Disorder of a Trigonally Planar Coordinated Water Molecule in Cobalt Sulfate Heptahydrate, C Os04 • 7 D20 (Bieberite)* Thomas Kellersohn*

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Disorder of a Trigonally Planar Coordinated Water Molecule in Cobalt Sulfate Heptahydrate, C oS04 • 7 D20 (Bieberite)* Thomas Kellersohn*. Robert G. Delaplane, and Ivar Olovsson** Institute of Chemistry. Department of Inorganic Chemistry, University of Uppsala, P.O. Box 531, S-75121 Uppsala Z. Naturforsch. 46b, 1635- 1640 (1991); received June 4. 1991 Cobalt Sulfate Heptahydrate. Bieberite. Disordered Structure. Hydrogen Bonding, Crystal Structure Cobalt sulfate heptahydrate (d-14), CoS04-7D:0 . Mr = 294.99. monoclinic, P2,/c, a = 1404.8(1), b = 649.41(6), c = 1092.5(2) pm,/? = 1057232(8) , V = 961.66- 106pm3, Z = 4. D v = 2.073 Mg-m"3, A(MoKa) = 71.073 pm. [(sin0)/x]max = 0.7035• 10"2 pm"1, ft = 20.26 cm ', F(000) = 580, T = 298 K. R(F) = 0.0264 for 2339 observed unique reflections. CoS04- 7 D:0 is shown to be isotypic to FeS04- 7 H ,0 (Melanterite). The deuterated compound is stable at am bient conditions in contrast to the normal hydrate. Its structure is built up by [Co(D:0 )6]2+ octahedra, S042~ tetrahedra, and “lattice” water molecules. One water molecule, which is al most exactly trigonally planar coordinated in its average position, exhibits a distinct oxygen disorder. The ‘lattice” water molecule accepts two strong hydrogen bonds and donates a li near and a bifurcated one. The hydrogen-bond lengths (O 0 distances) are in the range 271 - 302 pm.

Introduction hydrates when the mother liquor is removed, and, A large number of divalent metal sulfates are hence, it is extremely difficult to handle. The com known to form heptahydrates. Two different mercial product, upon closer examination, turned out to be a mixture of different phases. On prepar structure types are known [ 1 ]: the monoclinic ing C oS0 4 ■ 6 D20 for an electron density study [ 8 ], FeS0 4 - 7 H20 (Melanterite) type with M = Fe, Mn (Mallardite), Cu (Boothite), Zn (Zincmelanterite), we obtained single crystals of CoS0 4 -7D 20 as a by-product. The deuterated from was found to be and Co (Bieberite), and the orthorhombic much more stable than the normal hydrate so that M gS0 4 -7H 20 (Epsomite) type with M = Mg, Ni (Morenosite) and Zn (Goslarite). Common basic a structure determination could be performed. features of both structure types are [M"(H 2 0 ) 6]2+ Experimental octahedra and one additional "lattice” water mo Deep red crystals of CoS0 4 -7D20 were ob lecule, not coordinated to a metal ion. tained by slow evaporation of a neutral D20 solu However, precise structure determinations are tion of previously dried (600 K, 24 h) C oS0 4 at up to now only available for the two respective room temperature. A suitable, approximately te- principal minerals [2-6]. Therefore, no detailed trahedrally shaped fragment with maximum di comparative discussion could be given. mensions of 0.45x0.36x0.32 mm was broken

