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Synthesis & Spectral Studies of Fluorosulphates ofSome Lanthanides

SUDHINDRA N. MISRADepartment of Chemistry, University of Jodhpur, Jodhpur

Received 23 May 1980; revised 22 September 1980; rerevisedand accepted 9 February 1981

Fluorosulphates of La(III), Ce(llI), Pr(ID), Nd(III), Sm(III).Eu(III), Tb(III), DY(ID), Ho(lII), Er(m) and Y(ID) have beenisolated from the reaction of the anhydrous metal chlorides andfluorosulphurlc acid. These derivatives have been found to betetrameric in fluorosulphurlc acid. Diffused reflectance spectraof these chelates indicate partial covalent character of themetal-ligand bond. A comparison of tbe values of nephelauxeticratio (~), percentage covalency parameter ( 8) and bondingparameter (bl") shows that the covalency in these chelates increa-ses with increase in the atomic number of lanth nide. Infraredspectra ofthese compounds have been studied in the region 5000-100cnr? and the thermal decomposition behaviour has been investi-gated.

IN an earlier study", it was found that the reactionof lanthanide(IlI) oxides with peroxydisulphuryl

difluoride yielded oxyfluorosulphate with lighterlanthanides (upto samarium) while tris(fluoro)-sulphates were obtained with heavier lanthanides(beyond samarium). Since tris(fluoro)sulphatesof La(IIl), Ce(IlI), Pr(IlI), Nd(IlI) and Sm(IIl)could not be obtained by the above mentioned reac-tion, it was thought worthwhile to synthesize themfrom the reaction of fluorosulphuric acid itself.Fluorosulphates of La(IIl), Ce(IlI), Pr(IlI), Nd(IlI)Sm(IIl), EU(lll), Tb(IlI), Dy(IlI), Ho(III), Er(IIl)and Y(IIl) have been prepared now by the reactionof anhydrous metal chlorides and fluorosulphuricacid.

Fluorosulphuric acid was redistilled before use .in a pyrex apparatus.

NOTES

Anhydrous lanthanide chloride was taken in excessof fluorosulphuric acid and the reaction mixturerefluxed under a current of dry nitrogen. The micro-crystalline solid that separated was washed thorough-ly with fluorosulphuric acid in an inert atmosphereand dried by heating at 80°C in vacuo (0.001 mm)for 6 hr.

For metal analyses, fluorosulphates were decompo-sed by boiling with 4M NaOH for I hr, the precipi-tates filtered and dissolved in the minimum quantityof HCl. The metal was precipitated as oxalate by theaddition of ammonium oxalate. Sulphur was estima-ted as BaSO 4'

Fluoride content in the fluorosulphates was deter-mined by decomposition with phosphoric acidfollowed by steam distillation in the apparatus desig-ned by Seal".

Diffused reflectance spectra were recorded in LiFmedium. Infrared spectra were recorded in the region3000-100 cnr? using KBr and AgCl windows.Molecular weights of Nd(lll), Pr(IlI) and Sm(III)fluorosulphates have been determined by the ebullio-scopic method using freshly distilled fiuorosulphuricacid as the solve nt.

Electronic spectra of these derivatives in the visibleregion indicated the red shift of the bands. The mag-nitude of red shift (nephelauxetic effect) is dependenton the change in the interelectronic repulsion para-meter. The nephelauxetic ratio (~)3,4 and thepercentage covalency parameter (~) were calculatedusing well known equation='. The value of ~ hasbeen found to be positive in all these complexes indi-cating electron delocalization from 4/ orbitals. Henrieand Choppin" correlated the bonding parameterbl/2 to the nephelauxetic ratio by the relationship, bIIS

TABLE I-ANALYTICAL DATA OF LANTHANIDE FLUOROSULPHATES

Found (Calc.), %

Lanthanon S F

La(OSO.F)3 31.6 22.1 12.8(31.9) (22.0) (13.1)

Ce(OSO.F)3 31.7 22.0 12.7(32.0) (21.9) (13.0)

Pr(OSO.F)3 31.9 21.6 12.8(32.2) (21.9) (13.0)

Nd(OSO.F)3 32.3 21.9 12.6(32.7) (21.7) (12.9)

Sm(OSO.F)3 33.1 21.6 12.2(33.6) (21.5) (12.7)

Eu(OSO.F)a 33.4 21.3 12.3(33.9) (21.4) (12.7)

Gd(OSO.F)3 35.0 21.6 12.2(34.6) (21.3) (12.6)

Tb(OSO.F)3 34.2 21.0 12.0(34.7) (21.~ (12.5)

Dy(OSO.F)3 34.9 20. 11.9(35.3) (20.9) (12.4)

Ho(OSO.F)3 35.2 20.7 11.9(35.~ (20.8) (12.3)

