Dilead(II) trimercury(II) tetraoxide chromate(VI), Pb₂(Hg₃O₄)(CrO₄)

Pb₂(Hg₃O₄)(CrO₄) consists of [CrO₄]^2- tetra­hedra, linear O-Hg-O dumbbells and divalent Pb atoms in [3+5]-coordination. The HgO₂ dumbbells are condensed into [Hg₃O₄]^2- units and can be regarded as a section of the HgO structure. The [Hg₃O₄]^2- complex anions are connected by inter­stitial Pb²⁺ ions, while the [CrO₄]^2- tetra­hedra are isolated.

S1. Comment

Pb₂(Hg₃O₄)(CrO₄) consists of three different structural building units, viz. [CrO₄]²⁻ tetrahedra, [Hg₃O₄]²⁻ anionic strips and, connecting the latter, Pb²⁺ ions. The [CrO₄]²⁻ ion is only slightly distorted; all Cr—O distances are identical within the 1σ limit, and the angles are close to the tetrahedral value.

All crystallographically independent Hg atoms are linearly coordinated by O atoms, and the Hg—O distances decrease remarkably from the central [2.094 (9) and 2.097 (9) Å] to the terminal bonds [2.006 (10) and 2.0154 (10) Å??]. The Hg—O—Hg angles of 111.2 (4)° and 114.7 (4)° result in a zigzag shape of the [Hg₃O₄]²⁻ anion. The dihedral angle between the two terminal O—Hg—O dumbbells is ca 0.9°, so all atoms are approximately coplanar. These characteristic [Hg₃O₄]²⁻ units represent a section of the HgO structure and are linked by Pb²⁺ ions, forming a three-dimensional framework (Figs. 1 and 2).

The two Pb atoms exhibit a stereochemically active lone pair; each atom has three close O-atom contacts on one side [Pb1: 2.246 (10)–2.512 (11), Pb2: 2.309 (10)–2.380 (9) Å], and five considerably more remote neighbours on the other [Pb1: 2.868 (15)–3.610 (10), Pb2: 2.761 (12)–3.228 (11) Å; Table 1 and Fig. 3). Considered as pyramidal PbO₃ units, the polyhedra containing Pb2 form dimers via common O8—O8ⁱⁱⁱ [symmetry code: (iii) 2 − x, −y, 2 − z] edges, while those containing Pb1 are arranged as infinite chains along [001], linked via common vertices. The close contacts are formed exclusively with O atoms of the [Hg₃O₄]²⁻ group, each coordinating to three heavy metal cations (O5 and O8: 1 Hg + 2 Pb; O6 and O7: 2 Hg + 1 Pb).

Atoms O1–O4, belonging to the [CrO₄]²⁻ tetrahedra, find their closest Hg and Pb neighbours at distances of 2.57 (O1—Hg2) and 2.76 Å (O4—Pb2), respectively, i.e. atoms O1–O4 reside in the second coordination sphere of Hg and Pb atoms, only.

The title compound is closely related to Pb₂(HgO₂)(CrO₄) (Klein et al., 2002), which was found under similar synthetic conditions. The crystal structure also consists of isolated [CrO₄]²⁻ tetrahedra, [HgO₂]² dumbbells, and Pb²⁺ ions in a (3 + 5) O-atom coordination with closest contacts to O atoms which are not bonded to chromium. Both compounds described here belong to the series of Pb₂(Hg_nO_{n + 1})(CrO₄) compositions and differ in the chain length of the Hg_nO_{n + 1} fragment. Another formal member of this family with n = 0, the naturally occurring phoenicochroit, Pb₂OCrO₄ (Williams et al., 1970), was also observed in our experiments.

Inspecting oxochromates(VI) containing divalent Pb and/or Hg, such as PbCrO₄ (Quareni & de Pieri, 1965), HgCrO₄ (Stalhandske, 1978), Hg₃CrO₆ (Hansen et al., 1995) and the compounds mentioned above, general structural trends become obvious. With increasing amounts of the heavy metal cations, the [CrO₄]²⁻ tetrahedra tend to separate firstly from Hg-containing entities, and subsequently the number of short Pb—OCrO₃ contacts is reduced. As a second structural constant besides the [CrO₄]²⁻ tetrahedra, Hg appears in linear coordination in all compounds mentioned here, and structural motifs strongly reminiscent of the HgO structure are formed, i.e. [HgO₂]² dumbbells in Pb₂(HgO₂)(CrO₄), [Hg₃O₄]²⁻ zigzag chains in the title compound and two-dimensional networks in Hg₃CrO₆. Pb²⁺ ions, mostly occurring in a (3 + x) O-atom coordination (x around 5), exhibit the highest flexibility in its coordination sphere among the present cations, and separates and shortens the [HgO]_n chains and fragments.

