Mercury(II) polyphosphate, Hg(PO₃)₂

Single crystals of mercury(II) polyphosphate, Hg(PO₃)₂, were prepared from HgO in an acidic polyphosphate melt. The structure is isotypic with α-Cd(PO₃)₂ and comprises infinite polyphosphate chains with a period of four phosphate units. Chains of the form ¹_∞[PO₃⁻] are linked by Hg²⁺ to form a three-dimensional network. The Hg atom is located at the centre of a distorted octahedron of O atoms with distances 2.173 (5) < (Hg—O)_mean < 2.503 (6) Å. The [HgO₆] polyhedra form zigzag-like chains of the form ¹_∞[HgO₂O_4/2] parallel to the c axis.

S1. Comment

Anhydrous mercury(II) phosphates have been known for a long time (Gmelin, 1969), but only Hg₃(PO₄)₂ (Aurivillius & Nilsson, 1975) and Hg₂P₂O₇ (Weil & Glaum, 1997) have been characterized by single-crystal structure analysis so far. The first experiments to synthezize a compound with the molar ratio Hg:P:O = 1:2:6 were described by Glatzel (1880) and Warschauer (1903). Later, Thilo & Grunze (1957) showed by paper chromatography during their systematic investigations of the dehydration of acidic phosphates of divalent metals, that this phase is not a tetrametaphosphate. According to them, HgP₂O₆ is a high molecular polyphosphate, which is isotypic to the low temperature modification of cadmium polyphosphate, α-Cd(PO₃)₂. Some years later, for both polyphosphates, powder diagrams were indexed and lattice parameters given (Beucher & Tordjman, 1968). The structure of α-Cd(PO₃)₂ was solved from Weissenberg data by Tordjman et al. (1968) and refined some years later from single-crystal diffractometer data by Bagieu-Beucher et al. (1974).

In parallel with our investigations into the crystal chemistry of phosphates containing Hg₂²⁺, Hg₃⁴⁺ or Hg²⁺, we have focused our interest on the thermal behaviour of these compounds (Weil, 1999). In the case of Hg(PO₃)₂ this is of particular interest, because the isotypic compound α-Cd(PO₃)₂ is polymorphic and transforms into the cyclo-tetrametaphosphate Cd₂P₄O₁₂ (Laügt et al., 1973) and β-Cd(PO₃)₂ (Bagieu-Beucher et al., 1979). Therefore, detailed structural and thermal analyses of the title compound, Hg(PO₃)₂, have been undertaken and the results are presented here.

The structure of Hg(PO₃)₂ consists of polyphosphate chains of the type ¹_∞[PO₃⁻] extending parallel to the c axis. In a topological description, layers of polyphosphate chains and those formed by Hg atoms (Fig. 1) alternate along the a and b axes. The Hg atom shows sixfold distorted octahedral coordination by the terminal O atoms of the polyphosphate chains. The Hg—O distances are in the range 2.173 (5)–2.503 (6) Å, with a mean value of 2.346 Å, which is slightly longer and more irregular than for Cd—O in α-Cd(PO₃)₂, where the mean Cd—O distance is 2.301 Å (reference? Bagieu-Beucher et al., 1974)?

In the title compound, [HgO₆] units are linked via two cis edges, thus forming ¹_∞[HgO₂O_4/2] zigzag chains parallel to the c axis (Fig. 2). The phosphate groups show the typical distribution of P—O distances which is observed for many other polyphosphate structures (Durif, 1995). There are two longer P—O distances with a mean of 1.594 Å to the bridging atoms O5 and O6 and two shorter distances with a mean of 1.489 Å to the terminal atoms. The dihedral angle (P2/O5/P1/O6/P2) between two phosphate groups within the polyphosphate chain is 76.5 (3)°. The polyphosphate anion shows a period of four tetrahedra. The terminal oxygen atoms O1 and O4 with the shortest P—O bond distances also exhibit the shortest distances to the Hg. Except for O2 and O5, which are the bridging atoms within the Hg—O chains and which have CN = 3 (2 Hg, 1 P), all O atoms have CN = 2 (O5 and O6: 2 P, O1 and O4: 1 Hg, 1 P).