Er(OSO.F)3 35. 20.4 11.7(35.9) (20.6) (12.2)

Y(OSO.F)3 23.0 24.6 14. 3(23.1) (24.6) (14.7)

Molcc.wt

1800

1900

1860

845

INDIAN J. CHEM'J VOL. 20A, AUGUST 1981

TABLE2 - NEPHELAUXEnCRAno (~), PERCENTAGECOVALENCYPARAMETER(II) ANDBONDINGPARAMETER(b"') OF LANTHANIDE

FLUOROSULPHATES

Compound

Pr(OSO,F).Nd(OSO,F).Sm(OSO,F)3Eu(OSO,F)3Dy(OSO.F)3Ho(OSO.F).Er(OSO.F).

~0.98400.98340.98040.97840.97600.97400.9700

3

1.6261.6881.9992.2072.4592.6693.092

0.08940.09110.09900.10390.10950.11400.1225

= [(1_~)/2P/2. The value of parameter bi/2 indicatesthe extent of 4/ participation in the complexation.Greater the magnitude of this parameter, greater isthe contribution of the 4f orbital to complex forma-tion. The positive values of ;) and bi/2 and less thanunity values of f3 in the present complexes point to-wards the incidence of covalency in the matal-ligandbond. Comparing the values of these parameters(Table 2) in different lanthanides, one finds a gradualincrease in the covalent character with the increasein atomic number of lanthanide. This is in confor-mity with the lanthanide contraction. The infraredspectra of these fluorosulphates indicate the presenceof three bands below 300 cm-I which may be due tostrong cation-anion forces and lower crystal symmetrycausing different lattice vibrations to appear morestrongly and at higher frequencies than in potassiumfluorosulphate". This suggests increased covalentinteraction in the lanthanide fluorosulphates. Thetwo-fold degenerate '16 transition seems to increasein strength and split into two absorptions in the.spectra of lanthanide fluorosulphates. Similarlydoubly degenerate '14 transition occurs in the formof two bands. All these observations point towardssome covalent character in these compounds whichmay be due to the tendency for pn-dn bondingbetween sulphur and fluorine as a result of the with-drawal of charge through oxygen by the cation.

Fluorosulphate group can be attached to metalin the covalent monodentate"?", bidentate and brid-ging1hI2 and tridentateI3,I4 manner. The lanthanidefluorosulphate frequencies are generally in the rangereported for tri-and bidentate fluorosulphate groups,the differences in the v S-Omodes are lower than thosepreviously observed and v S-F is observed ata higherfrequency than usual.

The IR spectra of the complexes generally indicatea substantial increase (compared to free SOaF) inthev'S-F frequency (100-150 crrr"), a smaller increasein the v S-O,;) S-O modes (10-50 cm-l) and decreasein vS-O (20-70 crrr"). The position of ;)S-F modeis not significantly changed. Similar changes areexpected for bidentate groups. The splitting is expec-ted to increase with the increased anion-cationinteraction.

The molecular complexity of ,....4 for these lantha-nide fluorosulphates explains their sparingly soluble,non-volatile nature. The lanthanides can exhibit

846

coordination number greater than six. It is quiteprobable that 8 fluorosulphate oxygens may bearranged around metal thus O-S-O angle will besmaller than O-S-O angle for hexacoordinate geo-metry. Vibrational interactions between S-O bondsare decreased as O-S-O angles move closer to 90'and consequently the s-o stretching frequenciesmove closer together. On the other hand, the O-S-Fangle is nearer to 1800 leading to increased S-O toS-F coupling and higher S-F stretching frequencies.Vibrational changes based on this simple change ingeometry are manifested in the spectra of lanthanidecompounds as shown by continuously decreasingseparation of bands assigned to symmetric (VI)and asymmetric ('14) stretching of the tridentate-SOa groups with decreasing cation size. The vS-Fvalues generally increase from La to Er analogues.However, trends in frequency differences are likelyto be more reliable indicators of structural variationsthan absolute values of frequencies since the latterwill be more dependent upon environmental changes,especially for polar S-F and S-O bonds.

The thermal decomposition patterns of these deri-vatives show that the compounds are stable upto140-180aC, above which range slow decompositionsarts which becomes fast at 2S0-280aC. The productsobtained after removing the volatile fractions (S03'Sta05F2, S02F2 etc.) were metal sulphates and metalfluorides.

The decomposition pattern shows that the lighterlanthanide fluorosulphates which are more ionic innature yield disulphuryl difluoride as the main de-composition product. The amount of disulphuryldifluoride decreases while that of sulphur trioxideincreases with the increase in the atomic number oflanthanide. Though yttrium is not a member oflanthanide series, the decomposition pattern ofyttrium fluorosulphate is almost similar to that oferbium fluorosulphate.

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