). The solvate ethanol molecules in 5 are hydrogen bonded to the phosphate oxygen atom O(2), with O(2)[thin space (1/6-em)]⋯[thin space (1/6-em)]O(5) 2.680(2) and O(2)[thin space (1/6-em)]⋯[thin space (1/6-em)]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[thin space (1/6-em)]⋯[thin space (1/6-em)]O(2) 2.838(1) Å].

Table 3 Core geometries for [Ag(PPh3)2(H2PO4)]·2EtOH 5
Ag–P(1) 2.4309(4) P(1)–Ag–O(1) 123.96(3)
Ag–P(2) 2.4595(4) P(2)–Ag–O(1) 107.27(3)
Ag–O(1) 2.3280(12) P(1)–Ag–P(2) 127.671(14)
P(3)–O(1) 1.5051(12) P(3)–O(1)–Ag 103.77(6)
P(3)–O(2) 1.5169(13)    
P(3)–O(3) 1.5759(14)    
P(3)–O(4) 1.5759(14)    


Molecular structure of [Ag(PPh3)2(H2PO4)]·2EtOH 5. Details as for Fig. 1.
Fig. 3 Molecular structure of [Ag(PPh3)2(H2PO4)]·2EtOH 5. Details as for Fig. 1.

Solid-state NMR spectroscopy

The solid-state 13C CPMAS NMR spectra of complexes 1–4 exhibit only one broad unresolved resonance at δca. 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.

Table 4 Solid-state CPMAS 31P NMR parameters
Complex δ(31P)[hair space]a 1 J(Ag,P)/Hz 2 J(P,P)/Hz
a Relative to solid PPh3.
[Ag2(PPh3)4(SO4)]·2H2O 16.7 ± 0.2 453 ± 10 148 ± 5
  19.6 ± 0.2 492 ± 10 148 ± 5
[Ag2(PPh3)4(SeO4)]·2H2O 15.4 ± 0.1 352 ± 5 148 ± 3
  21.5 ± 0.1 531 ± 5 148 ± 3
  15.5 ± 0.1 414 ± 5 148 ± 3
  20.1 ± 0.1 484 ± 5 148 ± 3
[Ag(PPh3)2(HSO4)]·H2O 15.6 ± 0.2 469 ± 10 148 ± 5
  17.4 ± 0.2 484 ± 10 148 ± 5
  19.2 ± 0.3 492 ± 10 —
[Ag(PPh3)2(HSeO4)]·H2O 16.7 ± 0.3 437 ± 15 —
  19.5 ± 0.3 488 ± 15 —
[Ag(PPh3)2(H2PO4)]·2EtOH 14.1 ± 0.1 406 ± 5 148 ± 3
  21.6 ± 0.1 492 ± 5 148 ± 3
  14.8 ± 0.1 — —


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.
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.

Solid-state 31P 2-D CPCOSY spectra acquired at 9.40 T of complexes 1–5. Details as in Fig. 4.
Fig. 5 Solid-state 31P 2-D CPCOSY spectra acquired at 9.40 T of complexes 1–5. Details as in Fig. 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 temperature 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 ‘equivalence’ (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 overlapping 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 chemical shift dispersion (in hertz) is introduced at 17.62 T, the intrinsic resolution defining the 1J(Ag,P) and 2J(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 exhibiting 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 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 multiplets. The more strongly coupled ABX multiplet at lower field (δ ≈[hair space]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 structure of complex 3 as depicted in Fig. 2, which exhibits two quite different monomeric P2Ag moieties in the unit cell. The closeness 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, but in this case both multiplets closely resemble A2X-type manifolds. 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 one of the factors that determines the magnitude of 1J(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 considerable 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 difference in 1J(Ag,P) values in comparison to Ag(2). Also shown in Fig. 6 is a plot of 1J(Ag,P) vs. r(AgP) for [Ag(PPh3)n(NO3)] (n = 1–4) using previously reported data.29 From this it is clear that the dependence of 1J(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.


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 (⋄).
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 (⋄).