S2. Experimental

Single crystals of Pb₂(Hg₃O₄)(CrO₄) were obtained at elevated oxygen pressures using steel autoclaves. In a typical experiment, HgO, PbO and CrO₃ were used as starting materials, and distilled water (2 ml) was added to accelerate the reaction. The ground mixture, placed in gold crucibles, was annealed for 120 h at 753 K and 12 MPa oxygen pressure in stainless steel autoclaves (Linke & Jansen, 1997).

Computing details

Data collection: X-AREA (Stoe & Cie, 1996); cell refinement: X-AREA; data reduction: X-RED (Stoe & Cie, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A perspective view of Pb2Hg3O4(CrO4) along [001]. Drawn are Pb atoms, [Hg3O4]^2− units and [CrO4]^2− tetrahedra.
	Fig. 2. The surrounding of the [Hg3O4]^2− unit with six-coordinate PbO3 pyramids and a [CrO4]^2− tetrahedron. Displacement ellipsoids are shown at the 50% probability level. [Symmetry codes: (i) 1 + x, y, z; (ii) 1 + x, 1/2 − y, 1/2 + z; (iii) 2 − x, −y, 2 − z; (iv) −1 + x, y, z; (v) 1 − x, −y, 2 − z; (vi) x, 1/2 − y, 1/2 + z; (vii) x, 1/2 − y, −1/2 + z; (viii) −1 + x, 1/2 − y, −1/2 + z.]
	Fig. 3. The (3 + 5)-coordination of the two Pb^2+ ions; dashed bonds are longer than 2.52 Å. Displacement ellipsoids are shown at the 50% probability level. [Symmetry codes: (i) 1 + x, y, z; (ii) 1 + x, 1/2 − y, 1/2 + z; (iii) 2 − x, −y, 2 − z; (vi) x, 1/2 − y, 1/2 + z; (vii) x, 1/2 − y, −1/2 + z; (ix) 1 − x, −y, 1 − z; (x) 2 − x, −y, 1 − z; (xi) −1 + x, 1/2 − y, 1/2 + z.]

Dilead trimercury chromate(VI)

Crystal data

Pb2(Hg3O4)(CrO4)	F(000) = 1968
Mr = 1196.15	Dx = 8.417 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 17644 reflections
a = 6.5408 (9) Å	θ = 0.0–44.2°
b = 21.947 (3) Å	µ = 85.28 mm^−1
c = 6.9672 (10) Å	T = 293 K
β = 109.304 (11)°	Block, red
V = 943.9 (2) Å^3	0.2 × 0.2 × 0.2 mm
Z = 4

Data collection

Stoe IPDS-II diffractometer	2292 independent reflections
Radiation source: fine-focus sealed tube	2016 reflections with I > 2σ(I)
Plane graphite monochromator	Rint = 0.138
Detector resolution: 6.67 pixels mm^-1	θmax = 28.0°, θmin = 1.9°
ϕ or ω? scans	h = −8→8
Absorption correction: integration X-SHAPE (Stoe & Cie, 1996)	k = −28→28
Tmin = 0.003, Tmax = 0.023	l = −9→9
16447 measured reflections

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)] = 0.045	w = 1/[σ^2(Fo^2) + (0.079P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.119	(Δ/σ)max < 0.001
S = 1.01	Δρmax = 3.79 e Å^−3
2292 reflections	Δρmin = −4.97 e Å^−3
128 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00073 (10)

Special details

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.

