Refinement of ferrous sulfate heptahydrate (melanterite) with low-temperature CCD data

Refinement of the title compound, FeSO₄·7H₂O, with CCD data at 120 K has led to a fivefold increase in precision over the previously reported structure based on film data. The H atoms have been located and refined. Two independent octahedral [Fe(OH₂)₆]²⁺ cations lie on inversion centers, while one water mol­ecule is uncoordinated. Fe-O distances are in the range 2.0795 (9)-2.1873 (9) Å.

Key indicators

• Single-crystal X-ray study
• T = 120 K
• Mean [sigma](S-O) = 0.001 Å
• R factor = 0.029
• wR factor = 0.076
• Data-to-parameter ratio = 25.7

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

While attempting to prepare mixed-metal complexes of cysteine, we encountered crystals of the title compound. We discovered that the best available structure determination was based on 738 film data (Baur, 1964). We report herein the refinement of the structure using modern data-collection techniques. The excellent structure determination reported by Baur is confirmed, including the asymmetric pattern of Fe—O distances in the two centrosymmetric [Fe(OH₂)₆]²⁺ ions [2.068 (5)–2.188 (5) Å (Baur, 1964) and 2.0795 (9)–2.1893 (9) Å from our data]. From the film data, the H-atom positions were not directly obtained, but were placed from geometric hydrogen-bonding considerations. We confirm Baur's placement of the H atoms except for that involved in the bifurcated hydrogen bond involving uncoordinated water O7W as donor, and O3 and O6W as acceptors (Table 2). Placement of that H atom had been considered ambiguous, and the bifurcated interaction which we observe was mentioned as a possibility by Baur (1964), who adequately described both the structure of the individual ions and the packing.

S2. Experimental

The crystal used for data collection was taken from a commercial sample (J. T. Baker Chemical Co., lot 302120) and was reduced in size by partial dissolution using water.

S3. Refinement

The coordinates of Baur (1964) were used as an initial refinement model, with some atoms moved to form connected sets. H atoms were located from difference maps and were individually refined. O—H distances were in the range 0.79 (2)–0.90 (2) Å, and U_iso values for H atoms are in the range 0.028 (5)–0.047 (6) Ų.

Computing details

Data collection: COLLECT (Nonius 2000); cell refinement: DENZO and SCALEPACK; data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: coordinates of Baur (1964); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. A view of ferrous sulfate heptahydrate with the numbering scheme and ellipsoids at the 70% probability level.

Ferrous sulfate heptahydrate

Crystal data

[Fe(H2O)6](SO4)·H2O	F(000) = 576
Mr = 278.02	Dx = 1.918 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 13.9969 (3) Å	Cell parameters from 8709 reflections
b = 6.4803 (2) Å	θ = 2.5–36.3°
c = 11.0211 (2) Å	µ = 1.82 mm^−1
β = 105.5959 (11)°	T = 120 K
V = 962.85 (4) Å^3	Fragment, pale blue–green
Z = 4	0.25 × 0.20 × 0.17 mm

Data collection

KappaCCD (with Oxford Cryostream cooler) diffractometer	4580 independent reflections
Radiation source: fine-focus sealed tube	3686 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.021
ω scans with κ offsets	θmax = 36.3°, θmin = 3.0°
Absorption correction: multi-scan HKL SCALEPACK (Otwinowski & Minor, 1997)	h = −23→22
Tmin = 0.688, Tmax = 0.734	k = −10→10
14982 measured reflections	l = −17→18

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.029	All H-atom parameters refined
wR(F^2) = 0.076	w = 1/[σ^2(Fo^2) + (0.0305P)^2 + 0.2242P] where P = (Fo^2 + 2Fc^2)/3
S = 1.04	(Δ/σ)max < 0.001
4580 reflections	Δρmax = 0.64 e Å^−3
178 parameters	Δρmin = −0.64 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0140 (11)

