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Photolysis of Alkaline Earth Metal Tris(Oxalato)Ferrates(III): a Mossbauer Study

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Photolysis of Alkaline Earth Metal Tris(Oxalato)Ferrates(III): a Mossbauer Study Indian Journal of Chemistry Vol. 25A. January 1986, pp. 23-25 Photolysis of Alkaline Earth Metal Tris(oxalato)ferrates(III): A Mossbauer Study AS BRAR·, S BRAR & S S SANDHUt Chemistry Department, Indian Institute of Technology. New Delhi 110016 Received 19 April 1984; revised and accepted 30 August 1985 Photolysis of alkaline earth metal tris(oxalato)ferrates(llI) in solid and liquid states has been investigated using Mossbaucr and electronic spectroscopy. Iron(l I) species is formed in solid as well as liquid state. The effect of outer cations. viz. Mg, Ca. Sr and Ba has been observed on the formation of intermediates and quantum yields for Fe(II) species in the liquid state. In earlier papers the effect of outer cations (Li, Na, K, Results and Discussion Cs) on the photolysis and radiolysis of alkali metal Mossbauer parameters, IR data and magnetic moments tristoxalatojferratestl ll)"? and alkali metal of trisioxalato )ferrates(III) of alkaline earth bis'citratojferrates lf l):' in solid and liquid phases was M~ssbauer spectra of the alkaline earth metal reported. The photolysis of hexamminecobalt(III)tris- tris(oxalato)ferrates(III) of Mg, Ca, Sr and Ba are (oxalato)ferrate(III) has also been reported." As an broad, due to spin-lattice relaxation effect? The isomer extension the effect of outer cations, viz. Mg. Ca, Sr. shift value for magnesium, calcium, strontium and and Ba on the products of photolysis of alkaline earth barium tris(oxalato)ferrates(llI) are 0.24. 0.26. 0.25 tris(oxalato)ferrates(I1I) and a comparison of results and 0.31 mms-I respectively, the values for strontium with those of potassium ferrioxalate are reported in and barium tris(oxalato)ferrates(lII) being close to this paper. The feasibility of its use in actinometer for those reported by Gallagher and coworker. 8 The the measurement of intensity shall be discussed. isomer shift values indicate these complexes to be high- spin, with +3 oxidation state of iron in an octahedral Materials and Methods symmetry. Tris(oxalato)ferrates(III) of Mg(II), Ca(II), Sr(I1) Infrared spectra of magnesium and calcium and Ba(I1) were prepared as reported in our earlier tris(oxalato)ferrates(III) exhibit bands at 3300 (vOH), publications. The experimental details of photode- 1650 (\'"C=O) and 1425 and 1260cm-' (l'sC=O), composition in solid and liquid states were the same as In the case of strontium tris(oxalato)ferrate(III) these described earlier.'? The details of Mossbauer setup modes appear at 3300, 1640 and 1435, 1285cm-', were also same as reported earlier. Photolysis in liquid respectively, while in the case of barium tris(oxalato)- state was investigated under inert 02-free nitrogen ferrate(III) these modes appear at 3320, 1675 and atmosphere. The reaction mixture, taken in a quartz 1410, 1270cm-', respectively. These observations sup- photochemical reactor(200 ml) was irradiated at 20°C port the presence of coordinated oxalate ligand." with 300nm radiations using a 125 watt medium The elemental analyses of these ferrioxalates in- pressure mercury lamp (Hanovia). Doubly distilled dicate the presence of water of crystallization which is water and 0.2 M sulphuric acid were used for each run. further confirmed by TG. Reflectance spectra of these Oxygen-free nitrogen gas was bubbled both for de- complexes exhibit a continuous absorption band from oxygenation and stirring the mixture, prior to and 200 to 550 nm, which is presumably due to ligand to during the reaction. The intensity of the lamp which metal charge transfer, and a very weak (d-d, spin- was found to be 2 x 10 17 quanta ml-' s -I. and quan- forbidden) band in the range of 650-690 nm. These tum yield was measured using potassium ferrioxalate values are in agreement with the values reported in actinometer. 6 Thermogravimetry (TG) Stanton ther- literature for tris(oxalato)ferrate(III) complexes'?" . mobalance at a heating rate of lO°C/min was used to Magnetic moments for magnesium, calcium, stron- study the thermal behaviour. Magnetic measurements tium and barium tris(oxalato)ferrates(JII) are 5.65, were made on a Guoy balance using Hg complex as the 5.64,5.78 and 5.76 B.M. respectively, indicating that calibrant. Infrared spectra were recorded in KBr iron in these compounds is in +3 oxidation high spin matrix on a Perkin-Elmer 570B model spectrophoto- state. meter. tChemistry Department. Guru Nanak Dev University. Amritsar Solid state photolysis 143005 All the alkaline earth tris(oxalato)ferrates(lII) were 23 INDIAN J. CHEM .. VOL. 25A. JANUARY 1986 Table 1- M~ssbauer Parameters of Solid State Photolysis of Alkaline Earth Tris(oxalato)ferrates(llJ) SI.No. Complex Isomer shint Quadrupole splitting Tentative assignment (mms-') (mms-') 1 Mg3[Fe(C,04hJ,.6H,O 0.24 ± 0.03 Parent complex 2 Ca3[Fe(C,04hJ,.8H,O 0.26 ± 0.03 3 Sr3[Fe(C204hJ,.12H20 0.25 ±0.03 0.40 ±O.03 4 BaJ[Fe(C,H4hJ,.lOH,O 0.31 :!. 