Despite the fact that "CoS0 4 -7H 2 0 ” is availa from a larger crystal. It was quickly sealed in a ble as a commercial product, its crystal structure glass capillary tube and mounted on a STOE has not been previously determined. This is ob AED 2 four-circle diffractometer. viously due to experimental difficulties: although Cell parameters were determined from 16 well- the heptahydrate has been described to be the sta centered reflections with 35° < 20 < 45 and are given in the Abstract. The intensities of 5238 re ble hydrate at room temperature [7], it rapidly de flections were measured in step-scans in the a ; - 2 0 - mode with a step width of Aco = 0.012 and a mini + Hydrogen Bond Studies 156. Part 155: R. G. Dela mum number of 70 steps/scan. A maximum count plane, R. Tellgren, and I. Olovsson. Acta Crystal ing time of 3.0 s/step was employed, range of hki. logr. B46, 361 (1990). * On leave from Universität GH Siegen, Inorganic -2 0 < h < 20, - 2 < k < 10, 0 < / < 15. Six moni Chemistry I, P.O. Box 101240. D-W-5900 Siegen. tor reflections were measured every 3 h and exhib FRG. ited an average total intensity decrease of 2.5% for ** Reprint requests to Prof. Dr. I. Olovsson. the 260 h of data collection. A linear decay correc Verlag der Zeitschrift für Naturforschung, D-7400 Tübingen tion was therefore applied. Averaging over 202 re 0932 - 0776/91 /1200 - 163 5/$ 01.00/0 peatedly measured reflections gave an internal R = 1636 Th. Kellersohn et al. • Cobalt Sulfate Heptahydratc, CoSQ4 7DoO

0.0137 (on F2). A numerical absorption correction FeS0 4 -7H20 [4], a refinement with 0 5 on a split using Gaussian integration was performed where position was performed, which gave a small, but the crystal shape was approximated by four significant improvement, final R = 0.0264. The oc boundary planes; the transmission factor varied cupancy of each site was kept at 0.5. However, it between 0.780 and 0.892. Averaging over symme was not possible to include the corresponding hy try equivalent reflections gave R = 0.0490. drogen atoms in this refinement. The same space group (P2,/c, no. 14 [9]) and the In the final cycle, 187 parameters were refined, close resemblance of the lattice parameters to including an isotropic extinction coefficient, type those of FeS0 4 • 7 H20 were taken as a strong indi II with a Gaussian distribution of mosaic blocks cation that CoS0 4 -7D20 is isotypic. Therefore, according to Becker and Coppens [10] with a final the atomic coordinates as reported by Baur [4] were value of 1.07(4)-105. Seven low-angle reflections taken as starting values for the subsequent refine were obviously severely affected by extinction ments. In order to facilitate the comparison be (more than 50% difference) and were excluded. tween the two structures, the established atomic 2339 reflections with F 2 > 3

Table I. Fractional atomic coordinates and Atom .Y y z Ueq/A2 equivalent isotropic temperature factors for CoS0 4-7D-,0. Asterisks indicate isotropically Co(l) 0.0 0.0 0.0 0.0211(2) refined atoms. The occupancy of 0 5a and 0 5 b Co(2) 0.5 0.5 0.0 0.0208(2) was set to 0.5. The U have been estimated ac S 0.22675(2) 0.47241(5) 0.17647(3) 0.0196(2) cording to Ueq = 1 /3 EjjUya^a 0(1) 0.20467(8) 0.47122(16) 0.03652(10) 0.0289(5) 0(2) 0.13814(7) 0.54217(17) 0.21371(10) 0.0315(5) 0(3) 0.30951(2) 0.61333(17) 0.22694(10) 0.0327(5) 0(4) 0.25251(7) 0.26351(16) 0.22663(10) 0.0309(5) 0(5 a ) 0.1229(4) 0.4045(12) 0.4434(8) 0.031(2) 0(5 b) 0.0992(5) 0.3642(13) 0.4231(9) 0.041(3) 0(6) 0.09718(9) 0.96137(20) 0.18159(12) 0.0362(6) 0(7) 0.02906(8) 0.79182(17) 0.43215(11) 0.0337(5) 0(8) 0.47956(9) 0.44943(20) 0.17767(10) 0.0306(5) 0(9) 0.43148(8) 0.27931(18) 0.44138(11) 0.0329(5) 0(10) 0.35744(7) 0.85773(18) 0.44239(11) 0.0339(5) 0(11) 0.36435(10) 0.00542(19) 0.11696(14) 0.0338(6) D(51) 0.138(2) 0.293(4) 0.455(2) 0.040(5)* D(52) 0.117(2) 0.432(4) 0.369(3) 0.053(6)* D(61) 0.114(2) 0.850(4) 0.199(2) 0.059(7)* D(62) 0.145(2) 1.035(4) 0.209(2) 0.046(6)* D(71) 0.077(2) 0.866(3) 0.462(2) 0.031(4)* D(72) -0.010(2) 0.859(4) 0.389(2) 0.060(7)* D(81) 0.428(2) 0.514(3) 0.187(3) 0.050(7)* D(82) 0.522(2) 0.474(4) 0.225(3) 0.058(8)* D(91) 0.380(2) 0.281(4) 0.377(3) 0.071(8)* D(92) 0.420(2) 0.340(4) 0.486(3) 0.065(8)* D(101) 0.307(2) 0.923(4) 0.455(3) 0.063(7)* D (102) 0.341(2) 0.792(4) 0.379(2) 0.061(7)* D (lll) 0.324(2) 0.067(4) 0.139(2) 0.043(6)* D( 112) 0.347(2) -0.088(4) 0.096(2) 0.052(7)* Th. Kellersohn et al. • Cobalt Sulfate Heptahydrate, CoS04-7D;0 1637

