792
J. P. DISMUKES, L. H. JONES AND JOHN C. BAILAR,JR.
in the visible in tet(rahedra1 complexes, band B a t 12,900 cm.-' would have no reasonable assignment. Moreover, with a tetrahedral field of the magnitude required to give the 4Az(F)-t4T1(P) transition a t the position of band A a magnetic moment of 4.34.5 B.M. would be expected, in serious conflict with the experimental result. Experimental The magnetic susceptibilities were measured using a sensitive Gouy balance, following procedures already described .E The experimental data and some derived quantities are collected in Table I. The reflectance spectrum of Co2Si01was measured using a Beckman DU spectrophotometer equipped with the standard reflectance attachment, MgCOs serving as the reference. The reflectance spectrum is shown in Fig. 1. (6) R. H. Holm and F. A. Cotton, J . Chem. Phys., 31,788 (1959).
Temp., OK.
T-ol. 65
TABLE I MAGNETIC DATAFOR CozSiOc XCor Diamagnetic Mol x lo', c.g.s.u.
cor., c.g.s.u. X 106
reff
299 (3)" 17,382 f 165b 73 4.58 193 (2) 24,284 910 73 4.35 77 (2) 46,979 f 495 73 3.80 a Figures in brackets show number of measuremente deviation from used to calculate mean values. "verage mean.
*
We thank Dr. Kelley of the Berkeley Laboratory of the Bureau of Mines for his kindness in supplying the sample, and the United States Atomic Energy Commission for financial support under Contract No. AT(30-1)-1965.
THE MEASUREMENT OF METALLIGAND BOND VIBRATIOKS IS ACETYLACETONATE COMPLEXES1 BY J. P. DISMUKES, L. H. JONES AND JOHN C. BAILAR,JR. william Albert Noyes Laboratory of Chemistry, University of Illinois, Urbana, Illinois and the Los Alamos Scientific Laboratory Los Alamos, New Mexico Receiaed November 3,1960
The infrared spectra of a large number of metal-acetylacetonate complexes have been recorded. The abRor tion frequencies below 700 cm.-1 are discussed in terms of coupling of metal-oxygen vibrational modes with three low-Kequency xibrational modes of the acetylacetonate anion at 654,520 and 410 cm.-I. It is concluded that no absorption band between (00-350 crn.-' can be assigned to a pure metal-oxygen vibration.
Introduction The infrared spectrum of acetylacetone has been investigated in a large variety of coordination compounds, but only a few measurements of the low-frequency vibrations of the metal-ligand bond have been reported. Measurements of metalligand vibrations have two advantages over the measurements of the internal vibrations of the coordinated ligand. First, trends in the metalligand vibrational frequencies are more directly related to the strength of the metal-ligand bond than are the trends of the internal vibrations of the coordinated ligand. Second, from the frequency of the metal-ligand stretching vibration, the force constant of the metal-ligand bond can be calculated. Such force constants give a measure of the bond strength independent of values derived from stability constants. The present study of the low-frequency infrared spectra was undertaken to investigate metal-oxygen bond strengths in metal-acetylacetonate complexes. It was hoped that metal-oxygen stretching vibrations could be identified and that force constants could be calculated for the metal-oxygen bond. The infrared spectra of acetylacetone2-6 and (1) Most of the preparative work reported here was done a t the University of Illinois and the infrared work a t Los Alamos. One of the authors (J.P.D.) wishes to thank the National Science Foundation, Minnesota Mining and Manufacturing Company, and the University of Illinois for fellowship assistance which made this work possihle. Inquiries concerning this article should he addressed to J.P.D. a t R.C.A. Laboratories, Princeton, New Jersey. (2) K. W. F. Kohlrausch and A. Pongratz, Ber., 67, 1465 (1934). (3) H. W.Morgan, U. S. Atomic Energy Commission Report AECD 2659 (1949).
