K₄(CrO₄)(NO₃)₂, the first chromate(VI)-nitrate

K₄(CrO₄)(NO₃)₂ is the first chromate(VI)-nitrate to be reported. It contains isolated CrO₄^2- tetrahedra and NO₃^- groups linked together by K⁺ ions. The latter show a layered arrangement approximately parallel to the (001) plane. The average Cr-O, N-O and K-O bond lengths are 1.645, 1.242, and 2.905 Å, respectively. All atoms are on general positions.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](O-N) = 0.004 Å
• R factor = 0.038
• wR factor = 0.106
• Data-to-parameter ratio = 20.0

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

---

S1. Comment

We are currently investigating and classifying crystal structure types of both natural and synthetic kröhnkite [Na₂Cu(SO₄)₂·2H₂O]-type oxysalts and related compounds (Fleck et al., 2002a,b). In our recent research we have also included chromate compounds (Fleck and Kolitsch, 2002), which are environmentally important because of the toxicity of hexavalent chromium. The title compound, the first chromate(VI)-nitrate to be reported, was obtained as an unexpected by- product during synthesis from aqueous solutions at room temperature. Because it crystallized from an approximately neutral aqueous solution and was accompanied by the synthetic analogue of tarapacáite (naturally occurring K₂CrO₄), it might also occur in K-rich natural environments.

The asymmetric unit of K₄(CrO₄)(NO₃)₂ contains four K, one Cr, two N and ten O atoms, all located on general positions. The structure is characterized by isolated CrO₄ tetrahedra and NO₃ groups which are linked together by K atoms (Figs. 1, 2). The latter show a layered arrangement approximately parallel to the (001) plane (Fig. 1). The overall linkage is weak, and explains the low hardness and tenacity of the compound.

The K atoms show coordination numbers between eight and ten, with a grand mean K—O distance of 2.905 Å. The unique CrO₄ tetrahedron, although isolated, shows a slight angular distortion, with O—Cr—O angles ranging from 108.43 (10) to 111.21 (10)°. The oxygen ligands of the N2O₃ group, O4, O5 and, to a lesser extent, O6, show somewhat increased displacement parameters (unlike those of the N1O₃ group), and therefore indicate slight positional disorder.

The chemical composition of the title compound makes it unique. However, it is chemically related to the following four alkali sulfate nitrates, all with unknown crystal structures: K(NH₄)₃(SO₄)(NO₃)₂ (ICDD-PDF 20–853), K₂(NH₄)₂(SO₄)(NO₃)₂ (ICDD-PDF 20–852), K₄H₂(NO₃)₂(SO₄)₂ (Orlova et al., 1986; ICDD-PDF 39–722), and Na₂CrO₂F₂(NO₃)₂ (Brown & Gard, 1973; ICDD-PDF 30–1179). The first two of these compounds might be isostructural with the title compound, if one considers their equivalent stoichiometries.

S2. Experimental

The title compound was prepared by slow evaporation at room temperature of an approximately neutral aqueous solution containg dissolved HNO₃, K₂CrO₄, KOH, ZrCl₄ and Co(II)(OH)₂. It formed yellow transparent thin platelets with indistinct to pseudohexagonal outline. The platelets were arranged in rosette-shaped to spheroidal aggregates, and accompanied by large prismatic to blocky crystals of yellow K₂CrO₄ [synthetic tarapacáite], minor colourless KNO₃, and uninvestigated lilac grainy aggregates of a Co-compound.

Computing details

Data collection: COLLECT (Nonius, 2002); cell refinement: HKL SCALEPACK (Otwinowski & Minor 1997); data reduction: HKL DENZO, SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Shape Software, 1999); ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).

Figures

	Fig. 1. Perspective view of K4(CrO4)(NO3)2 along [010]. Isolated CrO4 tetrahedra (yellow) are linked to isolated NO3 groups (green, striped) via bonds to K atoms (bluish).
	Fig. 2. Anisotropic displacement ellipsoids (50% probability level) of the atoms in the asymmetric unit of K4(CrO4)(NO3)2. [Symmetry code: (i) −x, y + 0.5, −z + 0.5]

tetrapotassium chromate(VI) dinitrate(V)

Crystal data

CrK4N2O10	F(000) = 776
Mr = 396.42	Dx = 2.466 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 3227 reflections
a = 10.143 (2) Å	θ = 2.0–30.0°
b = 11.149 (2) Å	µ = 2.68 mm^−1
c = 9.837 (2) Å	T = 293 K
β = 106.31 (3)°	Thin platelet, pale yellow
V = 1067.6 (4) Å^3	0.20 × 0.10 × 0.01 mm
Z = 4

