D. T. Haworth and K. M. Elsen Marquette University Milwaukee, Wisconsin 53233
II
Within t h e last decade, inorganic chemistry courses have been including material concerning ligand field theory and t h e molecular orbital approach t o t h e discussion of coordination complexes. T h e basic ideas are usually treated in t h e lecture portion of t h e course a n d then, if possible, further strengthened by a meaningful laboatory experience. T h e greatest difficulty is finding a n experiment which can be performed by a n undergraduate within a reasonable amount of time resulting in observations which can he applied t o theoretical principles. T h e determination of 10 Dq a n d t h e placement of ligands i n t h e spectrochemical series h a s been successfully tested by Dunne ( I ) . Comparing t h e spectra of transition metals during ligand exchange processes is useful in calculating electronic transitions 72): T h e use of resin ion-exchange columns for sep= aration of various complexes has been proved t o be very successful a n d adaptable t o t h e undergraduate course (3). Described here is t h e interpretation of t h e spectra of transition metal complexes i n t h e distinction between cis a n d trans isomers. T h e intensity and position of t h e spectral bands are related t o t h e complex's structure. This experiment's advantages are t h a t it: ( I ) utilizes isomeric compounds t h a t can be prepared cheaply and safely i n a reasonable amount of time, (2) does not involve complex instrumentation, and (3) produces interpretable d a t a in agreement with published spectra. Preparation of Compounds
Cis- a n d Trans-Dichlorobis(ethylenediamine)cobalt(lll) Chloride ( 4 ) . [Co(enhCl~]CI Trons Isomer. Dissolve 40 g of CoCl~6Hz0in 125 ml of distilled H20. Dissolve 15 g of anhydrous enthylenediamine in 135 ml of distilled H20. Combine the two solutions. The reddish solution is added to a 500-ml suction flask fitted with a stopper and a glass tube which extends deep into the solution. Air is farced through the solution for 8 hr. The oxidized solution is placed in a large evaporating dish and 97.5 ml of eanc HCI are added. Concentrate the solution to half of its volume using a steam bath. This procedure should be done in a hood. Cool the remaining solution in an ice bath. The salt, [ C O ( ~ ~ ) ~ C ~ ~ ] C ~is. Hobtained C~~~H O as~ bright peen plates which are drained on a filter. The crystals are ground in a mortar with anhydrous methanol. A color change to light green occurs. After filtration the trans isomer is dried at 105-112°C. Cis 1.somer: The trans isomer is dissolved in a minimum af water which has been heated to 90°C. Evaporate the solutian to dryness aver an air bath at 103-105°C. Retreat the residue with the same volume of hot HzO and repeat the evaporation process. The violet colored residue consists of the cis isomer.
Cis- a n d Trans-Potassium Dioxaltodiaquochromate(llI), K [ C r ( C 2 0 d 2 ( H 2 0 ) ~ ] 3 H 2 (5) 0 Trans Isomer. Dissolve 12 g of HLh01.2H20 in a minimum (approx. 20 ml) of boiling distilled H20. Dissolve 4 g of KzCrz01 in a minimum (20 ml) of hot distilled HzO. Mix the two solutions in s 400-ml beaker. A violent reaction occurs, thus the large beaker is necessary. Over a steam bath evaporate the solution of half of its original volume. Allow the solution to stand. Slow natural evaporation will cause the less soluble trans isomer to crystallize fint. This process may take several days. The trans crystals are filtered and washed using a mixture of ice cold ethanol and water. 300 / Journal of Chemical Education
spectra1 Comparison of Geometrical k o m e r ~
Energy levels involved in the transitions responsible for the observed bands in coordination number six Co(li1)complexes (not to scale)
Note: Rapid evaporation of the equilibrium mixture of cis and trans isomers will cause crystallization of both isomers. Cis Isomer. Grind together 4 g of K2Ch07 and 12 g of H&0r.2H20 forming an intimate mixture. Heap the mixture in a &in. evaporating dish. Place one drop of distilled Hs0 in a small depression in the mixture. Cover. A violent reaction occurs. Absolute alcohol (20 ml) is poured aver the viscous liquid product. The mixture is ground in a mortar until the product solidifies. Filter and dry at a pump. The cis isomer will have a greenish tint. Preparation of Solutions and Procedure
