Magnetic Susceptibility A Practical Introduction A. M. Greenaway and L. E. Trail University of the West indies, Mona, Kingston 7, Jamaica T h e basic ~rinciplesof magnetic susceptibilitv (1.2) are generally covered a t an early stage in any cburse on transition metal chemistry. The understanding of these orinciples is important if the students are to comprehend fuily m&e advanced courses on bonding theory and the application of more complicated techniquessuch as electrod spin resonance, Mosshauer spectroscopy, etc. Despite this, very few laboratow courses include a practical introduction to this subject and those that do usually require the students only to measure the susceptihility of one sample with the emphasis being on the measurement rather than the interpretation of the data. We have develo~edthe followine ex~erimentto fill this pan. The students synthesize tr&cetylacetonato)iron(fIIj, tris idiethvldithiocarhamato)irouiIII). and chlorobisidiethvldit h i o c a ~ b a m a t o ) i r o n ( ~ ~ ~ ) ; a r e a sample of potas&m hexacvanoferrate(I11): and are then asked to measure the room temperature gr& susceptibility of each of these samples using the Gouy technique (I).These complexes have been chosen since they require the students to consider the effects of lieand field streneth. and orbital aneular mo" . svmmetrv. me&m when interpreting their data. Since tris(diethyldithiocarbamato)iron(III), a spin crossover system, and chlorobis(diethyldithiocarbamato)iron(III), an S = Y2 system, have effective magnetic moments which are very similar, the interpretation of the obtained data presents the students with some interesting problems. We allow the students two 6-hr laboratory sessions to complete this experiment and run it with groups of five students. We have found it to he both instructive and enjoyable. Experimental Tris(acetylacetonato)ironiIII), Fe(acac)a, trisidiethyldithiocarbamato)iron(III), FeiEtzdtc)~,and chlorobis(diethyldithiocarbamsto)iron(III), FeCI(Et*dtc)n, were synthesized by established literature techniques ( 3 , 4 , 5 )with only slight modifications. Quantities used, students' yields and modifications were Fe(acac)d3):5g of Fe(NOa):ySHzO,4 ml of 2,4-pentanedione,and 5 g of Na02CCH3 were reacted to give 3.8 g of the red orange crystalline product. Fe(Ehdtc)a(4):5g of Fe(N03)y9Hs0and 8.5 g of NaS2CNiCzHd2 were reacted in aqueous solution and the product extracted into 150 ml of chloroform. The volume of the organic layer was reduced by half and precipitation effected by the addition of ethanol. The product is recrystallized from chloroform/ethanolas above, yielding 4.5 g of product. FeC1(Et2dtc)2(5):1.5g of Fe(Et2dtc)a was dissolved in benzene [WARNING: Benzene has been shown to he carcinogenic and thus must be handled in an efficient HOOD], and then 10 ml of acetone containing 0.2 ml of concentrated HCI was added to give a green solution. Precipitation was effected by evaporating off the acetone and cooling. The yield of the green-black solid was 0.86 g. The Gouy tubes are calibrated using mercuryiI1) tetrathiocyanatocobaltate(I1) which is synthesized easily (6) in a pure state and whose gram susceptibility parameters are now well established (7). The students are then required to determine the gram susceptibilities of their three samples and of potassium hexacyanoferrate(III), K&WCN)s).
