Bpectrccklmlcr, Acti. 1967, Vol. 0, pp. 51 to 58. Pergmon PIWE Ltd., London

Infra+ed absorption spectra of inorganic co-ordination complexes*-IX I&wed spectra of oxalato complexes

M. JUDITH Scmumz,j’ TATSUO MIYAZAWA, SAN-ICHIRO MI~USHIIU$ T. J. LANE,§ and J. V. QUAQLIANO

Ab&ract-The in&wed qmotra of simple met& oxsletes have been measured end the essign- ment of bends hes been besed on the normal vibration oalculetion for the free oxalate ion. The i&m-red speotre of ox&to complexes of Fe(III), Co(III), Cr(III), Al(III), Pd(II), end Cu(II) hsve been measured in the NeCl and CsBr regions. Assignment of bends hes been made by the oomparieon of the spectra with those of elkeli mti ox&&s and dimethyl oxslste.

SIMPLE oxalate salts consist of a metal cation and the oxalate anion. Therefore, the infrared spectra of these compounds in the NaCl and CsBr regions are con- sidered to arise from the oxalate ion and from water of crystallization present in the compound. In most metal oxalato complexes the ligand is co-ordinated to the central metal ion through two oxygen atoms, whose oxygen-to-metal bonds have at least some covalent character. DOUVILLI$ DUVAL, and LECOMPTE [l] have carried out a number of absorption studies of simple and complex metal oxalates. Oxalato complex salts can be explained approximately by considering the vibrations of the structure, whiah in some respects is similar to dimethyl oxalate. However,

in a complex ion such as thetrioxalato metal complex, coupling of similar vibrations of the same structure must also be considered.

The results of this research, based on the normal vibration calculation of the free oxalate ion, explain the nature of the vibrations of simple oxalate salts. The infrared bands of the oxalato metal complexes are assigned by comparison with those of simple oxalate salts and of dimethyl oxalate.

Experimental Prepamtion and purQ2caGm of mwpoud. Calcium, lithium, end berium oxalates were

Fisher-certi5ed reagents; potassium oxelete was 8 Baker’8 C.P. product; and sodium oxelete w8s obtained from the Nntion8l Buresu of Stendards. &mples were dried et 110”. Iron(III), oobslt(III), chromium(III), end&mini um(III) triox&ato complexes were prepared end &31l8lysed by Dr. D. N. SEN [2]. Tran+W.sssium dioxaletodiaquochromte(II1) trihydrate and trane- sodium diox&todisquoohrom&e(III) pentahydrste were prepsred by themethod of Wxnmrx[2].

l Paper VIII in scrics, J. Amer. Clmtn. Sm. 1956 78 889. Prcscntcd in part before the Physics1 and Inorgenio Division of the American Chemical S&&y, Kanssc City, Misscuri, March 1954; contribution from the Department of Chemistry, University of Notre Dsme.

t Sr. M. Junrr~. R.S.M. $ Visiting Profcsscr from 4 Rev. T. J. Lm, C.S.C.

Faculty of Science, Tokyo University.

51

M. JUDITH SCAHELZ ti al.

Anal. c&d. for K[Cr(CI0,),(H,0)81’3HIO: Cr 14.56. Found: Cr 14.87. Calcd. for Se[Cr(C,O,),(H,O),]*sH,O: Cr 13.79. Found: Cr 13.75.

Sodium dioxaletodiiuocuprste(I1) wee prepared ee described by RILEY [4]. Anal. calcd. for Na&u(C,O,),(H,O),]: Cu 19.75. Found: Cu 19.50. Potassium dioxalatopalledate(I1) [5] was prepared by adding a concentrated solution of

K&&O, to a concentrated aqueous solution of qPdCl,]. [Pd(NH.J,(C,O,)] was prepared by dissolving cie-~d(N&)&ls] in a concentrated aqueous solution of K&,0, [6]. The complexes DreciDitated in the cold. _

A&. celcd. for KJPd(C,O,),]*ZH,O: C,O, 44.36. Found: 44.10. C&d. for [Pd(NH&C,OJ: C&O, 38.47. Found: 37.98. - _

Deuteration was effected by recrystellizetion of the metal ox&tea from 99.6% D,O.

Table 1. Observed frequencies of simDle ox&tee in cm-l* .

