Richard A. Potts The University of Michigan Dearborn, Michigan 48128
Synthesis and Spectral Study of Copper(II) Complexes
In chwsing experiments for the general chemistry laboratory, one dealing with the synthesis of a metal complex and one dealing with the spectral properties of the transition metal complexes are often sought. Rather than having separate unrelated experiments, it would he much more desirable to have a series of experiments in which the students would prepare complexes which would then he used in the spectral study. This coupling of experiments would illustrate to the student that syntheses are seldom carried out for the sake of obtaining a product but that the product ohtained is usually then submitted to further study. Also, the spectral study will he of more interest because the student is using a compound which he has prepared. Such an experiment is described here. The copper(I1) metal ion was selected because its complexes are numerous and relatively easy to prepare and its spectrum is simple and easily interpreted in terms of the spectrochemical series ( I ) . The students are generally familiar with the "blue" color of copper(I1) which helps to reinforce the theory, as the "blue" color is altered with a change in the ligands. The ligands used in the spectral study are water, ammonia, ethylenediamine, the acetylacetonate anion, and the glycinate anion. This selection gives some variety of oxygen and nitrogen donors as well as a mixed donor ligand. The acetylacetonate and glycinate complexes, namely bis(acetylacetonato)copper(II) and cis-bis(glycinat0)copper(II) monohydrate, are prepared and isolated prior to the spectral study. Experimental Preparation of cis-Bis(glycinato)copper(ll) Monohydrate The method used is similar to that reported by Condrate and Nakamato (21. A 3.0 g sample (12.0 mmole) of eopper(I1) sulfate pentahydrate
is dissolved in 17 ml of 1M hydrochloric acid. To this solution 1.5 g (20.0 mmole) of dycine is added and then warmed for about 1 hr. Sodium hydrogen carbonate is added until precipitation is complete (avoid a large excess). The precipitate is suction filtered, recrystallized from hot water, and dried in an oven. Preparation of Bis(acetylacetanatoJcopper(ll)
Several methods have been reported for the preparation of this compound (3-5). The method used in this experiment was selected because it uses techniques and equipment similar to the above preparation. A solution of acetylacetone is prepared by adding 2.5 g (25.0 mmole) of acetylacetone to 100 ml of 0.25 M NaOH solution (25.0 mmole). This solution is added to a solution of 3.1 g (12.5 mmale) ofcopper(I1)sulfate pentahydrate in 100 ml of water. The precipitate is suction filtered, recrystallized from dioxane, and air dried. Spectra of Copper(l1J Complexes
The procedure used is similar to that reported by Trapp and Johnson ( I ) . The spectra are determined with Bauseh and Lomb "Spectronic 20" spectrometers using 12-mm Pyrex test tubes as cells. The following stock solutions are made available to the students: 0.01 M in Cu(N03)2 and 2 M in NHnN03, 0.01 M in Cu(N03)~and 1 M in KNOI, 0.10 M in NHJ, and 0.10 M in ethylenediamine. The spectra of the following nine solutions are determined: CuZ+in water, CuZ+-NH3in a 1:1, 1:2, 1:3, and a 1:4 male ratio,
Cu2+-enin a 1:l and a 1:2 mole ratio, Cuz+-glycinecamplex in a 0.01 M aqueous solution, and Cu2+-acetylacetonecomplex in a 0.01 M chloroformsolution. Discussion
The two preparations, when carried out simultaneously, require from 4-6 hr. The students are given weighed quantities of acetylacetone and glycine which then allows a portion of their grade to he based on yield. Typical yields are 40% for the acetylacetone complex and 30% for the glycine complex. This portion of the experiment could he shortened by requiring each student to do only one of the preparations. In the other director, this portion can he extended or made open ended by suggesting that the student try a different preparation of the acetylacetone complex (3, 5), sublime the acetylacetone complex (4), or prepare the trans glycine complex by starting with copper hydroxide (6, 7). The preparation of the solutions is purposely not detailed, providing the student ex~eriencein preparina solut i o n s . ~ h ecalc;lations are treated as a pre:lab exercise so time is not lost in the laboratory. The spectra of nine solutions must be determined in the second part of the experiment. To complete these spectra in one laboratory period, the work can be divided among the students and then the data shared. Spectra ohtained by the students for the water, ammonia, and ethylenediamine solutions were similar to those reported (I). The glycine complex gave a maximum at 630 nm while the acetylacetone complex gave a maximum at 650 nm with a shoulder a t 570 nm. This portion could also be made open ended by suggesting that the students try other ligands or metal ions (1, 8), determine the spectra of Cu(NH3)s2+, Cu(NHa)s2+, and C ~ ( e n ) ~ ( I )~, +or investigate the effect "lvents On the acet~lacetonec O m ~ l e x'pett ~ m . The students are required to submit a report in which they are asked to analyze the spectra using crystal field theory. For simplicity of interpretation, the students are told to assume an octahedral structure for all complexes. Although the Cu2+ complexes in aqueous solution are elongated octahedra and Cu(acac)z in chloroform is probably a weakly solvated square planar structure, the above assumption allows a correct interpretation of the spectra in terms of the ligand field strengths. The students are asked to comment on the shifts in the spectra as the ligands change and then construct a spectrochemical series using the data for the similar complexes; Cu(Hz0)e2+, C U ( N H ~ ) ~ ( H ~ O ) ~Cu(en)z(Hz0)z2+, ~+, Cu(acac)z, Cu(gly)z. Equilibrium calculations for the solution CuZ+NH3 in a 1:4 mole ratio indicate that the species Cu(NH& (Hz0)z2+ is not the only CuZ+-NH3 species present in significant amounts. This fact is also true for the other aqueous solutions. However, the assumption that stoichiometric amounts of CuZ+ and the ligand produce the corresponding stoichiometric complex, gives results which yield a spectrochemical series with the expected order: en > NH3 > gly > acac > HzO. The students have been very receptive to this experiment because it is demanding, i t teaches them new techVolume 51. Number8. August 1974 / 539
niques, and it correlates with the theoretical discussions presented in the lecture. Literature Cited (11 Tlapp, C.. andSohnson, R.. J.CHEM.EDUC., 4 4 527(19671. 121 Condrete. R.A.. and Nakarnoto, K..J. Chem Phvs. 42.2590(19ffil.
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/ Journal of Chemical Education
131 Finn. A.E.. Hampton. G.C.. andSutton.L.E..J. Chsm Soc. 1254(19381. 141 A r m s t m n ~R S . . LeF&vro,C . G . a n d LoFhre. R.J.W.. J. Chrm. Soc.. 371 119571 (51 Wilmn, R.. and Kivelsan. D . J . Chem. I'hys. I,. 4445119661, (61 ~ ~ ~I.. and t %ida. h W ~ . ~, M ~ ~them., O . ~ ~ll.37311890). ~ . (71 Tornits. K., Bull. Chsm. SouJan., 31.280(13611. 181 Dunne.T. G..LCHEM. EDUC.. U, 101 11867).