Data collection

Nonius KappaCCD diffractometer	3107 independent reflections
Radiation source: fine-focus sealed tube	2139 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.027
ψ and ω scans	θmax = 30.1°, θmin = 2.1°
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997)	h = −14→14
Tmin = 0.617, Tmax = 0.974	k = −15→15
6048 measured reflections	l = −13→13

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)] = 0.038	w = 1/[σ^2(Fo^2) + (0.053P)^2 + 0.430P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.106	(Δ/σ)max < 0.001
S = 1.01	Δρmax = 0.52 e Å^−3
3107 reflections	Δρmin = −0.84 e Å^−3
155 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0020 (4)

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. The structure is strongly hypercentric. 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
K1	0.35921 (5)	0.12280 (5)	0.03356 (5)	0.02566 (15)
K2	0.36336 (5)	0.87896 (5)	0.53445 (6)	0.02938 (15)
K3	0.09503 (6)	0.62574 (5)	0.61292 (6)	0.02560 (14)
K4	0.08402 (6)	0.12720 (5)	0.60806 (5)	0.02717 (15)
Cr	0.20415 (4)	0.88240 (3)	0.80931 (4)	0.01722 (12)
N1	0.1539 (2)	0.37197 (17)	0.7980 (2)	0.0214 (4)
N2	0.5070 (2)	0.63781 (19)	0.7679 (2)	0.0261 (5)
O1	0.15608 (18)	0.46875 (16)	0.86197 (18)	0.0296 (4)
O2	0.14715 (18)	0.27453 (15)	0.85857 (18)	0.0287 (4)
O3	0.15162 (19)	0.37247 (15)	0.66926 (18)	0.0291 (4)
O4	0.3974 (3)	0.5874 (3)	0.7275 (3)	0.0819 (10)
O5	0.5486 (3)	0.7099 (2)	0.6946 (3)	0.0620 (7)
O6	0.4216 (3)	0.1246 (2)	0.6065 (2)	0.0508 (6)
O7	0.37171 (18)	0.8941 (2)	0.8475 (2)	0.0433 (5)
O8	0.15907 (19)	0.75791 (15)	0.87380 (18)	0.0291 (4)
O9	0.14067 (17)	0.88209 (14)	0.63575 (16)	0.0242 (4)
O10	0.14221 (17)	0.99815 (15)	0.87589 (17)	0.0238 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K1	0.0186 (3)	0.0294 (3)	0.0277 (3)	−0.0019 (2)	0.0043 (2)	−0.0008 (2)
K2	0.0246 (3)	0.0332 (4)	0.0318 (3)	−0.0004 (2)	0.0104 (2)	0.0011 (2)
K3	0.0340 (3)	0.0200 (3)	0.0251 (3)	−0.0029 (2)	0.0120 (2)	−0.0012 (2)
K4	0.0404 (3)	0.0192 (3)	0.0209 (3)	0.0034 (2)	0.0070 (2)	0.0002 (2)
Cr	0.0178 (2)	0.0173 (2)	0.01624 (19)	0.00061 (14)	0.00418 (13)	0.00013 (14)
N1	0.0272 (11)	0.0188 (10)	0.0193 (10)	−0.0031 (8)	0.0085 (8)	−0.0013 (8)
N2	0.0256 (12)	0.0279 (12)	0.0257 (11)	0.0032 (8)	0.0087 (9)	0.0005 (9)
O1	0.0378 (10)	0.0202 (9)	0.0297 (9)	−0.0037 (7)	0.0078 (8)	−0.0067 (7)
O2	0.0345 (10)	0.0217 (9)	0.0303 (10)	0.0034 (7)	0.0099 (8)	0.0062 (7)
O3	0.0356 (10)	0.0304 (10)	0.0254 (9)	−0.0005 (7)	0.0153 (7)	0.0007 (7)
O4	0.0421 (15)	0.102 (2)	0.104 (2)	−0.0279 (15)	0.0245 (15)	−0.0573 (19)
O5	0.0910 (19)	0.0566 (15)	0.0531 (14)	0.0181 (13)	0.0442 (14)	0.0178 (13)
O6	0.0642 (15)	0.0468 (14)	0.0357 (12)	−0.0108 (10)	0.0047 (11)	−0.0043 (10)
O7	0.0157 (9)	0.0647 (15)	0.0466 (12)	−0.0019 (8)	0.0039 (8)	−0.0052 (10)
O8	0.0425 (11)	0.0197 (9)	0.0271 (9)	−0.0012 (7)	0.0130 (8)	0.0041 (7)
O9	0.0275 (9)	0.0261 (9)	0.0181 (8)	−0.0003 (7)	0.0049 (7)	−0.0016 (7)
O10	0.0291 (9)	0.0201 (9)	0.0231 (8)	0.0022 (6)	0.0087 (7)	−0.0030 (7)