Table II. Selected bond lengths (pm) and angles ( ) with e.s.d.'s given in parentheses. The variances s of the average values were calculated as n X (.Yj-.Y)2 _ i=l n - 1 Symmetry operations: 0 = x, y, 1 = -V, - 1/2+y, 1/2—z; 2 = x, 1/2—y, -1/2+z; 3 = x, - 1 + v, z; 4 = -.y, -y , -z; m

Sulfate ion S0-O(l)° 147.9(1) O(l)0-S°--0(2)° 108.83(6) S0-O(2)° 147.9(1) O(l)0-S°- -0(3)° 108.79(6) S0-O(3)° 146.8(1) O(l)0-S°--0(4)° 110.15(6) S0-O(4)° 147.3(1) 0(2 )°-S°--0(3)° 110.28(6) O(2)°-S0--0(4)° 108.83(6) O(3)0-S°--0(4)° 109.96(6) average'1 147.5(5) 109.5(7)

Co( 1) coordination Co( 1)°—0(5a)'-2 207.6(7) 0 (5 a )-C o (l)-0 (5 b ) 12.1(3) (1-0-1, 2-0-2) Co( 1 )0-O (5b)12 201.0(8) 0(5a)-Co(l)-0(6) 86.1(2) (1-0-4, 2-0-3) Co(l)0-O(6)3-4 210.7(1) 0(5a)-Co(l)-0(7) 84.9(2) (1-0-1, 2-0-2) Co( 1 )0-O(7)'-2 211.4(1) 0(5b)-Co(l)-0(6) 84.4(2) (1-0-3, 2-0-4) 0(5b)-Co(l)-0(7) 88.4(3) (1-0-2, 2-0-1) 0(6)-Co( 1)—0(7) 85.04(5) (3-0-1, 4-0-2) average11 209(4) 90(5)bc

Co(2) coordination Co(2)°-0(8)0-5 206.2(1) 0(8)-Co(2)-0(9) 89.04(5) (0-0-2, 5-0-6) Co(2)0-O(9)6-2 207.4(1) 0 (8 )-C o(2)-0( 10) 89.69(5) (0-0-8, 5-0-7) Co(2)0-O( 10)7-8 214.4(1) 0(9)-Co(2)-0( 10) 87.95(4) (6-0-7, 2-0-8) average11 209(4) 90(1 )b