Surveys of the unique crystal chemistry of Hg²⁺ with its d¹⁰ electronic configuration have been published in the past (e.g. Grdenić, 1965; Aurivillius, 1965; Stålhandske, 1980; Müller-Buschbaum, 1995; Wessels, 1996), likewise the influence of relativistic effects (Norrby, 1991) for the preference of the linear coordination. Frequently, a [2 + x] coordination with two short axial distances or an [x + 2] coordination with two long axial distances is observed. In case of the anhydrous mercury(II) phosphates, for Hg₃(PO₄)₂ [2 + 3] and [2 + 4] coordination is found for the Hg atoms. The Hg—O distances range from 2.114–2.599 Å, with a mean of 2.389 Å, and the O_ax—Hg—O_ax angles range from 163.3–169.9° (Aurivillius & Nilsson, 1975)?. In mercury(II) diphosphate a [2 + 4] and a [4 + 2] coordination is observed, with Hg—O distances in the range 2.120–2.793 Å (mean 2.392 Å) and O_ax—Hg—O_ax angles in the range 166.0–176.1° (Weil & Glaum, 1997)?. For the title compound a [4 + 2] coordination and an O_ax—Hg—O_ax angle of 161.1° are observed. In HgHPO₄ (Dubler et al., 1981), two independent Hg atoms are surrounded in a [2 + 5] and a [2 + 4] coordination with Hg—O distances in the range 2.060–2.924 Å (mean 2.528 Å) and O_ax—Hg—O_ax angles in the range 160.6–166.6°. In other mercury(II)-containing phosphates listed in the ICSD (FIZ-Karlsruhe, 1998), such as HgLi₂(PO₃)₄ (Averbuch-Pouchot et al., 1976), HgNaPO₄ (Hata & Marumo, 1982), HgKP₃O₉ (Averbuch-Pouchot & Durif, 1986a), Hg(NH₄)₂Na₂(P₃O₉)₂ (Averbuch-Pouchot & Durif, 1986b) and HgV₂(P₂O₇)₂ (Boudin et al., 1994), similar coordination geometries around the Hg atom are found.

In contrast to α-Cd(PO₃)₂, which transforms into β-Cd(PO₃)₂ upon heating above 1008 K (Bagieu-Beucher et al., 1979), and under particular thermal conditions into Cd₂P₄O₁₂ (Thilo & Grunze, 1957; Laügt et al., 1973), neither a high-temperature modification of Hg(PO₃)₂ nor a transformation into the possible cyclo-tetraphosphate Hg₂P₄O₁₂ could be detected by thermal analysis nor temperature dependent Guinier photographs. Our own results confirm earlier observations performed by Thilo & Grunze (1957).

S2. Experimental

Microcrystalline Hg(PO₃)₂ was synthesized by heating a solution of HgO (e.g. 4.93 g, Merck, p·A) in half-concentrated nitric acid, together with a small excess (ca 2%) of the stoichiometric amount of 85% H₃PO₄ (5.34 g, Merck, pure) in a silica crucible up to 773 K for 3 d. After cooling the solid obtained was homogenized in an agate mortar and tempered again at 673 K for 1 d. According to Guinier photographs, the white product showed no impurities. Single crystals of Hg(PO₃)₂ were synthesized in a slight modification of the procedure given by Beucher (1968). HgO and H₃PO₄ (85%) were mixed in the molar ratio Hg:P = 1:11 (e.g. 0.747 g HgO, 4.46 g H₃PO₄) and heated in a glassy carbon crucible up to 693 K. The reaction mixture was kept at this temperature for about 3 d, then cooled within 10 h to 573 K and kept for 2 d. It was then quenched to room temperature. After extraction with hot water, colourless transparent crystals were obtained of mainly platelike crystal form with edge lengths up to 8 mm.

