Magnetic and Spectrgl Properties of an Air-Stable fitanium Complex Miles Pickering Princeton University, Princeton, NJ 08544 Almost no d l complexes of transition metals have been used in student experiments, except for a single illustrative experiment' involving solutions of Ti(H20)63+. This is unfortunate, since d l systems have one of t h e most easily interpreted visible spectra of any d" system and spin-only magnetic moments. T h e reason for this conspicuous gap in laboratory experiments is t h a t very few d' systems exist t h a t are air stable.. svmmetricallv coordinated, and easy t o work with. T h e experiment described here involves a Ti$+-urea comp l e ~ ,[ T ~ i ( ~ r e a ) ~ ]t Ih~a t, can be synthesized easily from commercially available TiC13. T h e complex prepared is reasonably air stable if dry, stabilized, it is believed, by H-bonding between t h e ureas. T h e stability is kinetic rather than thermodynamic. However, the complex has been stored without visible decomposition for a s long as six weeks in the author's lab. Students synthesized t h e complex from TiC13, urea, and KI in a simple, one-afternoon operation. Then they were asked t o answer t h e following questions through experiment: (1) What species are in the coordination sphere? One can envision
several possible compounds containing I-, urea, or H20. (2) If the urea is bound in the coordination sphere, is it bound through the oxygen or the nitrogen? (3) Is the d-d splitting more or less than that induced by HzO? (4) What is the magnetic moment of the compound? This verifies
the oxidation state of Ti, and, consequently,defines the number of iodide counter ions. This experiment was tested by juniors taking the inorganic chemistry lab a t Princeton. Approximately four 3-h periods were allocated for t h e experiment.
It is desirble that the vial be filled and tiebtlv oacked to exclude 01.
We have measured thesurceptibiiity h> (wing a (buy bniance.The percentage o f T ~ is needed for this calcudntim. Thia is ulrtaiued simply by heating aweighed sample of complex to red heat inacrucible. All components except the Ti are decomposed to volatile products, and Ti02 is left behind as a white (or yellowish white) solid. This operation emits verv moleasant ~,~ .~~~ smoke. and must be done under a hood. From ,tiamount of T102 produeid, and the mittai weigh! ofthe sample, it is Pas\ tu calculate the molrcular weight the ctmpound. ~~
Spectra One of the main questions is whether the ureas hind through the oxygen or the nitrogen. The IR spectrum of the complex can be taken on a typical organic chemistry lab IR instrument using a KBr pellet. Students should also take a reference spectrum of pure urea. This cheeksaut both the instrument and the student. Also in cases where the wavelength calibration is inaccurate such a comparison spectrum is essential, since the differences between free and hound urea are small. The IR spectrum will not show Ti-I or Ti-urea bond vibrations; these are in the far IR. However, the IR spectrum of the urea will change depending on whether it is banded to the Ti through the N or the 0 atom.3 Visible spectra of the compound in water solution are taken. If the comoound is dissolved in water or KI solution, the characteristic me&m of lavailahle in mast standard inoreanie ~-TilH,O@+ -~ , ~ - - , annears " ,~~ " textbooks). If the complexis dissolved in afreshly prepared urea solution, or in a solution containing bothureaand KI, then the X,of the compound shifts to the value reported in the table. This proves that the coordination sphere contains no I- and does contain urea.
~~.~.~~~-~
Pronerties of TilureaM,
Experimental Synthesis Since urea solutions do not keep well, the student must f i t prepare a solution of 25 g of ureain 25 ml of water. It may be necessary to warm the solution (to not more than 40°C) to get the solid todissolve. The beaker is placed on a beam balance in a hood, and 4 g of anhydrous Tic$ (Alfa) are added, poured directly from the bottle. Students must wear gloves, aprons, and eye protection for this step. Anhydrous Tic13 resemhles AIC13 in properties, and it fumes in moist air. After the TiC13 has been added, the rest of the synthesis can be done on an open bench with no special precautions. The reaction of Tic13 with water is very exothermic, and the resulting warm mixture (about 45-50PC) should be filtered through a fluted filter paper or through Whatman glass microfibre (Grade GFIC) to remove any TiOz solid. Then a solution of 50 g of KI in 30 g of HzO should be added. (This solution must be prepared the day of the lab by the student, and some warming to not more than 40 or 50°C will be needed to get all the KIto dissolve.) The reaction mixture should be cooled in an ice bath, and the deep blue crystals of [Ti(urea)s]I3can he collected on a suction filter and dried by continued
Accepted Value Molecular Weight P a
A_..
760. 1.7W 18000 em-'
Literature v a l ~The ~ . spin+nly value is 1.73EM.
swtinn. .
~
The matrrlal should t,c put i n a heaker and stored ovprnirht in a drasicator to remove the last wnter and thru pack4 in a small vial.
' Trapp. C., and Johnson. R.. J. C ~ E MEOJC . , 44, 527 (1967).
Clark. R. J. H., "Chemistv of Titanium and Vand~um."Elsevier. Amsterdam, 1968, p. 100. Penland. R. B., Mizushima. S.. Curran. C., and Ouagliano, J. V., J. Amer. Chem. Soc., 79, 1975 (1957).
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Journal of Chemical Education
IR spectra of pure urea (. . .)and Ti complex I-).
Median Student Value
*
668 (Range 80) 1.77 (Range + 0.10) 17900 cm-'
.. Results
The magnetic moments found by the students were close to the d l spin-only value for one electron of 1.73 BM. The assay, however, gave molecular weights that were lower than expected. The &dents interpreted this to mean that the coordination sphere contained fewer than six ureas or some combination of ureas and water. However, small quantities of nonvolatile impurities or slight hydrolysis of the coordinated urea into volatile hioroducta could also cause the discrepancy. I t is noteworthy that the magnetic moments renorted were essentiallv unaffected bv the assav discre~ancv: . . this argue8 for the decompositim hyporhesis. The IR soectrum shown in the figure araucs convincinglv for bonding through the oxygens of the k e a . The region around 30003500 cm-I is essentiallv the same in spectra of the complex and of pure urea (althouih there is a l&ge water peak in most student preparations). The C==O moves to lower energy as it takes oi mire single bond chacter, and so the C=O stretching and N-H bending bands, originally distinct,
merge in the complex. There are no new N-H hands around 3200 cm-1 as would be expected if the NH2 group were coordinated. (Coordinated NH2 is different from unbound NH2, giving rise to a splitting.) The peak at 1130 em-' arising from an N-H bend, and the CN stretch a t 1020 cm-I are also identical in hoth the comulex and in oure urea. The poaltlon of the peak 111the ~ b i ate5503 corresponds ~o an oxveen that causes less d-d qol~rt~ne than that ofH.0. This is i