Geometric parameters (Å, º)

K1—O7^i	2.657 (2)	K4—N1	3.271 (2)
K1—O10^ii	2.6952 (18)	K4—N1^iii	3.319 (2)
K1—O3^iii	2.7923 (19)	K4—Cr^xii	3.3908 (9)
K1—O5^iv	2.834 (2)	Cr—O7	1.6395 (19)
K1—O2^v	2.8934 (19)	Cr—O8	1.6434 (17)
K1—O6^iii	2.931 (2)	Cr—O9	1.6468 (16)
K1—O4^iii	2.978 (3)	Cr—O10	1.6494 (16)
K1—O7^ii	3.162 (2)	Cr—K4^vii	3.3908 (9)
K1—O5^i	3.178 (3)	Cr—K3^xiii	3.4656 (10)
K1—N2^i	3.360 (2)	Cr—K1^xiv	3.5461 (9)
K1—Cr^ii	3.5461 (9)	Cr—K2^xiii	3.7409 (9)
K1—K3^iii	4.0796 (9)	N1—O1	1.246 (2)
K2—O8^vi	2.698 (2)	N1—O2	1.250 (2)
K2—O9	2.7129 (18)	N1—O3	1.261 (3)
K2—O5	2.811 (3)	N2—O4	1.209 (3)
K2—O6^vii	2.849 (2)	N2—O5	1.231 (3)
K2—O1^vi	2.8598 (19)	N2—O6^xv	1.253 (3)
K2—O6^i	2.895 (3)	N2—K2^xiii	3.343 (2)
K2—O7	3.061 (2)	O1—K2^xiii	2.8598 (19)
K2—O4^vi	3.156 (3)	O1—K4^xvi	2.921 (2)
K2—N2^vi	3.343 (2)	O1—K4^x	3.089 (2)
K2—Cr	3.5173 (10)	O2—K1^xvii	2.8934 (19)
K2—Cr^vi	3.7409 (9)	O2—K4^xvi	2.918 (2)
K2—K2^viii	4.0600 (12)	O2—K3^ix	3.0416 (19)
K3—O10^ix	2.8239 (18)	O3—K1^xvi	2.7923 (19)
K3—O10^vi	2.8637 (18)	O3—K3^xi	3.172 (2)
K3—O8	2.8710 (19)	O4—K1^xvi	2.978 (3)
K3—O9	2.8939 (18)	O4—K2^xiii	3.156 (3)
K3—O3	2.9037 (19)	O5—K1^xviii	2.834 (2)
K3—O8^vi	2.9148 (19)	O5—K1^i	3.178 (3)
K3—O1	2.932 (2)	O6—N2^xix	1.253 (3)
K3—O4	2.986 (3)	O6—K2^xii	2.849 (2)
K3—O2^x	3.0416 (19)	O6—K2^i	2.895 (3)
K3—O3^xi	3.172 (2)	O6—K1^xvi	2.931 (2)
K3—N1	3.327 (2)	O7—K1^i	2.657 (2)
K3—Cr	3.4560 (8)	O7—K1^xiv	3.162 (2)
K4—O9^xii	2.7900 (17)	O8—K2^xiii	2.698 (2)
K4—O9^xi	2.808 (2)	O8—K4^x	2.9125 (19)
K4—O3	2.8422 (19)	O8—K3^xiii	2.9148 (19)
K4—O2	2.8800 (19)	O9—K4^vii	2.7900 (17)
K4—O8^ix	2.9125 (19)	O9—K4^xi	2.808 (2)
K4—O10^xii	2.9131 (18)	O10—K1^xiv	2.6952 (18)
K4—O2^iii	2.918 (2)	O10—K3^x	2.8239 (17)
K4—O1^iii	2.921 (2)	O10—K3^xiii	2.8637 (18)
K4—O1^ix	3.089 (2)	O10—K4^vii	2.9131 (18)
O7^i—K1—O10^ii	142.06 (6)	O10^ix—K3—O1	69.45 (5)
O7^i—K1—O3^iii	127.59 (7)	O10^vi—K3—O1	110.37 (5)
O10^ii—K1—O3^iii	71.41 (5)	O8—K3—O1	67.53 (5)
O7^i—K1—O5^iv	81.18 (8)	O9—K3—O1	122.21 (5)