S3. Refinement

The lattice parameters given were refined with the program SOS (Soose, 1980), using 45 reflections from a Guinier photograph. The highest and lowest electron density in the final difference electron density map were found to be 0.06 and 0.66 Å, respectively, from the Hg atom.

Computing details

Data collection: STADI4 (Stoe & Cie, 1995); cell refinement: STADI4; data reduction: STADI4; program(s) used to solve structure: ???; program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS for Windows (Dowty, 1998); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The projection of the Hg(PO3)2 structure along [001]. Polyphosphate chains are white, ^1∞[HgO2O4/2] chains are grey.
	Fig. 2. A plot with displacement ellipsoids of a section of ^1∞[HgO2O4/2] and ^1∞[PO2O2/2] chains running parallel to the c axis. Ellipsoids are drawn at the 90% probability level [symmetry codes: (i) x, 1/2 − y, z − 1/2; (ii) x − 1/2, 1/2 − y, −z; (iii) x − 1/2, y, 1/2 − z; (iv) x − 1/2, 1/2 − y, 1 − z; (viii) 1/2 − x, 1 − y, 1/2 + z].

Mercury(II) polyphosphate

Crystal data

HgO6P2	Dx = 5.006 Mg m^−3
Mr = 358.53	Melting point: 612(10) K
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
a = 9.709 (2) Å	Cell parameters from 45 reflections
b = 13.748 (2) Å	θ = 6.4–25.9°
c = 7.128 (1) Å	µ = 32.97 mm^−1
V = 951.4 (3) Å^3	T = 293 K
Z = 8	Irregular, colourless
F(000) = 1264	0.28 × 0.18 × 0.12 mm

Data collection

Siemens AED-2 diffractometer	1223 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.082
Graphite monochromator	θmax = 30.0°, θmin = 3.0°
θ/2θ scans	h = −13→13
Absorption correction: numerical HABITUS (Herrendorf, 1993)	k = −19→19
Tmin = 0.023, Tmax = 0.085	l = −10→10
10112 measured reflections	3 standard reflections every 120 min
1379 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.034	Calculated w = 1/[σ^2(Fo^2) + (0.0367P)^2 + 18.4224P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.093	(Δ/σ)max < 0.001
S = 1.11	Δρmax = 3.01 e Å^−3
1379 reflections	Δρmin = −2.88 e Å^−3
83 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0032 (2)

Special details

Experimental. The crystal shape consisting of 14 faces was optimized by minimizing the internal R-value of the ψ-scan data of 48 selected reflections with the programme HABITUS (Herrendorf, 1993). The derived HABITUS has been used for the numerical absorption correction.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Data collection by the peak/background method, reduction of data using the software package STADI4 (Stoe, 1995). Scattering factors, dispersion corrections and absorption coefficients were taken from International Tables for Crystallography, Vol. C (1992), tables 6.1.1.4., 4.2.6.8. and 4.2.4.2. respectively.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Hg	−0.00637 (3)	0.204514 (19)	0.04929 (4)	0.01364 (15)
P1	0.21142 (18)	0.39560 (12)	0.1799 (3)	0.0119 (3)
P2	0.30089 (18)	0.40325 (13)	0.5744 (3)	0.0119 (3)
O1	0.2966 (6)	0.3755 (4)	0.0131 (8)	0.0216 (11)
O2	0.1025 (6)	0.3245 (4)	0.2359 (8)	0.0186 (10)
O3	0.3997 (6)	0.3251 (4)	0.6302 (8)	0.0182 (10)
O4	0.1535 (5)	0.3973 (4)	0.6328 (8)	0.0187 (10)
O5	0.3126 (5)	0.4138 (4)	0.3515 (7)	0.0164 (10)
O6	0.1360 (5)	0.4979 (3)	0.1504 (8)	0.0145 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Hg	0.01344 (19)	0.0141 (2)	0.0134 (2)	0.00131 (8)	−0.00142 (8)	0.00056 (9)
P1	0.0124 (7)	0.0115 (7)	0.0118 (8)	0.0011 (6)	0.0007 (6)	−0.0006 (6)
P2	0.0111 (7)	0.0118 (7)	0.0128 (7)	0.0009 (6)	−0.0021 (6)	−0.0015 (6)
O1	0.023 (3)	0.027 (3)	0.014 (2)	0.008 (2)	0.006 (2)	−0.002 (2)
O2	0.026 (3)	0.013 (2)	0.016 (2)	−0.005 (2)	−0.001 (2)	0.001 (2)
O3	0.026 (3)	0.015 (2)	0.014 (2)	0.008 (2)	−0.004 (2)	0.0016 (19)
O4	0.015 (2)	0.021 (2)	0.020 (3)	−0.0041 (19)	0.001 (2)	−0.003 (2)
O5	0.014 (2)	0.025 (2)	0.011 (2)	0.0045 (19)	0.0018 (19)	0.009 (2)
O6	0.012 (2)	0.012 (2)	0.020 (2)	0.0002 (17)	0.0040 (19)	0.0036 (19)