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

	x	y	z	Uiso*/Ueq
Pb1	0.15994 (9)	0.29963 (2)	0.92668 (8)	0.03118 (18)
Pb2	0.97308 (9)	−0.02318 (2)	0.73910 (8)	0.03034 (17)
Hg1	−0.36147 (8)	0.21950 (2)	0.90351 (8)	0.03032 (17)
Hg2	0.06588 (9)	0.13842 (2)	0.86609 (8)	0.03149 (18)
Hg3	0.50352 (8)	0.05650 (2)	0.82352 (8)	0.03087 (18)
Cr	0.5853 (4)	0.11184 (9)	0.3614 (3)	0.0282 (4)
O1	0.326 (2)	0.1067 (5)	0.218 (2)	0.045 (3)
O2	0.736 (2)	0.1421 (6)	0.236 (2)	0.050 (3)
O3	0.596 (2)	0.1553 (6)	0.562 (2)	0.053 (3)
O4	0.674 (2)	0.0419 (5)	0.4350 (19)	0.043 (3)
O5	−0.6871 (16)	0.2253 (4)	0.7989 (17)	0.032 (2)
O6	−0.0348 (15)	0.2153 (4)	0.9888 (17)	0.031 (2)
O7	0.1767 (16)	0.0660 (4)	0.7303 (17)	0.030 (2)
O8	0.8240 (16)	0.0415 (5)	0.9186 (17)	0.035 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb1	0.0313 (3)	0.0221 (3)	0.0425 (3)	0.00053 (17)	0.0153 (2)	−0.00009 (18)
Pb2	0.0319 (3)	0.0233 (2)	0.0370 (3)	−0.00023 (18)	0.0130 (2)	−0.00055 (18)
Hg1	0.0270 (3)	0.0248 (3)	0.0405 (3)	0.00091 (17)	0.0128 (2)	−0.00137 (18)
Hg2	0.0329 (3)	0.0212 (3)	0.0424 (3)	0.00183 (19)	0.0153 (2)	0.00009 (18)
Hg3	0.0287 (3)	0.0249 (3)	0.0403 (3)	0.00141 (18)	0.0131 (2)	0.00062 (19)
Cr	0.0285 (10)	0.0220 (9)	0.0358 (10)	0.0017 (7)	0.0127 (8)	−0.0006 (8)
O1	0.036 (6)	0.039 (6)	0.054 (7)	0.000 (5)	0.006 (5)	0.004 (5)
O2	0.048 (7)	0.050 (7)	0.057 (7)	0.004 (5)	0.024 (6)	0.016 (6)
O3	0.062 (8)	0.043 (6)	0.051 (6)	0.006 (6)	0.015 (6)	−0.023 (6)
O4	0.049 (7)	0.023 (5)	0.052 (6)	−0.002 (4)	0.011 (5)	−0.006 (4)
O5	0.025 (4)	0.027 (4)	0.043 (5)	0.002 (4)	0.012 (4)	0.000 (4)
O6	0.022 (4)	0.019 (4)	0.053 (6)	0.002 (3)	0.014 (4)	−0.010 (4)
O7	0.025 (4)	0.018 (4)	0.053 (6)	0.000 (3)	0.018 (4)	−0.007 (4)
O8	0.017 (4)	0.039 (5)	0.050 (5)	0.007 (4)	0.011 (4)	−0.002 (4)

Geometric parameters (Å, º)