Discussion T h e syntheses are an integral part of the experiment. T o avoid congestion a t the balance, the students must organize
their laboratory time carefully and the syntheses allow them the flexibility to do this. Apart from this, they are also of interest in their own right. Fe(acac)s is invariably prepared in a pure state. The Fe(Ehdt& synthesis is messy but generally very successful. The precipitate which comes out of the aqueous solution is an intractable, curdy, black mass which becomes easier to handle if the student patiently stirs the residue. The ~ r o d u cneeds t to he recrvstallized and thus the
product. We have found that the use ofBcetone as a me& for addine the hvdrochloric acid rreatlv increases the chances of
product is to smearit on a piece of Filter paper (a dark green smear) and then add a drop or two of chloroform to give a green color. (It will he brown if the reaction has not occurred.) It is also possible, if the chloroform has chloride ions present in it, that the synthesis of Fe(Etzdtc)a will yield FeC1(Etzdtc)2 instead. Again, this can he detected easily by the appearance of the green color and does not present a problem as the student has thereby obtained one of the products and merely needs to repeat the synthesis of Fe(Et2dtc)susing a pure solvent. These syntheses then introduce the students to the need of making careful observations during their experiments and also to the idea of carrying out simple tests on their products to confirm their identities prior to carrying out time-consuming physical studies. The interpretation of the magnetic data will depend on the level to which the students have been taken in lecture courses. We ask the students to calculate the gram susceptihility of H ~ C O ( S C Na) t~ the temperature a t which they do their measurements, from now well-established parameters (7). This helps to emphasize the temperature-dependent nature of gram susceptibilities. Fe(acac)s is a useful complex to study since it has a6Al ground state and hence is a spin only system. (peff = 5.92 BM.). Since it is invariably obtained in a pure form, i t can be used by the instructor as a monitor of the students ability to carry out the measurements accurately. The K$Fe(CN)eJ system, octahedral with a 2T2ground state, has a moment of 2.20BM (I),and thus an orbital contribution is observed as expected. These two systems then illustrate the high spin and low spin configurations and can be easily interpreted from simple octahedral d orbital splitting diagrams. The five coordinate FeC1(Ebdtc)2has an effective magnetic moment of 4.00 BM (5) which can be interpreted by considering the d orbital splittings in a square pyramidal symmetry (2). The ground state is a 4A2 and thus no direct orbital contribution is to be expected. This complex, when compared to the previous two, illustrates the the effects of ligand field symmetry on the d orbital splitting patterns. The spin crossover system presents interesting interpretation problems to the students, provided they are not given Volume 60 Number 8 August 1983
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any hints. I t is a pseudo-octahedral system; thus, d orbital splitting patterns would suggest a magnetic moment of either 5.92 or 2.20 BM as found for the other two octahedral species studied. The observed moment of 4.24 BM, however, suggests an S = 312 configuration particularly since the students will have just considered the FeC1(Etzdtc)2data. This, however, is not possible for an octahedral, or trigonally distorted six coordinate (which Fe(Et~dtcj3is) complex (2). Without studvine the temnerature devendence of the susceutibilitv o i t with the grdup after the data has been collected and the calculations comvleted) is very instructive. ceptibility theory and measuremknt techniques. i f variable
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temperature auuaratns is available then the experiment could he fruitfully extended to utilize this and allow the theory to he illustrated in greater depth. One aspect of the experiment which needs to be emphasized to thestudents is that of the experimental uncertainty of the measurements. Literature Cited Figgis, B. N., and Lewis, J., "The Magnetnchemistry of Complex Cumpounds," in 'Modern Cwrdination Chemisiry: J. Lewis and R. G.Wilkins, (tid,t"i\), Interscience Publishers. Ine., New York. 1960, p. 400. (2) Cotton, F. Albert. snd Wilkinron. Geoffrey,"Aduanced Inorganic Chemistry." 31d ed., Interscience Publishers. New York, 1972, p. 528. (31 Fernelius, W. C.. and Bryant, B. E., "P~eparationofmetalderivatives of 13-diketnnes," . McCraw-Hill Book Company, Inc.. in "Inorganic Syntheses: T. M u e l l e ~(Editor). London, V. 105 119571. (4) Whiie,A. H..R"per.R., Kok0t.E.. waterman. H.,nndMarti",R. L..Ausiiolron Journal ofChrmirliy, 17,294 (19641. ( 5 ) Msrfin,R. L.,and Whiie,A. H.,Inorganic Chemistry, 6,712 119fi7). and Nyholm, R.S..Journal 01 the Ch~rnicolSociety, 4180 (18581. (61 Figgis. B. N.. (71 Bunzli. J. C. G.. Inorganic Chimm A d a , 36, L413 119791. (1)