Li,C,O,

- - -

1650 vs -

1420 m 1330 V8

- - -

782 ah 771 vs

- -

607 s 434 8 370 8

NB*C*O,

- - -

1640 vs -

1418 m 1338 v8 1320 VB

- -

i80 ah 774 vs - -

514 s -

360 m

K&,O,*H1O

3370 8 - -

1600 vs -

1408 m 1310 v8

- - -

772 s -

716 m 613 6 623 s -

349 m

cac*04*~o

3400 8 3300 8 3020 E 1620 vs

-

1366 m I316 VB

-

943 m 881 m 780 VB -

660 8 -

511 s 416 w 306 s

l vs = very strong, 8 -_ strong, m = medium, w = weak, ah = shoulder.

Ek3C*04*~0

3600 m 3400 m

- 1620 VB 1600~1 1470 m 1326 vs 1310 VB 896 w 866 m 771 VB -

662 m -

620 s 406 w 343 m

Table 2. Observed frequencies of trioxalato complexes in cm-l

I%MC,O,),l.3H,O

3440 w 1726 VB 1696 vs 1400 va 1290 a 1268 a 906 m 820 m 800m 677 s 478 8 434 m

3460 w 1710 vs 1680 vs 1400 vs 1300w 1265 m 903 w 823 m 804 m 540 8 480 m 409 B

L

3466 w 1710 VB 1680 vs 1390 vs 1266 m 1248 m 890 m 802 8 785 m 674 sh 628 B 493 a

3460 w 1710 VB 1680 va 1397 vs 1300 w 1266 m QOOW 823 m 806 m 661 B 470 B 445s

52

Infra-red apeotm of oraleta oomplexea

A&umydon +twatum. Spectra were obtained by merm~ of a Per&n-Elmer mode1 21 i&a-red ~~~~r u&g N&l and CkBr priamt~ The region from 2 to 3B microns wae examined and the kgectra obtained by the KBr d&k method were checked u&g Nujol mu&. The abeorption bands obeerved we Ii in Tables 1.2, and 3.

Table 3. Observed frequencies of mono- and dioxalato complexes in cm-1

32PX@ 3X0(”

-

liws Ys 1665 vs 1630 ah{“’ 1400 ve 1318 e(a) 12bOE

- 895 w 823 e 779 m(O)

- 649 B 467 8 374 m 363 m 299 m

3460 m 3400 lab

- 1708 ve 1680 vs

-

13SovfJ -

1240 e -

890 m 813 e

- 6Ooe 570 6 434m 374 a 367 m

35Ooa 3400s

-

1728 m 1680 vs

-

1412 vl! -

1275 s -

898 m 800s

- -

6&H s 4Q7 8 387 m 346 s

-

WWC,ol), @J$%iP@%Q

3430 8 3350 sh 2980 B 17111 vs 1090 v8 1660 vs 13911 VB

- 1255 8 967 m 88313 812 VB 663m 64(lm $43 e 424 B 398 a 316 m

-

wwIoI)* @%%1’3W’

3430 0 3360 ah 2960 a I715 VE 1690 WJ I66Ove 1396 vs

-

I257 a 967 m 886 e 815 vs 658 m

-

5408 422 6 399 B 3111 m

-

(*I These frequencies are attributed to NH, stmtohing and deformation vibrationa [7).

1. Ncan?& ~b~~~ ~~~~ of OX&i?0 ion and the a&Ggwne& of obS$f-#~

freque9uGecr. The calculation of normal vibrations of the free oxdate ion hadi been made on the basis of the Urey-Bradley type potential:

Table 4. ?dokcular oonstnn~ and force constants (106 dyne/cm) of the ox&&e ion

Mohouler COImtante Force con&at&

I

ef--a 1~6 A w--Q 1.27 A (O-C-O) 124” (c--G-o IlfY

K(c-a) 2.60 WC-01 7.20 Rfo-C-0) 0.15 R(c-c-0) 0.15 P(O *. .O) 3.00 P(C . . . 0) 040

M. JUDITH SCHMELZ et d.

V = Xd&(ArJZ + E*H. ,,T~~Y,~ ( Aaij)a + EQP,, ( AqJe + linear terms, where ri and r, are bond lengths with equilibrium values rio and r,O; ai, bond angles and qu distances between nonbonded atoms. In addition, K, H, and Prepresent stretching-, bending-, and repulsive-force constants respectively.

The atomic distances, bond angles, and force constants used in this calculation are shown in Table 4. The molecular constants have been taken from data obtained for ammonium oxalate monohydrate [8], and the stretching-force constant K, bending-force constant H, and repulsive-force constant P have been taken from the corresponding constants of ethane [9], formats ion [lo], and acetate ion [Ill

respectively. The frequencies calculated for the VI, model are shown in Table 6.

