Crystal and Molecular Structure of Hexaaquocobalt(II)-&xafluorosilicate(W), [Co(H20)6] [SiF6]

H. LYNTON A N D P-Y. SIEW Department of Chemistry, University of New Brunswick, Fredericton, New Brunswick

Received June 19, 1972

The structure of hexaaquocobalt(I1)-hexafluorosilicate(1V) has been determined by the heavy atom method. The compound crystallizes in the space group R3, with three molecules in the hexagonal unit cell of dimensions a = 9.358(16), c = 9.712(14) A. The atomic parameters were refined by block diagonal least squares using anisotropic thermal parameters. The hydrogen atom positions were established but the parameters were not refined. The final agreement residual for 151 observed reflections is R = 0.043.

The structure consists of discrete CO(H,O),~+ and (SiF6),- octahedra joined systematically by 0---W--F hydrogen bonds into chains parallel to the 3 axis.

La structure de l'hexaaquocobalt(I1) hexafluorosilicate(1V) a t t t determinte par la mkthode de l'atome lourd. Le compost cristallise dans le groupe R3 avec trois molecules par unite hexagonale de dimensions a = 9.358(16)& c = 9.712(14) A. Les parametres atomiques ont t t t raffints par la mtthode des moindres carrts appliquee aux matrices diagonales qui utilise les parametres d'anisotropie thermique. Les positions des atomes d'hydrogene ont ete etablies mais les parametres n'ont pas ete raffints. L'accord final apres 151 reflexions observees est R = 0.043.

La structure consiste en octaedre discret de CO(H,O),~' et (SF,)'- relit systematiquement par des liaisons hydrogene 0---H---F, en chaines paralleles aux axes 3.

[Traduit par le journal] Cdn J Chern . 51. 227 (1973)

Introduction

Cobalt fluorosilicate was prepared by the action of hydrofluorosilicic acid, H2SiF6, on cobalt carbonate, CoCO,, and was found to have the formula CoSiF6.6H20(l). The cell dimensions as given in the Powder line file (2) are a = 9.370 A and c = 9.732 A in space group R3in, No. 160, ref. 5. I t had been reported that in compounds of the type MSiF,.xH,O, the (SF6)'- species was discrete and formed chains with the M(H20):+ species either through F-M coordination, as in CuSiF6.4H20 (3), or F---0 hydrogen bonding as in [Fe(H,O),][SiF,] (4). An X-ray diffraction analysis was undertaken to determine the type of arrangement in CoSiF6.6H,0.

Experimental The crystal data are as follows:

[Co(HzO), I [SiF,I f.w. = 308.98 Trigonal, a = 9.358(16), c = 9.712(14) A, v = 736.6 A3, p, = 2.09(2), z = 3, p, = 2.089 (20 "C, MoKa, h = 0.71069 A).

The unit cell dimensions and their estimated standard deviations (in parentheses) were obtained from a least squares evaluation of the positions of 12 independent reflections. The space group is R3 (C,?) NO. 148 in ref. 5.

The crystals were pink, ditrigonal prisms, elongated in the c direction. They were air sensitive and slowly de- composed on prolonged air exposure.

A small crystal of size 0.6 x 0.4 x 0.3 mm3 was mounted, sealed inside a thin-walled Lindermann capil- lary, with the c axis parallel to the 4 direction of the diffractometer. Intensity data were collected on a Picker, computer-controlled, four-angle diffractometer with p- filtered MoKa radiation and scintillation detector using a 9/29 scan. Two octants hkl and hkl were explored to a 29 maximum of 40". There were 189 accessible reflections in this region of which 151 were considered observed. The threshold was taken as 20 counts; a scan speed of lc/min was used with a scan range of IS", corrected for dispersion. The background was taken for 20 s at each end of the range. Data was also corrected against a standard reflection taken at intervals of 20 readings. The linear absorption coefficient for MoKa radiation is 19.6 cm-'. No corrections were made for absorption or extinction.

Structure Analysis

The cobalt and silicon atoms were located from a three dimensional Patterson synthesis and are found to be in special positions 3a and 3b respectively. A Fourier synthesis established coordinates for the oxygen and fluorine atoms in general positions 1 Sf.

Refinement was carried out by block-diagonal least squares, using 3 x 3 matrices for atomic coordinates and 6 x 6 matrices for anisotropic thermal parameters for each of the atoms. The quantity minimized was Cw(lFol -

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228 CAN. J. CHEM. VOL. 51. 1973

TABLE 1. Final parameters (estimated standard deviations) x, y, z x lo4, uij x lo3 t.f. = exp [ - 2 . r r ' ( ~ * ~ h ~ u ~ ~ + ... + 2b*c*kluZ3 + ...)I

Atom x Y z ul l (or u) uzz u3 3 u2 3 u13 ~ I Z

TTo attain charge neutrality the fluorine atom was assigned a charge of 113- and the values for its scattering factor curve were obtained from interi?olating those for F0 and F-.

