Hexaammine Complexes of Cr(lll) and Co(lll) A Spectral Study
Downloaded via UNIV OF WINNIPEG on January 27, 2019 at 12:02:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
D. R. Brown and R. R. Pavlis Division of Science and Mathematics, College of the Virgin Islands, St. Thomas, U.S.V.I. 00802
Many of the undergraduate coordination chemistry experiments that are currently popular emphasize mixed ligand complexes (e.g., (1)), 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(III) chloride (2) and hexaamminechromium(III) nitrate (3), so that students can interpret fully the UV/visible 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. Synthesis Hexaamminecobalt(llt) chloride [Co(NH3)6]CI3 (4) Dissolve 48 g CoCl2 6H20 and 32 g NH4C1 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 yellow/brown. 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% HC1. Filter the solution hot and precipitate the product by adding 80 ml concentrated HC1 and cooling to 0°C. Wash with 60% alcohol and 95% alcohol and dry at 80-100°C.
Hexaamminechromium(lll) nitrate [Cr(NH3)6](N03)3 (5) 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 NH4C1 and 150 ml concentrated NH3 solution. Stopper the flask but arrange a gas outlet tube, terminating under water, to permit the escape of evolved H2. Ensure adequate ventilation to prevent the build up of high concentrations of H2. Coot the flask until gas evolution ceases (24 h). At this point some [Cr(NH3)3]Cl3 is 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
UV/Visible Spectral Data Complex Ion
^max(kK)
Assignment
[Co(NH3)6]3+
21.2 29.6 21.6 28.5
Vi
.3
[Cr(NH3)6]3+
.
"
.
Transition
v2
Mi9 —
V2
‘‘Alg-*
"3
4A>9
This band Is partially masked by the charge-transfer spectrum.
—
1
T2g
4r,g(F)
Tl;|(P)
nitric acid:water mixture. Filter the product, wash it with alcohol, and dry it in air. Caution: care should be 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 be isolated as follows. Treat the NH4C1 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 crystals as above.
or
after several hours. Wash and dry the
Spectral Measurements UV/visible spectra of 1 X 10-2 M aqueous solutions of both salts are conveniently recorded in 1.0-cm cells over the range 200-750 nm (see the table). Each band in the d-d spectra is assigned to a transition between two spectroscopic states, and values of Dq and B are calculated as described below. [Co(NH3)]6CI3 The appearance of two distinct bands in the d-d spectrum of Co(IlI) is indicative of a low-spin electron configuration t2g6eg. The lower in energy of the two, iq, can be assigned to the transi—* tion lT2g. lTig and the higher, v2, to iAig The value of 10Dq can be 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 by the nephelauxetic ratio (7, 8) to account for ligand effects) or by solving the appropriate simultaneous equations (9). —*
iq
=
10 Dq
+ &6B2/10Dq
(1)
f2
=
10Dq + 12B + 2B2/lODq
(2)
—
4
B
These equations are most easily solved using an iterative process, by first, ignoring the final terms in both equations and solving for approximate values of Dq and B and then back-substituting these values into the final terms only of the complete equations and resolving for refined values of Dq and B. The sequence is repeated until the refined values of Dq and B are not significantly different from the previous values. A useful exercise for students is to write a computer program to perform this iteration. The values of B and Dq can now be compared with predicted values such as those obtained by Jorgensen’s method of g and f values (8).
[Cr(NH3)6](N03)3 The ground state electron configuration of Cr(IlI) is and the three observable transitions can be assigned iA2g 47T2i and 4A^ 4Tig(P) M- The last of (n),4A2e-4rls(F) these appears as a shoulder on the intense charge-transfer absorption and is difficult to assign directly. The equations describing the three transitions are (7) —
-
jq
=
rq
=
7.5B + 15Dq
v3
=
7.5B + 15Dq + 0.5(225B2 + lOODq2
lODq
(3) -
0.5(225B2 + 100Dq2
-
-
180BDq)0-5
(4)
180BDq)°*
(5)
The spectral parameters B and Dq can be found by solving eqns. (3) and (4), using the positions of the two clearly defined absorption maxima, iq and iq. It is then possible to predict v3 by substituting these values for B and Dq into eqn. (5). The predicted value of iq can then be compared with an estimate of v3 taken directly
Volume 62
Number 9
September 1985
807
from the spectrum. If the two are at least roughly in agreement (within 200 cm-1), the exercise provides some justification for assigning the shoulder to the transition.
Literature Cited (1) (2) (3) (4)
(5) (6)
Suggested Further Work The magnetic susceptibilities of the two compounds can be readily measured using the Gouy method (JO). Subsequent calculations of numbers of unpaired electrons on the metal ions allow students to verify the conclusions already drawn from spectral data.
808
Journal of Chemical Education
(7) (8)
Greenaway, A. M., and Lancashire, R. F., J. CHEM. EDUC., 59,419 (1982). Linhard, M. Z., Electrochem., 50, 224 (1944). Schlafer, H. L., Z. physik. Chem. (Frankfurt), 11,65 (1957). Bjerrum, J., and McReynolds, J. P., in “Inorganic Synthesis,” (Editor: Fernelius, W. C.), McGraw-Hill, New York, 1946, Vol. II, p. 217’. Jorgensen, S. M.,J. prakt. Chem., 30, 2 (1884). Tomlinson, W. R., Ottoson, K. G., and Andrieth, L, F., J. Amer, Chem. Soc., 71, 375 (1949). Huheey, J, E., “Inorganic Chemistry,” 3rd ed., Harper and Row, New York, 1983, pp. 413, 446-448. Jorgensen, C. K., “Oxidation Numbers and Oxidation States,” Springer, New York, 1969, p. 106.
(9} Nathan, L. C., “A Laboratory Project in Modern Inorganic Chemistry,” Wadsworth, Belmont, CA, 1981, pp. 31-47. (10) Angelici, R. J., “Synthesis and Technique of Inorganic Chemistry,” 2nd ed., W. B. Saunders, Philadelphia, 1977, pp. 42-50.