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
Fe1	0.0000	0.5000	0.5000	0.01048 (6)
Fe2	0.5000	0.5000	0.0000	0.01082 (6)
S1	0.225907 (19)	0.47057 (4)	0.17436 (3)	0.00979 (6)
O1	0.20391 (6)	0.46976 (12)	0.03470 (8)	0.01296 (15)
O2	0.13563 (6)	0.53433 (12)	0.21052 (8)	0.01308 (15)
O3	0.30711 (6)	0.61746 (13)	0.22568 (8)	0.01529 (16)
O4	0.25504 (6)	0.26083 (12)	0.22414 (8)	0.01460 (16)
O1W	0.11878 (7)	0.39585 (16)	0.43692 (10)	0.0220 (2)
H11	0.1457 (14)	0.284 (3)	0.4568 (19)	0.035 (5)*
H12	0.1220 (14)	0.439 (3)	0.369 (2)	0.036 (5)*
O2W	−0.09794 (7)	0.45678 (15)	0.31502 (9)	0.01677 (17)
H22	−0.1139 (15)	0.332 (3)	0.2951 (19)	0.042 (6)*
H24	−0.1478 (16)	0.529 (3)	0.295 (2)	0.036 (6)*
O3W	0.02900 (7)	0.79563 (13)	0.43354 (9)	0.01622 (16)
H31	0.0790 (14)	0.876 (3)	0.4570 (17)	0.028 (5)*
H32	−0.0120 (16)	0.874 (3)	0.383 (2)	0.042 (6)*
O4W	0.47869 (7)	0.45429 (15)	0.17987 (9)	0.01569 (16)
H41	0.4297 (17)	0.511 (3)	0.193 (2)	0.033 (5)*
H47	0.5233 (15)	0.460 (3)	0.243 (2)	0.031 (5)*
O5W	0.56928 (7)	0.78270 (14)	0.06196 (9)	0.01697 (17)
H54	0.6209 (15)	0.784 (3)	0.129 (2)	0.036 (5)*
H57	0.5860 (15)	0.847 (3)	0.010 (2)	0.045 (6)*
O6W	0.64607 (6)	0.35354 (14)	0.05669 (9)	0.01744 (17)
H61	0.6967 (14)	0.414 (3)	0.0416 (18)	0.033 (5)*
H63	0.6655 (15)	0.280 (3)	0.129 (2)	0.047 (6)*
O7W	0.36626 (7)	0.00643 (13)	0.11265 (10)	0.01654 (17)
H74	0.3234 (15)	0.070 (3)	0.134 (2)	0.039 (5)*
H76	0.3428 (15)	−0.107 (3)	0.0893 (19)	0.041 (6)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe1	0.01138 (10)	0.01003 (10)	0.01074 (11)	0.00081 (7)	0.00421 (8)	0.00047 (7)
Fe2	0.01027 (10)	0.01242 (11)	0.00948 (11)	−0.00049 (7)	0.00216 (8)	−0.00016 (7)
S1	0.00908 (11)	0.01007 (11)	0.01016 (12)	−0.00007 (8)	0.00246 (8)	−0.00041 (8)
O1	0.0138 (3)	0.0148 (3)	0.0104 (4)	−0.0019 (3)	0.0033 (3)	−0.0015 (3)
O2	0.0120 (3)	0.0146 (3)	0.0139 (4)	0.0024 (3)	0.0057 (3)	0.0016 (3)
O3	0.0131 (3)	0.0163 (4)	0.0161 (4)	−0.0040 (3)	0.0032 (3)	−0.0049 (3)
O4	0.0150 (4)	0.0113 (3)	0.0167 (4)	0.0024 (3)	0.0028 (3)	0.0020 (3)
O1W	0.0270 (5)	0.0233 (5)	0.0205 (5)	0.0136 (4)	0.0146 (4)	0.0112 (4)
O2W	0.0174 (4)	0.0137 (4)	0.0172 (4)	0.0001 (3)	0.0014 (3)	−0.0011 (3)
O3W	0.0148 (4)	0.0120 (4)	0.0191 (4)	−0.0013 (3)	−0.0001 (3)	0.0035 (3)
O4W	0.0141 (4)	0.0217 (4)	0.0111 (4)	0.0004 (3)	0.0031 (3)	0.0003 (3)
O5W	0.0186 (4)	0.0176 (4)	0.0134 (4)	−0.0043 (3)	0.0022 (3)	0.0000 (3)
O6W	0.0132 (4)	0.0203 (4)	0.0181 (4)	−0.0007 (3)	0.0029 (3)	0.0072 (3)
O7W	0.0177 (4)	0.0130 (4)	0.0182 (4)	−0.0004 (3)	0.0036 (3)	−0.0011 (3)

Geometric parameters (Å, º)