Successive cycles gave an agreement residual, R = C(IIFol - IFcll)/CIFol, of 0.049 for the observed reflections for all atoms except the hydrogens. The weighting scheme used was w = 1/11 + [(IF,l - P , ) / P ~ ] ~ ) withPl = 35andP, = 45.

Scattering factor curves used were those given in ref. 6. The Co2+ and Si were corrected for anomalous dispersion.

A difference synthesis showed, in the vicinity of the oxygen positions, peaks of height around 0.5 e AW3. They grouped into two sets of general positions, 18J These were taken to be the hydro- gens. No other peaks with height greater than 0.3 e A-3 were present. The hydrogen coordinates were included in the least squares calculation and assigned an isotropic temperature parameter equal to that of the oxygen. The parameters for the hydrogens were not refined. The final agree- ment residual for all atoms was R = 0.043.

A table of structure factors for the final parameters as listed in Table 1 has been placed in the Depository of Unpublished Data.'

C O ( H ~ O ) , ~ ~ and (SF,)'- ions are rotated by 27.4(3)" with respect to each other about the 3 axis. Both are distorted such that the 0-O(1) and F-F(l) distances are shorter than those of the 0-O(2) and F-F(2). Similar distortion is found in the F e ( ~ , 0 ) , ~ + ion of [Fe(H,O),]- [SiF,]. The bond distances and angles are listed in Tables 2 and 3.

Figure 1 gives a projection of the structure along the b axis. Hydrogen atoms are included to illustrate the bonding. Each of the fluorine atoms is involved in two hydrogen bonds.

Table 2 also compares [Co(H,O),] [SiF,] with the related [Fe(H,O),] [SiF, ] structure. The bond lengths and bond angles for the two structures are close except for a longer 0-F(2) distance of 2.785(6) A and a correspondingly shorter Si-F

TABLE 2. Structural data for [Co(H,O),] [SiF6j and IFe(H20)61 [SiF,I*

Distance (A) -

Bond C O ( H ~ O ) ~ S ~ F ~ Fe(H20)6SiF6

Discussion M-0 2.079(6) 2.146(12)

The decrease in the residual R obtained by inclusion of the hydrogen atoms is significant by Hamilton's test (7) at the 0.025 (2.5%) level. These hydrogen positions are consistent with presence of hydrogen bonding in the structure.

The structure consists of discrete C O ( H , ~ ) , ~ + and (SF,)'- octahedra which are systematically joined by 0-H---F hydrogen bonds into chains parallel to the 3 axis. The 0-H---F distances

Si-F 1 .643(5) 1 .706(9)

0-F(2) 2.785(6) 2.681(13) 0-F(3) 2.711(8) 2.720(13)

0-O(1) 2.924(9) 2.991(19) 0-0(2) 2.955(6) 3.063(21)

F-F(1) 2.292(9) 2.390(15) F-F(2) 2.355(6) 2.396(14)

Angle (deg)

2.785(6) A within, and 2.71 l(8) A between chains Bonds Co(H20),SiF6 Fe(H20)6SiF6 are reasonable for such bonds (8). The discrete

'Complete set of tabular data is available, at a nominal charge, from the Depository of Unpublished Data, National Science Library, National Research Council of Canada, Ottawa, Canada. K I A OS2. *Estimated standard deviations in parentheses.

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LYNTON AND SIEW: STRUCTURE OF [Co(H2O)6][SiF6] 229

TABLE 3. Bonds involving hydrogen in [Co(H20)6 I ISiF,I

Bond* Distance (A)

Bonds Angle (deg)

*Intra, bonds connecting atoms of same chain; inter, bonds connecting atoms of adjacent chains.

We wish to thank Dr. M. J. R. Clark for the cobalt fluorosilicate crystals and the National Research Council of Canada for their financial support. We also thank Dr. F. R. Ahmed of the Biochemistry Laboratory, National Research Council of Canada for use of his computer programs.

1. M. J. R. CLARK. Ph.D. Thesis. University of New Brunswick, Fredericton, New Brunswick, 1971.

2. National Bureau of Standards (U.S.) Monograph 2 5 , Section 3 (1963).

3 . M. J . R. CLARK, J. E. FLEMING, and H. LYNTON. Can. J. Chem. 47, (20) 3959 (1969).

4. W. C. HAMILTON. Acta Cryst. 15, 353 (1962). 5. International tables for X-ray crystallography. Vol. I.

The Kvnoch Press. Birmingham, England. 1965. 6. ~nternational tables for x-ray crystallography. Vol.

FIG. 1. [CO(H,O)~] [SF6] : projection along the b 111. The Kynoch Press, Birmingham, England. 1962. axis. 7. W. C. HAMILTON. Acta Crvst. 18, 502 (1965).

8. W. C. HAMILTON and J. A. IBERS. Hydrogen bonding distance of 1.643(5) A for the [Co(H,O),][SiF,] in solids. W. A. Benjamin, Inc. N.Y. 1968. structure. The F-F distances are also different for the two compounds.

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