Cis- a n d Trans-Dioxalatodiaqoochromate(lll~ Solutions (50 ml) of each isomer are prepared at a concentration of 0.015 M. The trans isomer solution is prepared immediately before determining its spectrum. Ice cold water is advised to lessen the conversion of the trans to the cis isamer. Cis- a n d Trans-Dichlorobis(ethylenediamine)cobalt(lll) Solutions (50 ml) of each isomer are prepared at a concentration of 0.01 M. The trans isomer should be weoared iust before . . dererm~nmgits spectrum ustng ire cold uatrr The spectrum o i each wmcr is determined w n g n Beckman nH Specrmphotornrter using astrip recorder. The rnngeof 7 l U JX! mu is chosen. ~~
~~
~
Results and Discussion
T h e absorntion maxima (cm-l) for each complex are given i n t h e table. From crvstal field theorv. .. i t is found t h a t of t h e two ahsorption hands for geometrical Co(JII) complexes of the type CoAdB? t h e one of smaller wave number (longer wave length) may he split into two hands. This is accounted for by a n examination of t h e energy levels involved in t h e possible transitions for t h e observed band. I n t h e figure the variation with cis a n d trans symmetry of
Absorption Maxima of Isomers
Co(en)nCI?+ ustrans-
Cr(CeOn)AHzO)?+ ers-
tram-
Expt. (wave no. -%T)
Lit (wave no.) (6, 71
19,850(80%), 27,000(88%) 16,425(70%), 22,250(63%), 25,000(71%)
18,700, 21,000 16,400, 22,200, 25,000
17,000(62%), 24,100(74%) 18.350(18%), 24,200(22%) -
17,700, 24,000 18,000, 24,100
crystal field splitting for spin paired Co(III) complexes is shown. For a cis-CoAs complex two transitions are possible ~
~
IA~,-, 'T,I
13,-
The cubic symmetry of CoAs is changed to tetragonal by renlacement of two of the A lieands hv B lieands. Theorv " shows that for the resulting geometrical isomers cis- and trans-CoA1Bz the IT,. state is split into two states with the splitting for the trans isomer being approximately twice that of the cis isomer (8). If the splitting is large, three absorptions hands are now possible. This splitting can occur in trans-CoA4Bz complexes. With cis isomers only broadening of the band occurs. While the IT*. state is also split into two states, no resolution is observed even for trans complexes. As seen from the data, two absorptions are observed for cis-Co(en)zClz+ and three for trans-Co(en)~Cl~+. The first two bands of the trans complex are a result of the splitting of the IT, state. The extent of splitting is related to the ligand's relative position in the spectrochemical series
-
1m \-,.
CN- > NOz- > dipy > en > NHs > py > HzO > C20r2- > 0 H - > F>Cl->Br>I-
large splitting
small splitting
The further the two ligands are apart, as are en and CI, the better chance one will have to observe a three band spectrum for a trans complex. One also observes that the two bands of t r a n s - C ~ ( e n ) ~ C (16,425 I~+ cm-', and 22,250 em-') together correspond to an average position and in total intensity, to the lower band (19,850 cm-') of cisCo(en)~Cl~+.
In the Cr(III) complex, the oxalato and aquo ligands occupy adjacent positions in the spectrochemical series, thus a three hand spectrum is not obsewed for the trans isomer. The intensity of the first hands can also give some guide to the distinction between cis and trans isomers; for it has been observed that the cis compound usually has the more intense hands (10). This may he related to lack of a center of symmetry in cis isomers of the type MA4Bz. Complexes in which neither geometrical isomer has a center of symmetry such as CoA3B3. COAIBC and Co(en)z(NOz)CI+ equal areas under the absorption curves are to be expected. It is interesting to note that of two isomers of CoA3B3, the 1,2,3-isomer has cubic symmetry whereas the 1,2,6-isomer has rhombic symmetry. Due to additional splitting of the IT,#and T*., states, a six band spectrum for 1,2,6-CoA3B3 is possible as compared to the two band spectrum for 1,2,3-CoAzB~ (11). Literature ,Cited 11) Dunne.T. G.. J.CHEM. EDUC., 41.101 (19671. (2) Trspp, C..and Johnson, R . J . CHEM.EDIJC.,44.527 119671. 131 Angolici. R. J.. "Synthesis and Technique in lnolganie Chemistry," Saunders, Philadelphia, 1969. p. 58. 14; Sehlesringer, G.."lnomanic Laboratow Premration," Chemical Publishine Co.. 151 Pass, G:, and Sutcliffe, H., "Practical lnorgsnir Chemistry... Chapman and Hall. Ltd.. London. 1968, p. 91. (61 Cunnhgham, G , E.. Burley. R.W.,and Friend, M.T.,Noturp, 169,1103 11932;. 17) Baaolo, F.,Bsllhsusen.C. J . , andBjenum, J..Aefo C h e m Scond.. 9,810(1955). 181 Fires, B. N.."lntiodueiion to Liganda Fields." Inteneionee. New York. 1966, p.
Volume 50, Number 4, April 1973
/
301