0.03 5 Product 1* 1.20± 0.03 1.80± 0.03 r-c,o •.2H,0 6 PrO<luctII* 1.20±0.03 1.96±O.03 FeC,o •. 2H,o 7 Product III* 1.18±0.03 2.30 ±0.03 Fe"(C,O.),(H,o)/- 8 Product IV* 1.20± 0.03 2.68 ± 0.03 Fe" (C 0 ) (H 0) z . *P d I IV btai d . ,.," ro ucts - are 0 tame , respectivelyon irradiation of Mg,[Fe(C,04hJ,.6H ,0. Ca,[Fe(C,04hJ,.8H,O. Sr,tFe(C,04hJ,.12H,O and Ba,[Fe(C,OJ,J,.lOH,O for 300hr. tAli isomer sliin values are with respect to natural iron. Table 2-Quantum Yields for Formation ofIron(lI) Species During Photolysis of Alkaline Earth Tris(oxalato)ferrates- (III) in Solution Phase SI.No. Complex Quantum yield I Mg3[Fe(C20.hJ,·6H20 0.47 2 Ca3[Fe(C,04)3J,·8H,O 0.54 3 Sr,[Fe(C,O.hJ2.12H,O 0.91 4 Ba,[Fe(C,04hJ,·IOH,O 0.91 tris(oxalato)ferr.ate(III), [FeZ(Cp4)S]6- has been re- ported to be formed. This shows that outer cation affects the nature of intermediates and stability of products to a great extent. Photolysis in solution phase The quantum yields for the formation of iron(II) "8L--~~-----4-------0~----~L------~4~~ species during the photolysis of alkaline earth 1 VELOCITY (m m 5- ) tris(oxalato)ferrates(III) are given in Table 2. The Fig. 1- Mossbauer spectra of photolysis of calcium quantum yield1314 for the formation Fe(II) species in tris(oxalato)ferrate(lII) [(a) Unirradiated; (b) irradiated for the case of potassium tris( oxalato )ferrate(III) in acidic lOOhr; and (c) irradiated for 300hrj medium is (i.e. cp = 1.20). The quantum yields for the photoreduction of alkaline earth tris(oxalato)ferrates- irradiated at 300 nm upto 400 hr. Mossbauer spectra of (III) are in the range of 0.91 to 0.47. This may be due to all these samples show an asymmetrical doublet. The the use of polychromatic radiations and change of values of isomer shift and quadrupole splitting in the outer cation. The quantum yields are higher for case of magnesium and calcium show the formation of strontium and barium complexes and compared LO those ferrous oxalate (Fig. 1). The formation of for magnesium and calcium complexes. The quantum [Fe(Cp 4)iH P)zF- has been observed in the case of yields of magnesium as calcium complexes show that barium and strontium tris(oxalato)ferrates(III). The physical deactivation is predominant in these com- values of Mossbauer parameters are given in Table 1. plexes. The quantum yields less than unity clearly While the intermediate formed in the solid state indicate that there is no possibility of chain reaction. photolysis of potassium tris(oxalato)ferrate(lII) gets The quantum yield for photoreduction to iron(II) JII converted into K Fe (CZ04)Z<HzO)z on long stand- species is less than that of potassium tris(oxalato)- ing in air I z, in the case of alkaline earth tri~( oxalato)- ferrate(III). So it is not desirable to replace potassium ferrates(I1I), the intermediates are stable and do not ferrioxalate as a chemical actinometer. The inter- undergo oxidation on irradiation. In the solid state mediates formed in solid state photolysis are stable in photodecomposition, [Fe(C 20 4)2(H P) 2]Z- is formed air. The rates of photochemical reduction follow the in the case of strontium and barium complexes and order Sr ~ Ba >Ca > Mg. showing thereby that the size iron oxalate in the case of calcium and magnesium of the cation affects the rate of photochemical re- complexes. But in the photolysis of potassium duction of alkaline tristoxalatojferratesrlf l). 24 BRAR et al.: PHOTOLYSIS OF ALKALINE EARTH METAL TRIS(OXALATO)FERRATES(III) Isomer shift values are dependent on the nature of 5 Brar A S, Brar S & Sandhu S S Polyhedron, 2 (1983) 45. the outer cation. This shows the change of s-electron 6 Calvert J G & Pitts J N, Photochemistry (John Wiley, New York) density at iron nucleus. The changes in electron density 1966 pp. 785. 7 Blume M, Phys Rev Leu, 14 (1965) 96. and size of outer cation affects the photochemical 8 Gallagher P K & Kurkjian C R Inorg Chem, 5 (1966) 214. reduction of complexes in the solid and liquid states. 9 Naka Moto K, Infrared spectra of inorganic and co-ordination compounds (Wiley-lnterscience, New York) 1970 pp. 245. References 10 Parker C A, Trans Faraday Soc, 50 (1954) 1213.
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