Water molecules and hydrogen bonds o U 0 O O o 1 O -D O O -D D-O O-D-O D-O-D O ••• Ow ••• o S O(6)0-D (61)°--0(2)° 77(3) 278.6(2) 203(3) 169(3) 107.27(6) 107(3) 121.49(5) 0(6)°- D(62)° ••• 0(4) 82(2) 287.9(2) 209(2) 163(2) 110.75(6) O(7)0-D (71)°-O (l) 0 82(2) 287.7(2) 206(2) 175(2) 124.39(5) 107(3) 113.38(5) 0(7)°- D(72)° ••• 0(2) 1 76(3) 295.5(2) 220(3) 170(3) 118.92(5) O(8)°-D(81)0-O(3) 87(3) 279.4(2) 193(3) 169(2) 117.27(6) 112(3) 109.58(5) O(8)0-D (82)°-O (l l)6 70(3) 271.8(2) 203(3) 169(3) 118.30(6) O(9)0-D(91)°-O(4) ’ 87(3) 295.6(2) 209(3) 176(3) 116.33(5) 105(3) 101.32(5) O(9)0-D (92)°-O (l 1 )9 68(3) 273.4(2) 206(3) 171(3) 116.56(6) 0(10)°-D(101)°- 0(1)10 87(3) 283.8(1) 200(3) 162(3) 117.50(5) 109(3) 119.59(5) 0( 10)°- D( 102)° • • • 0(3)° 80(3) 277.1(2) 198(3) 175(3) 119.73(5)

0( 11)°—D(111)° ■ • • 0(4)° 78(2) 277.5(2) 202(2) 165(2) - 0(11)°-D(112)°-0(10)2 67(3) 301.8(2) 246(3) 143(3) 111(3) 144.76(6)d - 0(1 l)0-D(112)°-O(3)3 300.1(2) 255(3) 84(1) 96.06(5)e -

a For the average values, the variances are given in parentheses; b symmetry-related values (180 - a ) included; c 0 (5 a)-C o- 0 ( 5 b) not included; d 0(4)°■ 0(1 l)° - 0(10)2;e0(4)°-•O (ll)0 "O(3)3. 1638 Th. Kellersohn et al. ■ Cobalt Sulfate Heptahydrate, CoSQ4 7D-,0

Discussion the angles of the Co( 1) octahedron significantly, as can be seen from the variances of the average val A comparison of the structural parameters with ues (see Table II). The averages themselves are re those of the iron compound reveals the very close quired to be exactly 90 due to the I symmetry. resemblance of the two structures, even in the finer The Co —O bond lengths average to 209 pm for details. It should be noted that the hydrogen posi both Co(l) and Co(2); however, the distributions tions as determined in this study correspond quite of the individual values around these averages are well to those derived by Baur [4, 13] from mainly electrostatic considerations. different for the two cobalt atoms.

General aspects of the structure The disordered water molecule The structure consists of two crystallographical- Trigonally coordinated water molecules are of

ly independent [Co(H 20 )6]2+ octahedra which are ten observed in the presence of highly charged, arranged in a pseudo face-centered lattice, a sulfate small cations, e.g. Be2+, Al3+ or Cr3+. In contrast, ion, and a “lattice” water molecule which is not to our knowledge no example of trigonal coordi bound to a metal ion, see Fig. 1. These principal nation has been found up to now for monovalent elements are linked via an extensive hydrogen- ions or an incoming hydrogen bond on the lone- bond system. pair side of the water oxygen. Classical examples The two cobalt atoms are both located on a for this distinction are all alumns, where both ex symmetry centre. Their coordination is basically tremes occur within the same compound due to the octahedral (see Fig. 2 and 3) with the respective presence of both M 1 and Mm cations. In an inter bond angles differing only to a small extent from mediate range of cations, for the heavier alkaline 90 for Co(2), whereas the disorder of 0(5) distorts earth ions and low charged transition metals, both

Fig. 1. Structure of CoS04 ■ 7 D ,0 in polyhedral representation. The octahedra around Co(l) are thin, those around Co(2) bold hatched. Co and S atoms are not included, O and D are drawn with arbitrary radii. The disorder of 0(5) has intentionally been neglected. Th. Kellersohn et al. • Cobalt Sulfate Heptahydrate, CoSQ4-7D:Q 1639

seems to be dynamical as deduced from vibration al spectroscopic measurements [17], In the title compound, however, this instability is even more pronounced, and the crystal structure analysis yielded evidence for a disorder of the wa ter molecule. The refined oxygen positions are sep arated by 44( 1) pm (see Fig. 2 b).