many acetylacetonate have been measured by a number of investigators. The vibrations above 700 em.-', where there is little difference in the spectrum of acetylacetone and that of any metal-acetylacetonate complex, have been generally assigned to various normal modes of the acetylacetonate group. The lowering of the carbonyl frequency is the most noticeable change above 700 em.-' produced by coordination. Several investigators have found absorption bands in acetylacetone and its metal complexes in Morgan,* the region between 700-400 cm.-'. L e c ~ m t eand ~ ~ ~Mecke and Funk6 noted three strong absorption bands for acetylacetone in this region, and Lecomte assigned these bands to vibrational modes of two coupled acetone molecules. Morgan and Lecomte observed a number of bands in the spectra of divalent, trivalent and tetravalent metal acetylacetonates between 700-400 ern.-', but they could find no correlation between these bands and the oxidation state or character of the metal. Costa and Puxeddu'O obtained almost identical spectra for the acetylacetonate complexes of Cr(I1) and Cr(II1) in the 700-400 cm.-' region, and therefore assigned these bands to vibrations of the acetylacetonate ion. Martell," however, has (4) J. Lecomte, Disc. Faraday Soc.. 9, 125 (1950). (5) J. Lecomte, C. Duval and R. Freymann, B d . uoc. chim., France, 19, 106 (1952). (6) R. Mecke and E. Funk, 2. Elektrochem., 60, 1124 (1956). (7) L. J. Bellamy and R. F. Branch, J . Chem. Soc., 4491 (1954). (8) H. F. Holtzclaw, Jr., and J. P. Collman. J . Am. Chem. Soc.. 79, 3318 (1957). (9) R. West and R. Riley, J . fnorg. Nucl. Chem., I,295 (1958). (10) G. Costa and A. Puxeddu, rbid., 8, 104 (1958).
METAL-LIGAND VIBRATIONS IN ACETYLACETONATE COMPLEXES
May, 1961
assigned a strong band at 420-460 cm.-l in divalent metal acetylacetonate complexes to a metaloxygen vibration. At present, there appears to be no general agreement on the origin of the lowfrequency infrared absorption bands in metalacetylacetonate compounds. Experimental a. Preparation of Compounds .-The compounds used in these studies were prepared from acetylacetone and the appropriate metal salt by methods reported in the literature. Reference to the method of preparation in each case is given in Table I. Where melting points are reported, these were used to check the purity of the compounds. ANALYTiCAL Compound,
Be(acac)e Mn(acac)l Fe(acac)t.H20 Co( acac)n Xi( acac)g Cu( acac)? Cr(acac)~ Mn(acacia Fe( acac)n Co( acac)s hl( acac)r Ga( acac), In( acac)J La(acac)a.H:O Ce( acac)3.2Hz0 Kd(acac)? Na(acac) K(acac)
TABLE I DATAO N THE COMPLEXES STUDIED Ref.
12
13 13 13 14 13 15 16 14
17 18 19
19 20 20 20 9 9
Analyses. ir, -HydrogenCalcd. Found
-Carbon-Calcd.
Found
57.95 47.43 44.11 46.69 46.67 46.90 51.52 51.20 50.95 50.50 55.85 49.05 43.70 39.70 38.18 40.82 49.20 43.50
58.41 47.28 44.02 45.75 46.52 46.16 51.80 51.05 51.18 50.77 56.05 49.28 43.60 39.61 38.44 40.69 49.62 41.86
6.76 5.53 5.88 5.49 5.45 5.36 6.02 5.96 5.95 5.90 6.48 5.72 5.13 5.06 5.27 4.76 5.74 5.07
6.96 5.58 5.35 5.54 5.50 5.37 6.40 6.17 6.07 6.10 6.53 5.90 5.52 5.03 5.00 5.02 6.14 5.51
b. Measurement.-The spectra reported in Table I1 were observed on a F’erkin-Elmer Model 112 Spectrometer equipped with interchangeable LiF, CsBr, and CsI prisms, and on a Perkin-Elmer Model 21 Spectrometer equipped with a NaC1 prism. The spectra of the complexes were measured in mineral oil mulls, and where possible also ae single crystals and in benzene solution. No significant differences in the spectra were noted for the three methods. The reported frequency values are accurate to better than A 2 cm.-’. References to the measurements of other investigators are given in Table I1 along with the spectra of acetylacetone and its metal complexes.
