J. Rlbas. J. Casabo a n d J. M. Coronas Departhento Quimica Inorghica Zona Universitaria Pedralbes Barcelona, Spain
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Infrared Spectroscopy of Tetrahedral Ligands An advanced experiment in inorganic chemistfy
In recent years, ir spectroscopy and its applications to the determination of molecular structure have become widely used in Inorganic Chemistry programs. At the basic level, where students do not have the mathematical background required for a rigorous treatment of ir spectroscopy, it is instructive and interesting to coordinate some hasic theoretical aspects with easy and pedagogical experiments. In this field lies the analysis of the predicted variations in their spectrum of a particular ion or molecule when it acts as mono or hidentate ligand; this kind of study lets the student familiarize himself with the important role played by ir spectroscopy in the determination of the stereochemistry of coordination compounds. Various typical examples suitahle for these studies have been described (I).Probably, the more convenient groups are those with Td symmetry (Moan-; M = S,P,CI,Mo,W,As, Cr . . .). In our Department, the students spend laboratory time studying the ir spectra of the Sod2- group in the following molecules: M2SOa (as free ion); [Co(SOd)(NHs)s]Br (2) or [Cr(SOa)(HzO)s]C1.1/2H~0(3)(as monodentate ligand); and [CO(NH~)~-NH~SO~-CO(NH~)~(NO~)~ ( 2 )or [Cr(SO& (NH3)a][Ce(SOa)3] (4) (as bridging or chelate ligand). In the above mentioned cases, the preparation of complexes with monodentate sulfate ligand is simple, and can be carried out without difficulty by the students of first courses. Therefore, the student can obtain the complex in the laboratory himself, appropiately purify it, and get the ir spectrum. This is not the case of the complexes with chelate or bridging sulfato ligand; the synthesis is then laborious and difficult for most students not yet trained in laboratory techniques, and the complexes are not usually obtained pure enough to permit a satisfactory interpretation of ir spectra. This fact spoils the whole experiment, because the student must work with the spectra of complexes previously synthesized by the instructor. More easily obtainable are the complexes: [CoCrOd(NH3)sIBr and [CoCrOa(NH3)4](N03) (5, 6),but in these compounds the rocking band of coordinated NH3 appears in the same region (810-830 cm-') (7,s)as the new bands of chromate due to coordination of this group, which evidently complicates the study and interpretation of ir spectra. On the other hand, in penta and tetraammine complexes of Cr(III), the rocking band of coordinated NH3 is shifted toward lower frequencies (750-770 cm-') (9);thus, in these complexes the coordinated chromate vibrations are clearly different from those of coordinate NHs. The complexes previously described by the authors (10) have been chosen as characteristic examples because they fit with the proposed objectives: to study the bands attributable t o the chromate ion, and its variations in the coordinated (mono or bidentate) chromato group, particularly in the range of 1000-600 cm-' where more typical effects are observed. Preparation of the Complexes .[CrCrOd(NHa)s](NOa).Two gramsof [Cr(HsO)(N&)&NOd3(11) are mixed with 10 g of NaN03in 20 ml of water. The mixture is gently heated until complete dissolution of the reactives, giving a solution ofpH = 4. Solid, finely powdered NanCrOdisaddeduntilpH = 6.5-7 is reached. A reddish brown compound precipitatesalmost immediately; it is filtered, washed successively with ice cold-water, ethanol and ether, and is finally air-dried. Anal.: Found: Cr(V1): 16.70;Cr(II1):
