Hexaammine Complexes of Cr(lll) and Co(lll) A Spectral Study D. R. Brown and R. R. Pavlis Division of Science and Mathematics. College of the Virgin Islands. St. Thomas. U.S.V.I. 00802 Manv of the undernraduate coordination chemistrv experiments t h a t are cu&ently popular emphasize mixed ligand complexes (e.g., (I)),and students often have difficulty relating concepts developed in class based on octahedral crystal fields with the interpretation of spectra and magnetic measurements made on these systems with reduced symmetry. Two simple compounds have been chosen for this experiment containing complex ions with octahedral symmetry, hexaamminecobalt(111) chloride (2) and hexaamminechromium(111) nitrate (31, so t h a t students can interpret fully the UVIvisihle spectra of the complex cations in terms of the ligand field parameters, 10 Dq, the Racah interelectron repulsion parameters, B, and the relevant Tanabe-Sugano diagrams.
Hexaammlnecobalt(lll) chloride [Co(NH3)&13
(4)
Dissolve 48 g CoClz .6Hz0and 32 g NH4Cl in 40 ml water. Add 4-5 g activated charcoal followed by 100 ml concentrated NH3 solution. Bubble a stream of air through the solution until the red color changes to yellowlbrown. Ensure that the air stream does not appreciably deplete the concentration of NH3 in solution. If this occurs, add additional NH3 solution. Filter off the charcoal and the salt, and dissolve the residue in hot 1-2% HCI. Filter the solution hot and precipitate the product by adding 80 ml concentrated HC1 and cooling to O'C. Wash with 60% alcohol and 95% alcohol and dry at 80-100°C.
Add 15 g K2Cr207(caution: carcinogen) to 50 ml concentrated HC1 plus 20 ml ethanol in a 0.5-L flask. Stir the mixture until a green solution forms. Maintaining a nitrogen atmosphere over the solution, add an excess of zinc powder (10 g) and stir the solution until it turns blue. Add this solution to a mixture of 130 g NH4Cl and 150 rnl concentrated NH8 solution. Stopper the flask but arrange a gas outlet tube, terminating under water, to permit the escape of evolved Hz. Ensure adequate ventilation to prevent the build up of high concentrations of Hs. Cod the flask until gas evolution ceases (24 h). At this point some [Cr(NH3)6]C13is present in solution and some is deposited on undissolved NH4C1. The two portions of the product are treated separately from here on. Decant the red solution, and treat it with an equal volume of 95%alcohol. The chloride salt separates in a few hours. Wash the precipitate twice with alcohol, and dry it in air. Dissolve the chloride salt in lukewarm water, and filter it into well-cooled nitric acid. The nitrate salt separates as long yellow needles. Wash the precipitate several times with nitric acid followed by a 1:2
nitric aeid:water mixture. Filter the product, wash it with alcohol, and dry it in air. Caution: care should he exercized in the handling and storage of this salt, which has been demonstrated to have explosive properties (6).For example, the use of sintered glass filters should be avoided. ~ ~ ~ The other portion of the product, held on the solid NH4C1,can he isolated as follows. Treat the NH&L with 75-ml portions of water at room temperature until the extracts are no longer yellow. Add an equal volume of concentrated nitric acid to the combined extracts, and cool the solution. Yellow needles of the nitrate salt appear either at once or after several hours. Wash and dry the crystals as above. ~~
Spectral Measurements UV/visihle spectra of 1X lo-= M aqueous solutions of both salts are conveniently recorded in 1.0-cm cells over the range 200-750 n m (see t h e table). Each band in t h e d-d spectra is assigned t o a transition between two spectroscopic states, and values of Dq and B are calculated a s described below.
[CO(NH3)lsCk The appearance of two distinct hands in the d-d spectrum of Co(II1) is indicative of a law-spin electron configuration t2&p. The lower in energy of the two, "1, can be assi ned to the trans,. tion ]Alg ITlg and the higher, vz, to 'A,, 'T*~ The value of lODq can he obtained from the data either by matching the observed transition energies to transitions on the Tanabe-Sugano diagram (using the free ion value for B modified bv the nephelauxetie ratio (7.8) to account for lieand effects) or by solving the appropriate simultaneous equations (9)
-
+
uz = lODq + 12B + 2B2/10Dq (2) These ~quationsare most easily abed using an iterative prureas, hy first ignoring the final terms m both pqtrationsandsol\.ing for appnnimate valuer of Dq and B and then hnck-substituring these values inu, the final terms only of the complete rquatiuns and resolving for relined valuw uf Dq and R. The sequence is repeated until the irfinedvalues of DUand Bare not fiigniiicantlv different from the previous values. A "seful exercise fir students is to write a cornouter oroeram to oerform this iteration. The values of B and d q can now be compared with predicted values such as those obtained by Jorgensen's method of g and f values
(8).
[crfNH3)8IfNOd3 The ground state electron configuration of Cr(II1) is t&O, and the three observable transitions can be assigned 4TzZ (v,), 4Aa 4T1,(F) (ur) and 4Azg 4T~g(P) (us). The last of these appears as a shoulder on the rntense charge-transfer ahsorption and is difficult to assign directly. The equations describing the three transitions are (7)
-
-
-
UVlVlslble Spectral Data Comdex
Ion
A-.lkKI
Assianrnsnt
-This band is panialiy masked by the dlargarmnrfer s p d r u m .
Transition
The spectral parameters B and Dq can be found by solving eqns. (3) and (4), using the positions of the two clearly defined ahsarption maxima, v l and "2. It is then possible to predict us by substituting these values forB andDq into eqn. (5).The predicted value of vs can then be compared with an estimate of u3 taken directly Volume 82
Number 9
September 1985
807
the spectrum. If the two are at least roughly in agreement 200 em-'), the exercise provides some j u s t i f i c a t i o n f o r assigning the shoulder to the us transition. from
(within
Literature Clted (I) G ~A. M., ~ hi^, ~ R. F.,J. CneM. ~ ~ 59,419 ~(1982). ~ EDUC., 12) inh hard. M. 2.. ~lertrochem..50.224 (19441. (3) Sehlafer. H. L.. 2. physik. Chem. (Fronklurt), 11.65 (1957). 14) Bjerrum. J., and McReynolds, J. P.. in "Inorganic Synthosis."lEditocFerndiua. W. C.),McGrau-Hill,NewYork, 1946,Vol. 1I.p. 217. M.. J . ,orokt.Chern..80.2llRRdi~ 131 ~,~~ ~~~, . . Jnreensen.S. " (61 Tomlinson, W. d., Ottoson, K. G.and.4~drieth.L.F., J.Amer. Cham.Soc., 71,375 1~9491. 17) Huheey. J. E.."InorgsnicChemiatry,'. 3rded.. Hamerand Row, New York. 1988, pp. 413.44~8. IS) Jargensen, C. K.. "Oxidation Numbers and Oxidation States." Sprinper, New York, ~
Suggesled Further Work
The magnetic susceptibilities of the two compounds can be readilv measured usina the Gouv method (10).Suhsequent caiculations of numbers of unpaired electrons on the ions students to verify the already drawn from spectral data.
808
Journal of Chemical Education
.""", * .-.
~
,mco " ,m
19) ~ a t h a nL. , c.,"A ~abomtoryproject in ~ o d e r n~norgsniechemistry." wadsworth, Belmont,CA. 1981, pp. 3147. 110) Angdiei, R. J.,"Syntheais and Technique of Inorganic Chemistry." 2nd ed.. W. B. saundera, phiiadeiphia. 1977. pp. 4250.
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