Geometric parameters (Å, º)

Hg—O4^i	2.173 (5)	P2—Hg^vi	3.3356 (19)
Hg—O1^ii	2.251 (6)	P2—Hg^vii	3.4270 (18)
Hg—O3^iii	2.285 (5)	P2—Hg^ix	3.5905 (19)
Hg—O2	2.369 (5)	O1—Hg^v	2.251 (6)
Hg—O3^iv	2.493 (5)	O2—Hg^vi	2.503 (6)
Hg—O2^i	2.503 (6)	O3—Hg^vii	2.285 (5)
P1—O1	1.475 (6)	O3—Hg^ix	2.493 (5)
P1—O2	1.494 (6)	O3—Hg^vi	4.005 (6)
P1—O5	1.589 (6)	O4—Hg^vi	2.173 (5)
P1—O6	1.600 (5)	O4—Hg^ix	4.243 (6)
P1—Hg^v	3.4742 (18)	O5—Hg^vii	3.444 (5)
P1—Hg^vi	3.6469 (18)	O5—Hg^v	3.728 (5)
P1—Hg^vii	4.2587 (18)	O5—Hg^vi	3.771 (5)
P2—O4	1.493 (6)	O6—P2^x	1.586 (5)
P2—O3	1.494 (5)	O6—Hg^xi	3.773 (5)
P2—O6^viii	1.586 (5)	O6—Hg^vi	4.212 (5)
P2—O5	1.599 (6)
O4^i—Hg—O1^ii	110.3 (2)	O6^viii—P2—O5	103.6 (3)
O4^i—Hg—O3^iii	155.0 (2)	O4—P2—Hg^vi	29.9 (2)
O1^ii—Hg—O3^iii	84.5 (2)	O3—P2—Hg^vi	105.6 (2)
O4^i—Hg—O2	88.6 (2)	O6^viii—P2—Hg^vi	138.1 (2)
O1^ii—Hg—O2	146.2 (2)	O5—P2—Hg^vi	92.9 (2)
O3^iii—Hg—O2	89.3 (2)	O4—P2—Hg^vii	123.5 (2)
O4^i—Hg—O3^iv	84.5 (2)	O3—P2—Hg^vii	31.2 (2)
O1^ii—Hg—O3^iv	77.89 (19)	O6^viii—P2—Hg^vii	124.05 (19)
O3^iii—Hg—O3^iv	119.1 (2)	O5—P2—Hg^vii	77.15 (19)
O2—Hg—O3^iv	76.27 (18)	Hg^vi—P2—Hg^vii	96.92 (5)
O4^i—Hg—O2^i	80.80 (19)	O4—P2—Hg^ix	105.6 (2)
O1^ii—Hg—O2^i	96.0 (2)	O3—P2—Hg^ix	33.9 (2)
O3^iii—Hg—O2^i	77.57 (18)	O6^viii—P2—Hg^ix	84.08 (19)
O2—Hg—O2^i	115.1 (2)	O5—P2—Hg^ix	137.9 (2)
O3^iv—Hg—O2^i	161.07 (18)	Hg^vi—P2—Hg^ix	108.84 (5)
O1—P1—O2	119.3 (3)	Hg^vii—P2—Hg^ix	65.08 (3)
O1—P1—O5	107.7 (3)	P1—O1—Hg^v	136.7 (4)
O2—P1—O5	109.5 (3)	P1—O1—Hg	67.9 (2)
O1—P1—O6	108.4 (3)	Hg^v—O1—Hg	111.8 (2)