16.57; Cr(t0tal):33.27; NH3: 26.73; NOa-: 19.30%.Calcd. Cr(V1):16.60; Cr(II1): 16.60; Cr(tota1): 33.20; NHB:26.98; NO3-: 19.69%. .[CrCrOdNHa)rlCIOa.Twogramsaf [Cr(H20)2(NHz)d(ClOda (12) and 10 g of NaClOl are dissolved together in 10 ml afwater, giving a solution of pH = 3. Solid, finely powdered NazCrOd is added until solution reaches pH = 6.L7. A deep brown compound immediately precipitates; it is filtered, washed with ice cold water, ethanol and ether, and air-dried. Anal: Found. Cr(V1): 15.60; Cr(II1): 15.43; Cr(tota1): 31.03; NHB:20.05; C104-: 29.42%. Calcd: Cr(V1): 15.50; Cr(II1):15.50; Cr(tota1):31.00; NHs 20.27; C104-: 29.66%. IR Soeetra. IR soectra were obtained with an IR-20-A Beckman specmophmorneter. The samplei aerr prepared ernplwing KRr pressed pellet ur Nujol nwll technique*. Results and Discussion IR spectra of both complexes and potassium chromate, in the region from 1000 to 650 cm-1 can he compared from the fieure. The free chromate ion has T d symmetry, and upon coordination lowers its symmetry to C , , or CzLpoint groups, der~endinron whether it actsasmonoor bidentate ligand; the co;relatioi between these point groups appears in the table. The characteristic hands of coordinated ammonia and the corresponding anions can he seen in the spectra of the new compounds. Moreover, these spectra clearly indicate if the chromate acts as a mono or bidentate ligand. T h e following assignment of normal modes have been established for the free ion: ul(Al) = 847 cm-' (ir inactive); u ~ ( E=) 348 cm-' (ir inactive); us(T2) = 884 cm-' (ir active) and ua(T2) = 368 (ir active) (13).According to the correlation table, the coordination of the chromate groups as monodentate ligand causes the u l and u p modes to he actives, and us and ud, each to be split into two hands. This is indeed the behavior of [CrCrOa(NHs)~]NOs,which exhibits a new band a t 870 cm-1, and splitting of us into two frequencies: 915 (900) and 810 cm-l; additional splitting observed in the E frequency a t 915-900 cm-' should probably be attributed to effects of crystal symmetry, as seen in related cases (14). ~4(T2)mode a t 460 cm-' have not been split but is shifted and appears with greater amplitude,most likely due to overlapping with rooO 9w 7ao M-NH, stretching band; uz(E) infrared spectra ~ f : K&r04: b) mode can he seen as a weak band a t 370 cm-' in a very [CrCrOANHd~l(NOd 4 [C~C~OI(NHJ).I(CIOI). concentrated Nujol mull. ~
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Volume 54, Number 5, May 1977 1 321
Correlation Table P o i n t group
V,
4
440 em-', due to overlapping with M-NHs stretching band,
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as shove -. ~2
~1
In this complex, too, uz(E) mode can be seen at 370 cm-I only with very concentrated Nujol mulls, as in the case of pentaamrnine complexes. Literature Cited
Coordination of chromate groups a9 bidentate ligand lowers its symmetry to point group'As stated in the table, in C2, point group all normal modes are ir actives but A?. The appearance of the band at 890 cm-' (corresponding to u1(A1) mode, inactive for the free ion), and thesplittingof u3(T2)mode giving bands at 935,905 and 810 cm-l, confirm the nature of In this case, splitting of u d ( T 2 ) is not observed either, appearing as a broad band at
(1) Nakmdo, K., " l n f d S e a din-ieand Cmrdinatim, Campounds," 2nd Ed., Wilcy-Intersdence, London, 1970. (2) Nakamoto, K., and Fujita K., J. Amen Chem. Sac., 79.4904 (1957). (3) Finholf. J..and Anderaon. R..lnorr?. Chrm.. 4.43 (1965). (41 Ribss, J., Daetoral Thesis, ~arcelona,1974. (5) c m m b u , ~ . .and ~ ~ i f f i tw., h , J. cham Soe. (A), 1128 (1968). (61 Puglisi, C.. J. InorgNuel. Chem. 32,692 (1970). (7) h*in, &and Panland, R., J. Amer Chem. Soc., 81,3618 (1959). (81 ~ i ~ ~s.,and ~ N h i ~ ~ ~I., ~ J.~,c h pEm phys. ~ 23,1368 ~ ~ (1955). , (9) Tanaka, N., and Kamada, M., Bull, Cham. Soe. Japan, 31 (11) 1222 (19MI. (10) CslaM.J.,Ribss.J..and Coronss J.. J. lnorg. Nucl. Chem.. 38.886 (19761. 40 (11) 1 6 ~ ~ 1 9 6 7 ) . (11) K ~ Y U E O.,, B D K ~ ~ ~ ~ ~ ~ , M . . B U ~ I . (121 ~ o p p e n j a n a , o . , s n d ~ u n d~t.,. ~ n o rCham., g. 8,506, (19691. (13) Nyquist. R.. and K a d . R., '"lnfrered Spectra of Inorganic Compounds..' Academic Press.Naw York. 1971.~.16. (14) J., coronas, J.. and nrrer, M..lnorg. chim. ~ e t o . .18.47 (1976).
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