Discussion The infrared absorptions of all of the metal complexes of acetylacetone above 700 em.-’ occur at approximately the same frequencies as those of free acetylacetone. The most significant deviation above 700 ern.-’ occurs in the region of the carbonyl absorption. Since it is generally agreed that (11) A. E. hlari,ell, K. Piakamoto and P. J. McCarthy, Nature, it.?, 459 (1959). (12) A. Arch and R. C. Young, “Inorganic Syntheses,” McGrawHill Book Co , Inc , New York, N Y., Vol. 11, 1946, pp. 17-19. (13) T. Moeller, “Inorganic Syntheses, ’ Vol. V, 1957, pp. 105113. (14) F. Gach, Monatsh. Chem., 21, 98 (1900). (15) W. C. Fernelius and J. E. Blanch, “Inorganic Syntheses,” Vol. V, pp. 13C-131. (16) G. H. Cartledge, U. S. Patent 2,566,316, June 12. 1951: C. A.. 46, 1585 (1952). (17) W. C. Fernelius and B. E. Bryant, “Inorganic Syntheses,” Vol. v, pp. 188-189. (18) R. C. Young, “Inorgania Syntheses,’’ Vol. 11, pp. 25-26. (19) G. T. Morgan. J . C‘hem. Soc., 1058 (1921). (20) T. Moeller and W. F. Ulrich, J . Inorg. Nuel. Chem., 2, 164 (1956).
793
the absorptions above 700 cm.-l involve mainly internal vibrations of the acetylacetonate g r , o ~ p , ~ discussion ~ ~ J - ~ ~ of this region of the spectra will be omitted. The spectra of metal acetylacetonate complexes below 700 ern.-’, however, differ markedly from that of acetylacetone. There are more bands in the spectra of the metal complexes, and the absorption bands of acetylacetone apparently are strongly shifted. It is possible, however, to explain the principal features of the absorption below 700 cm.-l on the basis of the following two assumptions: (1) The three strong lowfrequency vibrational modes of the acetylacetonate anion are those of the potassium salt of acetylacetone at 654,520 and 410 em.-’; and (2) the metaloxygen vibrational modes interact with the three strong low-frequency modes of the acetylacetonate anion. The splitting of these three peaks into several peaks is caused partly by the coupling of the acetylacetonate vibrations within the complex, and partly by coupling with the metal-oxygen vibrations. It was observed in the sodium and potassium salts of acetylacetone that there are three sharp, intense vibrations in the 700-300 cm.-l region. Since these vibrations show only a slight shift in frequency from the three strong vibrations of the acetylacetone molecule, it is reasonable to assign these three vibrations to an uncoordinated acetylacetonate anion. The frequencies of the potassium salt are chosen for this assignment. It is then possible to arrange the strong absorptions of metal-acetylacetonate chelates in the 700-350 cm.-l region into three groups, each group being associated with one of the low-frequency absorption bands of the acetylacetonate anion. I n all the complexes there is a split band in the 650-690 cm.-l region which appears to be associated with the strong 654 ern.-’ absorption of the acetylacetonate anion. The upward shift of the 654 cm.-’ band shows no apparent correlation t o the character of the metal forming the complex. The splitting of this shifted band is no doubt caused by coupling of acetylacetonate vibrations within the complex. For the trivalent rare earth complexes, however, there is no shift in this band from the position in the potassium salt. This lack of interaction reflects the weak complexing nature of rare earth metal ions. A similar effect has been noted for the carbonyl absorption band in rare earth acetylacetonate complexe~,~ where there is little lowering compared to that shown by complexes of other trivalent metals. It is evident that the metal-oxygen vibrations in the rare earth complexes do not interact strongly with the 654 em. vibration of the acetylacetonate anion. I n all the metal acetylacetonate complexes, except the beryllium complex, there is a strong absorption band which appears to be associated with the 520 cm.-’ vibration of the acetylacetonate anion. The frequency of this band is shifted upwards upon complexing. The small shift for the rare earth complexes again indicates the weak complexing behavior of the rare earth metal cations. The shift in the 520 cm.-’ band of the acetylacetonate anion can be directly related to
the stability constant of the complexes, where these values are known; the frequencies of the shifted 520 cm.-l bands and the stability constants are listed in Table 111. The strong band a t 819 cm.-' in the beryllium complexg may be associated with the 520 cm.-l band of the acetyluretonate anion, the large shift
Hacac
3004(s) 2966(s) 2921(s) 2832(w) 1728(m) 1710(m) 1620(s)
TABLEI1 IXFRARED SPECTRA OF ACETYLACETONE .4UD ‘In FC?
7 25 8 67 9 51 10 38 14 95
((0
Si
cu -4 1 G3 Fe La CC?
