G. Aravamudan J. Gopalakrishnan and M. R. Udupa Indian Institute of Technolow -. Madras 36, India
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II
Preparation and Properties of Potassium ~ri~xabtoferrate(lll) '~rih~drate A laboratory exercise
The comnletion of the uhotoreduction is indicated by the Recently,' a simple method for the preparation of podisappea;ance of the yellow color. Fe(II) thus formed is tassium trioxalatoferrate(II1) trihydrate has been reported determined by titration against sodium vauadate(V) using as an illustrative exercise in general chemistry laboratory diphenylamine indicator-in the presence of phosphorii courses. We have been giving an exercise involving the acid, thus affording a simple method for quantitative detitle compound to graduate students of chemistry not only termination of iron in the original complex. The students from the preparative aspects hut also from the point of are made aware that conventional oxidants such as view of structure and reactivity of coordination comKMn04 and KzChO7 cannot he employed for the deterpounds. The exercise involves preparation, characterizamination of iron(II), since the remaining oxalate in solution, and study of spectral, magnetic, photochemical, and tion would react with such strong multi-equivalent oxithermal behavior of the title compound. dants in the presence of Fe(I1) even in the cold. The reaThe compound is stable and can be prepared in a state sons for induced reactions occurring in MnOl- (or Crz0r2-) of high purity from easily available chemicals.' It is a -Fe2+-C2042systems and their absence in the vanadategood example of a high spin iron(II1) complex with D3h Fe2+-C2042system are explained to the students. configuration of ligands around the metal ion. The ir ahThe thermogravimetric curve, obtained with 500 mg of sorption snectrum of the solid shows that oxalato groups the sample in air reveals that the compound loses three (which can ligate variously) are present as chelating bimolecules of water around 110°C and the anhydrous comdentate entities. The electronic spectrum of the solid and pound undergoes thermal reduction around 260°C to an of its solution in dilute acid media are also recorded and oxalate of iron(II), identified to be K8[Fez(Cz04)5] which interpreted. Whereas d-d transitions in the high spin d5 subsequently undergoes oxidative decomposition around system are weak, the moderately intense green color of the 400°C to a mixture of K2C03 and F e ~ 0 3 . ~ solid and the yellow color of its solution in acid medium The preparation and analysis of the compound comprise are characteristic of charge transfer transition from oxaone laboratory experiment of 6-hr duration, while the late to iron(II1). The magnetic moment fief! a t room temstudv of mametic suectral. ~hotochemical,and thermal perature determined by the Gouy method 1s almost equal behavior for& another exercise of equal duration. All the to the "spin only" value of the d5 system in keeping with the high spin configuration of iron(1II) in the c ~ m p o u n d . ~ students are able to comulete the exercise within the specified time with excellentresults. The large value.of the magnetic moment is an added advantage in an i n t r o d u c t o ~exercise. The complex affords an interesting comparison between 1 Johnson, R. C . , J. CHEM. EDUC., 47,702 (1970). the thermal and uhotochemical ~ t a h i l i t i e s ~of, ~oxalato Earnshaw. A,. "Introduction to Maenetochemistrv," . Academcomplexes of iron(II1). In the dark, solutions of the comie hess, ond don, 1968, pp. 34 and 57. plex in acid medium (-2 N HzS04) are stable even up to 3Balzani, V., and Carassiti, V., "Photochemistry of Caordina100°C. However, when the same solution is exposed to tion Compounds," Academic Press, London, 1970, p. 167. bright sunlight for 5-10 min or to radiation from a 400-W 'Raa, G. G., Aravsmudan, G., and Venkatamma, V. C., Z . mercury-vapor lamp for 15-20 min, quantitative photoreA d . Chem., 146,161 (1955). duction of Fe(III) to Fe(II) occurs as SBancroft, G. M., Dharmawardena, K..G., and Maddock, A. hr G., Inorg. Chem., 9,223 (1970). 2Fe(C20,)," --+ 2FeS+ + 5C20,'2C0,
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Volume 51, Number 2, February 1974
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