O2—P1—O6	106.6 (3)	P1—O2—Hg	128.4 (3)
O5—P1—O6	104.3 (3)	P1—O2—Hg^vi	129.9 (3)
O1—P1—Hg^v	26.4 (2)	Hg—O2—Hg^vi	101.63 (19)
O2—P1—Hg^v	115.1 (2)	P2—O3—Hg^vii	129.0 (3)
O5—P1—Hg^v	86.4 (2)	P2—O3—Hg^ix	126.6 (3)
O6—P1—Hg^v	130.37 (19)	Hg^vii—O3—Hg^ix	104.38 (19)
O1—P1—Hg	89.1 (3)	P2—O3—Hg^vi	53.3 (2)
O2—P1—Hg	32.0 (2)	Hg^vii—O3—Hg^vi	103.80 (18)
O5—P1—Hg	134.1 (2)	Hg^ix—O3—Hg^vi	118.39 (19)
O6—P1—Hg	110.40 (19)	P2—O4—Hg^vi	130.0 (3)
Hg^v—P1—Hg	93.10 (4)	P2—O4—Hg^ix	54.6 (2)
O1—P1—Hg^vi	146.8 (2)	Hg^vi—O4—Hg^ix	119.31 (19)
O2—P1—Hg^vi	31.8 (2)	P1—O5—P2	135.0 (4)
O5—P1—Hg^vi	82.1 (2)	P1—O5—Hg^vii	110.0 (3)
O6—P1—Hg^vi	99.27 (19)	P2—O5—Hg^vii	75.94 (19)
Hg^v—P1—Hg^vi	130.33 (5)	P1—O5—Hg^v	68.5 (2)
Hg—P1—Hg^vi	63.78 (3)	P2—O5—Hg^v	139.1 (3)
O1—P1—Hg^vii	83.6 (2)	Hg^vii—O5—Hg^v	63.41 (9)
O2—P1—Hg^vii	86.0 (2)	P1—O5—Hg^vi	73.3 (2)
O5—P1—Hg^vii	49.5 (2)	P2—O5—Hg^vi	62.05 (17)
O6—P1—Hg^vii	153.7 (2)	Hg^vii—O5—Hg^vi	88.98 (12)
Hg^v—P1—Hg^vii	57.41 (3)	Hg^v—O5—Hg^vi	119.05 (15)
Hg—P1—Hg^vii	92.65 (4)	P2^x—O6—P1	128.4 (3)
Hg^vi—P1—Hg^vii	79.22 (3)	P2^x—O6—Hg^xi	71.19 (18)
O4—P2—O3	120.1 (3)	P1—O6—Hg^xi	137.7 (3)
O4—P2—O6^viii	108.8 (3)	P2^x—O6—Hg^vi	157.4 (3)
O3—P2—O6^viii	106.1 (3)	P1—O6—Hg^vi	58.71 (16)
O4—P2—O5	110.5 (3)	Hg^xi—O6—Hg^vi	90.29 (11)
O3—P2—O5	106.5 (3)

Symmetry codes: (i) x, −y+1/2, z−1/2; (ii) x−1/2, −y+1/2, −z; (iii) x−1/2, y, −z+1/2; (iv) x−1/2, −y+1/2, −z+1; (v) x+1/2, −y+1/2, −z; (vi) x, −y+1/2, z+1/2; (vii) x+1/2, y, −z+1/2; (viii) −x+1/2, −y+1, z+1/2; (ix) x+1/2, −y+1/2, −z+1; (x) −x+1/2, −y+1, z−1/2; (xi) −x, y+1/2, −z+1/2.