Vibrational spectroscopy

The mid-range IR spectra (4000–400 cm−1) of complexes 1–5 contain bands due to the coordinated oxyanions, together with 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 coordinated PPh3, leaving bands due to the SO42− and SeO42− 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 HSO4− and HSeO4− bands an alternative method was adopted which was also applicable to the H2PO4− complex 5. This method involves extraction of the spectrum of coordinated PPh3 from the spectrum of the selenate 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)).
Mid-range IR difference spectra showing bands due to the oxyanions 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.
Fig. 7 Mid-range IR difference spectra showing bands due to the oxyanions 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.

The wavenumbers of the bands assigned to vibrations of the oxyanions are compared with those for the uncomplexed oxyanions[hair space]34–36 in Table 5. The symmetry types and activities of the normal modes of the free EO4n− species (point group symmetry Td) are A1(R) + E(R) + 2T2(IR, R). The binding of a proton to these ions causes considerable band shifts and splittings, as is evident in the spectra of HSO4− (Table 5). The most significant effect of protonation is a lowering of the frequency of one of the T2ν(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 HSO4− is assigned at 895 cm−1.35 Similarly, two ν(P–OH) bands occur for H2PO4−, at 945, 880 cm−1.36 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 T2ν(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 compounds 3 and 4, the ν(E–O) bands derived from ν3 of the parent EO42− 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 HSO4−. Thus, the band at about 850 cm−1 for 3 correlates with ν(S–OH) at 895 cm−1 for HSO4−, the frequency again being lowered by coordination to silver. The group of bands around 1200 cm−1 correlates with ν(S–O) at 1195 cm−1 for HSO4−. 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 cm−1 correlates with the band at 1040 cm−1 of HSO4−, 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 hydrogenselenate complex 4 (Table 5). The assignments of the PO4 group vibrations in the dihydrogenphosphate complex 5 follows readily from those of free H2PO4− (Table 5).

Table 5 Wavenumbers (cm−1) of the vibrations of the EO4 (E = S, Se or P) groups in complexes 1–5,a and of the correspondng uncomplexed oxyanions
Compound ν 1(A1) ν 3(T2) ν 2(E) ν 4(T2)
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 L−1 aqueous solution of KHSeO4. e Ref. 33.
[Ag2(PPh3)4(SO4)]·2H2O 1 942(R) 1134, 1103, 1076, 1055   614, 600
[Ag2(PPh3)4(SeO4)]·2H2O 2 818(R) 882, 838 357 435
[Ag(PPh3)2(HSO4)]·H2O 3 1060 1221, 1186, 1167, 1153, 873, 847   586
[Ag(PPh3)2(HSeO4)]·H2O 4 866, 860(R) 932, 894, 723, 698 328 389
[Ag(PPh3)2(H2PO4)]·2EtOH 5 1050, 1045(R) 1132, 947, 883, 876 378 536
SO42−[hair space]b 983 1105 450 611
SeO42−[hair space]b 833 875 335 432
HSO4−[hair space]c 1040 1195, 895 411 594
HSeO4−[hair space]d 866 921, 743 324 396
H2PO4−[hair space]e 1070 1150, 945, 880 393, 360 515

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 to occur below 300 cm−1.5,6 All of the spectra show a weak, relatively sharp band at about 210 cm−1 which is attributed to the PPh3 ligand, together with weak, broader bands (or multiple bands) in the 150–300 cm−1 region which are possibly due to ν(Ag–O). This is consistent with a shift in the position of the band from approximately 200 cm−1 for the hydrogensulfate complex 3 to about 260 cm−1 for the dihydrogenphosphate 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 cm−1 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 cm−1 in 5.


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.
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.

The present vibrational results for the coordinated oxyanions EO4n− 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 Raman spectroscopy (SERS) study of the adsorption of PO43− and HPO42− ion on silver resulted in the assignment of ν(Ag–O) at 230 and 240 cm−1 respectively,18 quite close to the value of ca. 260 cm−1 observed in the present study for the H2PO4− complex 5. In many of the studies of sulfate on metal surfaces, there is some uncertainty about whether the adsorbed species is SO42− or HSO4−.12,14,16,17,38–43 This is rather surprising in view of the results described herein, which show that the effect of a bound proton on the SO4 vibrations is much greater than that of an attached metal atom, so that a distinction between SO42− and HSO4− on the basis of vibrational spectroscopy should be possible. 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−.

Acknowledgements

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 sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory.

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