O10^ii—K1—O5^iv	96.82 (7)	O3—K3—O1	43.61 (5)
O3^iii—K1—O5^iv	146.38 (7)	O8^vi—K3—O1	153.81 (6)
O7^i—K1—O2^v	143.47 (6)	O10^ix—K3—O4	135.17 (8)
O10^ii—K1—O2^v	66.96 (6)	O10^vi—K3—O4	81.35 (7)
O3^iii—K1—O2^v	74.87 (5)	O8—K3—O4	77.61 (7)
O5^iv—K1—O2^v	71.57 (7)	O9—K3—O4	89.26 (7)
O7^i—K1—O6^iii	80.56 (7)	O3—K3—O4	70.23 (7)
O10^ii—K1—O6^iii	136.18 (6)	O8^vi—K3—O4	86.00 (8)
O3^iii—K1—O6^iii	90.27 (6)	O1—K3—O4	69.25 (8)
O5^iv—K1—O6^iii	76.58 (7)	O10^ix—K3—O2^x	63.39 (5)
O2^v—K1—O6^iii	69.95 (6)	O10^vi—K3—O2^x	131.46 (5)
O7^i—K1—O4^iii	74.30 (8)	O8—K3—O2^x	67.88 (5)
O10^ii—K1—O4^iii	84.32 (8)	O9—K3—O2^x	64.53 (5)
O3^iii—K1—O4^iii	71.84 (7)	O3—K3—O2^x	129.37 (5)
O5^iv—K1—O4^iii	139.88 (8)	O8^vi—K3—O2^x	101.57 (5)
O2^v—K1—O4^iii	141.49 (7)	O1—K3—O2^x	103.60 (6)
O6^iii—K1—O4^iii	128.25 (8)	O4—K3—O2^x	144.46 (6)
O7^i—K1—O7^ii	89.68 (6)	O10^ix—K3—O3^xi	64.28 (5)
O10^ii—K1—O7^ii	53.85 (5)	O10^vi—K3—O3^xi	64.00 (5)
O3^iii—K1—O7^ii	116.76 (5)	O8—K3—O3^xi	134.73 (5)
O5^iv—K1—O7^ii	75.06 (6)	O9—K3—O3^xi	97.55 (5)
O2^v—K1—O7^ii	105.73 (6)	O3—K3—O3^xi	103.78 (5)
O6^iii—K1—O7^ii	151.10 (6)	O8^vi—K3—O3^xi	65.84 (5)
O4^iii—K1—O7^ii	73.57 (8)	O1—K3—O3^xi	131.64 (5)
O7^i—K1—O5^i	69.40 (7)	O4—K3—O3^xi	143.62 (7)
O10^ii—K1—O5^i	141.86 (6)	O2^x—K3—O3^xi	67.57 (5)
O3^iii—K1—O5^i	70.71 (6)	O9^xii—K4—O9^xi	98.21 (5)
O5^iv—K1—O5^i	112.38 (5)	O9^xii—K4—O3	152.98 (6)
O2^v—K1—O5^i	98.70 (6)	O9^xi—K4—O3	107.82 (5)
O6^iii—K1—O5^i	40.36 (6)	O9^xii—K4—O2	119.01 (5)
O4^iii—K1—O5^i	88.17 (8)	O9^xi—K4—O2	133.14 (5)
O7^ii—K1—O5^i	155.54 (6)	O3—K4—O2	44.57 (5)
O8^vi—K2—O9	68.95 (5)	O9^xii—K4—O8^ix	129.70 (5)
O8^vi—K2—O5	103.39 (8)	O9^xi—K4—O8^ix	64.71 (5)
O9—K2—O5	107.30 (6)	O3—K4—O8^ix	70.34 (5)
O8^vi—K2—O6^vii	139.54 (6)	O2—K4—O8^ix	69.56 (5)
O9—K2—O6^vii	91.84 (6)	O9^xii—K4—O10^xii	56.23 (5)
O5—K2—O6^vii	116.48 (8)	O9^xi—K4—O10^xii	131.60 (5)
O8^vi—K2—O1^vi	70.89 (6)	O3—K4—O10^xii	107.94 (5)
O9—K2—O1^vi	70.19 (5)	O2—K4—O10^xii	64.40 (5)
O5—K2—O1^vi	174.22 (7)	O8^ix—K4—O10^xii	98.73 (5)
O6^vii—K2—O1^vi	69.10 (6)	O9^xii—K4—O2^iii	111.22 (5)