Cr—O1	1.661 (12)	Pb1—O7^v	3.610 (10)
Cr—O2	1.656 (13)	Pb1—Cr^v	3.544 (2)
Cr—O3	1.675 (12)	Pb1—Hg2^v	3.5840 (9)
Cr—O4	1.661 (11)	Pb1—Hg2	3.5920 (8)
Hg1—O5	2.015 (10)	Pb2—O8	2.309 (10)
Hg1—O6	2.022 (9)	Pb2—O8^vii	2.349 (11)
Hg1—Hg2	3.3956 (8)	Pb2—O7^ii	2.380 (9)
Hg2—O6	2.094 (9)	Pb2—O4	2.761 (12)
Hg2—O7	2.097 (9)	Pb2—O1^viii	2.771 (13)
Hg2—O1^i	2.576 (12)	Pb2—O4^ix	2.972 (14)
Hg2—O5^ii	2.639 (10)	Pb2—O2^ix	3.202 (13)
Hg2—Hg3	3.4749 (8)	Pb2—O7^viii	3.228 (11)
Hg2—Pb1^iii	3.5840 (9)	Pb2—Pb2^ix	3.6105 (11)
Hg3—O7	2.029 (10)	Pb2—Hg2^ii	3.6572 (8)
Hg3—O8	2.006 (10)	Pb2—Pb2^vii	3.6783 (11)
Pb1—O5^ii	2.246 (10)	Pb2—Cr^ix	3.736 (2)
Pb1—O6	2.367 (10)	O1—Hg2^x	2.576 (12)
Pb1—O5^iv	2.512 (11)	O5—Pb1^xi	2.246 (10)
Pb1—O3^v	2.868 (15)	O5—Pb1^xii	2.512 (11)
Pb1—O6^iii	2.910 (11)	O5—Hg2^xi	2.639 (10)
Pb1—O1^v	2.928 (13)	O7—Pb2^xi	2.380 (9)
Pb1—O2^vi	2.943 (14)	O8—Pb2^vii	2.349 (11)
O1—Cr—O2	112.0 (7)	O8—Hg3—Hg2	149.0 (3)
O1—Cr—O3	107.1 (7)	O7—Hg3—Hg2	33.2 (3)
O1—Cr—O4	107.7 (6)	O5^ii—Pb1—O6	80.9 (3)
O2—Cr—O3	110.1 (8)	O5^ii—Pb1—O5^iv	100.0 (3)
O2—Cr—O4	109.1 (7)	O6—Pb1—O5^iv	72.3 (3)
O3—Cr—O4	110.7 (7)	O5^ii—Pb1—Hg2^v	147.0 (3)
O5—Hg1—O6	176.0 (5)	O6—Pb1—Hg2^v	83.4 (2)
O5—Hg1—Hg2	143.5 (3)	O5^iv—Pb1—Hg2^v	47.4 (2)
O6—Hg1—Hg2	35.1 (2)	O5^ii—Pb1—Hg2	47.1 (3)
O6—Hg2—O7	175.6 (4)	O6—Pb1—Hg2	33.8 (2)
O6—Hg2—O1^i	91.6 (4)	O5^iv—Pb1—Hg2	84.3 (2)
O7—Hg2—O1^i	90.3 (4)	Hg2^v—Pb1—Hg2	113.76 (2)
O6—Hg2—O5^ii	77.5 (4)	O8—Pb2—O8^vii	75.7 (4)
O7—Hg2—O5^ii	98.3 (3)	O8—Pb2—O7^ii	81.3 (4)
O1^i—Hg2—O5^ii	96.5 (3)	O8^vii—Pb2—O7^ii	91.9 (4)
O6—Hg2—Hg1	33.7 (3)	O8—Pb2—Pb2^ix	120.9 (3)
O7—Hg2—Hg1	148.0 (3)	O8^vii—Pb2—Pb2^ix	142.2 (3)
O1^i—Hg2—Hg1	111.8 (3)	O7^ii—Pb2—Pb2^ix	61.2 (3)
O5^ii—Hg2—Hg1	101.7 (2)	O8—Pb2—Pb2^vii	38.2 (3)
O6—Hg2—Hg3	146.2 (3)	O8^vii—Pb2—Pb2^vii	37.5 (2)
O7—Hg2—Hg3	32.0 (3)	O7^ii—Pb2—Pb2^vii	85.8 (3)
O1^i—Hg2—Hg3	68.7 (3)	Pb2^ix—Pb2—Pb2^vii	145.83 (3)
O5^ii—Hg2—Hg3	77.8 (2)	Cr—O1—Hg2^x	141.0 (7)
Hg1—Hg2—Hg3	179.37 (2)	Hg1—O5—Pb1^xi	117.3 (5)
O6—Hg2—Pb1^iii	102.1 (3)	Hg1—O5—Pb1^xii	111.8 (5)
O7—Hg2—Pb1^iii	73.7 (3)	Pb1^xi—O5—Pb1^xii	119.3 (4)
O1^i—Hg2—Pb1^iii	132.0 (3)	Hg1—O5—Hg2^xi	121.8 (4)
O5^ii—Hg2—Pb1^iii	44.5 (2)	Pb1^xi—O5—Hg2^xi	94.3 (4)
Hg1—Hg2—Pb1^iii	104.174 (19)	Pb1^xii—O5—Hg2^xi	88.1 (3)
Hg3—Hg2—Pb1^iii	75.197 (18)	Hg1—O6—Hg2	111.2 (4)
O6—Hg2—Pb1	39.0 (3)	Hg1—O6—Pb1	119.4 (4)
O7—Hg2—Pb1	136.7 (3)	Hg2—O6—Pb1	107.1 (4)
O1^i—Hg2—Pb1	96.9 (3)	Hg2—O7—Hg3	114.7 (4)
O5^ii—Hg2—Pb1	38.6 (2)	Hg3—O7—Pb2^xi	116.5 (4)
Hg1—Hg2—Pb1	65.677 (16)	Hg2—O7—Pb2^xi	109.4 (4)
Hg3—Hg2—Pb1	113.936 (19)	Hg3—O8—Pb2	120.3 (5)
Pb1^iii—Hg2—Pb1	69.856 (14)	Hg3—O8—Pb2^vii	122.8 (5)
O7—Hg3—O8	176.4 (4)	Pb2—O8—Pb2^vii	104.3 (4)

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