Fe1—O1W	2.0795 (9)	S1—O4	1.4813 (8)
Fe1—O1W^i	2.0795 (9)	O1W—H11	0.82 (2)
Fe1—O2W	2.1474 (9)	O1W—H12	0.81 (2)
Fe1—O2W^i	2.1474 (9)	O2W—H22	0.85 (2)
Fe1—O3W^i	2.1287 (9)	O2W—H24	0.82 (2)
Fe1—O3W	2.1287 (9)	O3W—H31	0.856 (19)
Fe2—O4W	2.1034 (9)	O3W—H32	0.85 (2)
Fe2—O4W^ii	2.1035 (9)	O4W—H41	0.82 (2)
Fe2—O5W^ii	2.0992 (9)	O4W—H47	0.80 (2)
Fe2—O5W	2.0992 (9)	O5W—H54	0.89 (2)
Fe2—O6W^ii	2.1873 (9)	O5W—H57	0.79 (2)
Fe2—O6W	2.1873 (9)	O6W—H61	0.87 (2)
S1—O1	1.4864 (9)	O6W—H63	0.90 (2)
S1—O2	1.4822 (8)	O7W—H74	0.81 (2)
S1—O3	1.4738 (8)	O7W—H76	0.82 (2)
O1W—Fe1—O1W^i	180.0	O4W^ii—Fe2—O6W	88.38 (4)
O1W—Fe1—O3W^i	93.81 (4)	O6W^ii—Fe2—O6W	180.0
O1W^i—Fe1—O3W^i	86.19 (4)	O3—S1—O4	110.08 (5)
O1W—Fe1—O3W	86.19 (4)	O3—S1—O2	110.00 (5)
O1W^i—Fe1—O3W	93.81 (4)	O4—S1—O2	109.02 (5)
O3W^i—Fe1—O3W	180.0	O3—S1—O1	108.87 (5)
O1W—Fe1—O2W	90.23 (4)	O4—S1—O1	109.88 (5)
O1W^i—Fe1—O2W	89.77 (4)	O2—S1—O1	108.98 (5)
O3W^i—Fe1—O2W	94.43 (4)	Fe1—O1W—H11	123.3 (14)
O3W—Fe1—O2W	85.57 (4)	Fe1—O1W—H12	116.4 (14)
O1W—Fe1—O2W^i	89.77 (4)	H11—O1W—H12	114.5 (19)
O1W^i—Fe1—O2W^i	90.23 (4)	Fe1—O2W—H22	115.2 (14)
O3W^i—Fe1—O2W^i	85.57 (4)	Fe1—O2W—H24	117.5 (15)
O3W—Fe1—O2W^i	94.43 (4)	H22—O2W—H24	109.0 (19)
O2W—Fe1—O2W^i	180.0	Fe1—O3W—H31	131.5 (12)
O5W^ii—Fe2—O5W	180.0	Fe1—O3W—H32	127.2 (14)
O5W^ii—Fe2—O4W	90.78 (4)	H31—O3W—H32	100.6 (18)
O5W—Fe2—O4W	89.22 (4)	Fe2—O4W—H41	115.9 (16)
O5W^ii—Fe2—O4W^ii	89.23 (4)	Fe2—O4W—H47	122.7 (14)
O5W—Fe2—O4W^ii	90.77 (4)	H41—O4W—H47	109 (2)
O4W—Fe2—O4W^ii	180.0	Fe2—O5W—H54	118.9 (12)
O5W^ii—Fe2—O6W^ii	88.35 (3)	Fe2—O5W—H57	114.8 (16)
O5W—Fe2—O6W^ii	91.65 (3)	H54—O5W—H57	105.6 (19)
O4W—Fe2—O6W^ii	88.39 (4)	Fe2—O6W—H61	119.9 (13)
O4W^ii—Fe2—O6W^ii	91.62 (4)	Fe2—O6W—H63	121.2 (13)
O5W^ii—Fe2—O6W	91.65 (3)	H61—O6W—H63	109.9 (18)
O5W—Fe2—O6W	88.35 (3)	H74—O7W—H76	106.2 (19)
O4W—Fe2—O6W	91.62 (4)

Symmetry codes: (i) −x, −y+1, −z+1; (ii) −x+1, −y+1, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1W—H11···O1^iii	0.82 (2)	1.93 (2)	2.7394 (12)	169 (2)
O1W—H12···O2	0.81 (2)	1.91 (2)	2.7208 (13)	177 (2)
O2W—H22···O2^iv	0.85 (2)	1.95 (2)	2.7879 (12)	167 (2)
O2W—H24···O4^v	0.82 (2)	2.09 (2)	2.8972 (12)	166 (2)
O3W—H31···O1^vi	0.856 (19)	1.995 (19)	2.8426 (12)	170.4 (18)
O3W—H32···O2^v	0.85 (2)	2.04 (2)	2.8734 (12)	164.5 (19)
O4W—H41···O3	0.82 (2)	1.97 (2)	2.7915 (12)	174.2 (18)
O4W—H47···O7W^vii	0.80 (2)	1.92 (2)	2.7175 (13)	172.8 (19)
O5W—H54···O4^vii	0.89 (2)	2.03 (2)	2.9167 (13)	173.9 (18)
O5W—H57···O7W^ii	0.79 (2)	1.91 (2)	2.7052 (13)	177 (2)
O6W—H61···O1^ii	0.87 (2)	1.96 (2)	2.8058 (12)	166.0 (19)
O6W—H63···O3^viii	0.90 (2)	1.87 (2)	2.7708 (12)	174.3 (19)
O7W—H74···O4	0.81 (2)	1.99 (2)	2.7712 (13)	162 (2)
O7W—H76···O3^ix	0.82 (2)	2.47 (2)	3.0242 (12)	126.0 (18)
O7W—H76···O6W^x	0.82 (2)	2.30 (2)	2.9633 (13)	138.4 (18)

Symmetry codes: (ii) −x+1, −y+1, −z; (iii) x, −y+1/2, z+1/2; (iv) −x, y−1/2, −z+1/2; (v) −x, y+1/2, −z+1/2; (vi) x, −y+3/2, z+1/2; (vii) −x+1, y+1/2, −z+1/2; (viii) −x+1, y−1/2, −z+1/2; (ix) x, y−1, z; (x) −x+1, −y, −z.