O8^vi—K2—O6^i	105.65 (6)	O9^xi—K4—O2^iii	67.21 (5)
O9—K2—O6^i	173.28 (6)	O3—K4—O2^iii	73.74 (5)
O5—K2—O6^i	77.53 (7)	O2—K4—O2^iii	117.19 (3)
O6^vii—K2—O6^i	90.04 (7)	O8^ix—K4—O2^iii	104.66 (6)
O1^vi—K2—O6^i	104.55 (6)	O10^xii—K4—O2^iii	155.43 (5)
O8^vi—K2—O7	114.85 (6)	O9^xii—K4—O1^iii	68.26 (5)
O9—K2—O7	54.59 (5)	O9^xi—K4—O1^iii	67.09 (5)
O5—K2—O7	69.40 (6)	O3—K4—O1^iii	115.23 (5)
O6^vii—K2—O7	75.61 (6)	O2—K4—O1^iii	150.54 (6)
O1^vi—K2—O7	111.84 (6)	O8^ix—K4—O1^iii	130.33 (6)
O6^i—K2—O7	132.12 (6)	O10^xii—K4—O1^iii	122.30 (5)
O8^vi—K2—O4^vi	77.24 (7)	O2^iii—K4—O1^iii	43.57 (5)
O9—K2—O4^vi	132.52 (6)	O9^xii—K4—O1^ix	64.96 (5)
O5—K2—O4^vi	112.34 (7)	O9^xi—K4—O1^ix	65.67 (5)
O6^vii—K2—O4^vi	93.04 (8)	O3—K4—O1^ix	132.83 (5)
O1^vi—K2—O4^vi	67.76 (6)	O2—K4—O1^ix	103.67 (6)
O6^i—K2—O4^vi	40.89 (7)	O8^ix—K4—O1^ix	64.95 (5)
O7—K2—O4^vi	167.46 (8)	O10^xii—K4—O1^ix	66.18 (5)
O10^ix—K3—O10^vi	97.34 (5)	O2^iii—K4—O1^ix	131.30 (5)
O10^ix—K3—O8	101.84 (6)	O1^iii—K4—O1^ix	104.90 (5)
O10^vi—K3—O8	158.09 (6)	O7—Cr—O8	111.21 (10)
O10^ix—K3—O9	127.88 (5)	O7—Cr—O9	108.43 (10)
O10^vi—K3—O9	118.68 (5)	O8—Cr—O9	109.16 (9)
O8—K3—O9	55.43 (5)	O7—Cr—O10	109.43 (10)
O10^ix—K3—O3	68.14 (5)	O8—Cr—O10	109.22 (9)
O10^vi—K3—O3	67.48 (5)	O9—Cr—O10	109.37 (8)
O8—K3—O3	110.22 (5)	O1—N1—O2	120.52 (19)
O9—K3—O3	157.95 (6)	O1—N1—O3	119.75 (19)
O10^ix—K3—O8^vi	129.87 (6)	O2—N1—O3	119.64 (19)
O10^vi—K3—O8^vi	55.35 (5)	O4—N2—O5	123.1 (3)
O8—K3—O8^vi	116.56 (3)	O4—N2—O6^xv	119.5 (3)
O9—K3—O8^vi	63.65 (5)	O5—N2—O6^xv	117.1 (2)
O3—K3—O8^vi	120.65 (5)

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x, y−1, z−1; (iii) x, −y+1/2, z−1/2; (iv) −x+1, y−1/2, −z+1/2; (v) x, y, z−1; (vi) x, −y+3/2, z−1/2; (vii) x, y+1, z; (viii) −x+1, −y+2, −z+1; (ix) −x, y−1/2, −z+3/2; (x) −x, y+1/2, −z+3/2; (xi) −x, −y+1, −z+1; (xii) x, y−1, z; (xiii) x, −y+3/2, z+1/2; (xiv) x, y+1, z+1; (xv) −x+1, y+1/2, −z+3/2; (xvi) x, −y+1/2, z+1/2; (xvii) x, y, z+1; (xviii) −x+1, y+1/2, −z+1/2; (xix) −x+1, y−1/2, −z+3/2.