Since the situation in CoS0 4 -7D20 is compara

ble to that in S r(I0 3 )2 H20 and Sr(Br0 3)2 -H 20 due to the very similar arrangement of the metal ion and the two hydrogen-bond accepting anionic oxygen atoms, it seems likely that the hydrogen (deuterium) atoms in the title compound do like wise not or only to a small extent participate in the disorder, which has been shown for the Sr com pounds by neutron diffraction. Additionally, S042~ ions are even stronger hydrogen bond ac ceptors than halate ions, which supports this as sumption. However, final evidence must be pro vided by neutron diffraction. It should be pointed out that this disorder dif fers substantially from the type commonly found in solid hydrates, which involves a distribution of the hydrogen atoms over different hydrogen- bonded sites, usually with (crystallographically) different acceptors. A hydrogen disorder is also frequently observed for water molecules in zeolites when these are not too loosely bound; a recent ex ample is given in [18]. On the other hand, the clas sical structure of Ice Ih offers an even more com-

Fig. 2. [Co(D-,0 )6]2+ octahedron around Co(l), without (Fig. 2a) and including (Fig. 2b) the disorder of the wa ter molecule 0(5). The vibrational ellipsoids are scaled to include 50% probability [14], the hydrogen atoms are drawn with arbitrary radii.

cases as well as intermediates may be observed. The limits of this group are, however, not well de fined at present. But that the trigonal coordination within this last group is unfavourable is revealed by the “structural instabilities” which may arise, as

recently described for Sr(I0 3)2 H20 [15] and

Sr(B r0 3)2 -H20 [16]. For these two compounds, it was not possible to refine a disordered position of the water molecule from room temperature X-ray Fig. 3. [Co(D20)6]2+ octahedron around Co(2). See Fig. 2 and neutron data. The nature of this “instability” for further explanations. 1640 Th. Kellersohn et al. • Cobalt Sulfate Heptahydrate. CoS04-7D^O plicated situation which involves a disorder of the hydrogen as well as of the oxygen atoms [19]. All these examples, however, represent cases where the disorder is not so severe to prevent a location of the atoms by diffraction methods.

Hydrogen bonds The hydrogen-bond lengths (0 --0 distances) vary between 271 and 302 pm. The water mole cules around Co (except 0(10)) belong to the tri gonal classes 1 and 1 ' [2 0 ], respectively, with only one coordinated partner on the oxygen lone-pair side. The deviation from the ideal trigonal coordi nation varies considerably, however (see Table II).

0 ( 1 0 ) is the only water molecule coordinated to Co which also accepts a hydrogen bond (see Ta Fig. 4. Environment of the “lattice” water molecule. ble II), and this is reflected in the somewhat en Open lines represent hydrogen bonds. See Fig. 2 for fur larged Co(2)-0(10) distance. The resulting geom ther explanations. etry for 0 ( 1 0 ) is, however, quite irregular. The ‘'lattice” water molecule (O il) is acceptor of two strong hydrogen bonds. It donates one hy ness of this concept for the analysis of hydrogen- drogen bond to a sulfate ion, the second one is bi bonded networks, as has already been pointed out furcated with 0 ( 1 0 ) as well as one sulfate oxygen, [22,23], 0(3), as acceptors, see Fig. 4. That this bond is bi furcated can be understood in terms of the bond- This work has been supported by the Deutsche valence concept [21]. Applying the valence sum Forschungsgemeinschaft (in the form of a post rule yields a clear deficiency for the D(112) site if doctoral fellowship for T. K.) and the Swedish only one of the two possible acceptors is included, Natural Science Research Council which is grate but a good agreement when both are considered. fully acknowledged. We also wish to thank Mr. This might serve as another example for the useful- Hilding Karlsson for technical assistance.

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