Sd
Log K I K Z K S
22.3 23 6 26 2 11 9 12 6 13 1
Frequency of shifted 520 cm.-1 band
543 cm:’ 551 558 582 610 574 580 558 526 526 530
with the weak vibrations of acetylacetone below 400 cm.-l. In view of the complexity of the infrared spectra of metal-acetylacetonate complexes, the origin of these weak vibrations is uncertain. Since many of the complexes were prepared from aqueous solution, it is worthxhile to consider the effect, of the substitution of hydroxide ions for avetylacetonate anions upon the infrared spectra of these complexes. I n attempts to prepare olated metal acetylacetonate complexes during the course of these investigations, a polymeric olated chromium(II1) acetylacetonate compound was obtained. Chemical analysis and molecular weight determination in beiixene suggest the molecular formula Cr4(OH)6(acac)6. Comparison of the infrared spectrum of this roniplex with that of the normal trivalent chromium aretylaretonate indicates few changes upon even this extensive introduction of hydroxo groups. I t seenis unlikely, therefore, that a small percentage substitution of hydroxo groups would appreciably alter the infrared spectra of metal acetylacetonate complexes below 700 cm.-’.
the -1.10 em.-’ band in the acetylacetonate anion, but the splitting and frequency shifts are irregular. ‘the splitting of the 410 cm.-l peak into several peakb is no doubt caused by the coupling of the acetylacetonate vibrations within the complex. It appears that the interaction of the metal-oxygen vibrations is strongest in the case of the 410 cm.-l band. The most regular behavior is observed for the rare earth complexes, where there are four closely summary spaced bands in the 350-425 cm.-’ region. Here There is no simple relation between the infrared again the rare earth complexes exhibit weaker interaction than the other metal complexes. In spectra of metal-acetylacetonate complexes below the aluminum, gallium, indium series there are 700 cm.-l and the oxidation state or chemical three widely spaced bands, and the band of highest character of the metal or the structure of the comfrequency decreases in frequency from aluminum to plex. Although the strong frequency band in the gallium to indiurn. Since the stability constants 520-630 cm.-l region shows the same trend as the of the aluminum and the gallium complexes are stability constants of the complexes, the other approximately the same, this shift probably re- strong absorption bands below 700 cm.-’ do not flezts the change in mass of the central metal ion show this trend. It seems improbable that any in the series. The complexes of trivalent chro- one band in the 700-350 cm.-l region pan be atmium, manganese, iron and cobalt also show three tributed to a pure metal-oxygen vibration. The widely spaced bnnds, with the lowest frequency origin of several weak bands below 350 ern.-’ is bands displaced further toward lower frequencies uncertain. The spectra of metal-acetylacetonate in the cobalt and chromium complexes than in the complexes below 700 cm.-l are best explained in manganese and iron complexes. In the complexes terms of the following two assumptions: (1) the of divalent manganese, iron, cobalt, nickel and three strong low-frequency vibrational modes of copper there are several bands in the 400-450 the acetylacetonate anion are those of the potassium cm.-l region. The relation of frequency to salt of acetylacetone at 634, 520 and 410 cm.-’; stability constant observed by Martellll from co- and (2) the metal-oxygen vibrational modes interbalt to copper wa5 also noted here, but the relation act with the three strong low-frequency modes of does not hold for the whole series of divalent transi- the acetylacetonate anion. The splitting of these tion metals. I n the divalent beryllium complex three peaks into several peaks is caused partly by there are two strong bands, with the highest fre- the coupling of the acetylacetonate vibrations quency band showing a strong shift. This be- within the complex, and partly by coupling with havior probably is due to the small mass and strong the metal-oxygen \+rations. The vibrations in metal-acetylacetonate complexes between 700-350 complexing ability of the beryllium cation. I n addition to the vibrations which appear to cam.-’ can then be arranged into three groups, each be due to interactions of metal-oxygen vibrational group being associated with one of the low-fremodes with the three low-frequency vibrations of quency vibrational modes of the acetylacetonate the acetylacetonate anion, several weak absorp- anion. I n view of the vibrational interaction tions are observed in the infrared spectra of metal- noted in the acetylacetonate complexes, determination of metal-oxygen force constants must (21) J. Bjerrum, G. Schwarzenbach and L. G. Sillen, “Stability be done on complexes with simple oxygen-donor Constants,” Part I: Orgrinic Ligands, The Chemical Society, London, ligands, such as aquo or hpdroxo complexes. 1957. pp. 29-30.