‘Table 6. Calculated and observed frequencies of the ox&&e ion with point group V,*

Symmetry of vibration Type calcul8ted Obaervedt

VI VI (O--c--Q 1407 1489 A,(R) P P v (C-C) 973 902

“S 6 (O-C-O) 412 460

“4 C-C torsion I inactive

B,,(R) "6 VI3

va P--c--o) 1696 CO, rocking 432

1647 317

B,,(I) V? CO, We&X43 - - -

B,,(R) % CO, W%&% - -

B,,(I) 1’ S “lo

v, (O-C-O) 1644 1600 CO, rocking 222 -

B,,(I) Vll v, (O--c-o) 1336 1310

VlS 6 (O-C-O) 777 772

I I

* R = Raman active; I = infra-red active; v = stretching; v, = symmetric stretching; v,, = anti- symmetric stretching; b = in-plane deformation vibrations.

t Observed values are those for K,C,O,.H,O.

In Na,C,O, three absorption bands have been observed at 1640, 1329, and 774 cm-l. These three bands can undoubtedly be assigned to Y*, yll, and wX1, respec- tively. In the case of K,C&O,*H,O, LiaCaOr, CaC,O,*H,O, and BaCpO,*H,O the corresponding three bands occur at about 1010, 1314, and 775 cm-l. The band arising from the deformation frequency of water in the monohydrates overlaps the y0 band. Furthermore, the spectra of the monohydrates show a broad absorption band at about 680 cm-l which in some way may be related to the water molecule, since this band disappeared upon deuteration.

Raman lines of K&,0, in aqueous solution were reported by EDSALL [12] at

1647 (ib), 1489 (7), 902 (6), 456 (3) cm-l, and they can be assigned to Y&, yl, vz, and

64

I&a-red epeatra of oxslat complexes

Ye, respectively. Since the latter three lines are strong Raman lines, they can be assigned quite reasonably to the A, class.

The vli (B,,) and Y@ (B,,) bands are forbidden in the Raman effect by the selection rule for the Vh symmetry. Since they were observed in the aqueous solution, however, the VIh symmetry may be changed to V in this state. That is to say, the oxalate ion is no longer planar, and consequently yll and vp have been observed in aqueous solution. In this connection, Table 6 shows the corresponding vibrations for the planar V* model and the nonplanar V model. It is to be noted that for all the B, vibrations, frequencies become independent of the azimuthal angle 8 of the internal rotation about the central C-C axis, since the G and F matrices for B, do not include 8. Consequently, the B, frequencies of ?‘a become Raman-active for V without a change of frequency.

Table 6. CYkaificetion of vibration typea for the plansr V, and nonplanar V mod& with selection rulea

pl@ner ( VA) Nonplaner (V)

3A,W) 14 4A(R)

2%,(R) l%(Z) 3B,(R, 4

lB,,(R) 2&u(Z) I 3B,(R, I)

-

04,(R) 2%(Z) 2B,(R, I)

The G and F matrices of the B,- and B,-type vibrations contain 8 and therefore change considerably with a change of internal rotation. This change is most marked for the O-C-O rocking and wagging frequencies. It would be interesting to compare the Raman effect of K,C,O,*H,O in the solid state with that in aqueous solution.

Probably in aqueous solution the energy of conjugation between the two O-C-O groups is reduced and thus steric repulsion between them becomes more important, resulting in an out-of-plane twisting about the C-C bond. It is easily seen that the steric repulsion potential is a minimum when one O-C-O group is at right angles to the other.

2. Aesignmem? of IR band.8 of oxalato compleaw. To assign the bands observed in the oxalato complexes, the spectrum of dimethyl oxalate in the CW form together with that of the oxalate ion must be considered. Dimethyl oxalate in the liquid state was shown by one of us (B~IYAZAWA) to exist in both the trana and cis forms. Of these, the c&3 form may be considered similar in vibrational spectrum to chelated oxalato metal complexes. The assignment of the spectrum of the tie ester was made as shown in Table 7 [ 131, listing only the frequencies necessary for comparison.

55

M. JUDITH SCENELZ e4 al.

From these data the assignment of infrared bands for the chelated trioxalato metal complexes can be made as follows:

(1) An intense doublet band observed at about 800 cm-l .ie considered to correspond to the 772~am-l (B,,) band of the oxalate ion of the potassium salt, and the 851~cm-l (B,) band of the tie ester and can be assigned to the (O-C-O) deformation vibration (B,) . The doublet structure arises from coupling of vibrations of the three oxalato groups, as will be explained later.

(2) The band at about 900 cm-l corresponds to the band at 902 (A,) of the oxalate ion and that of the cti eater at 862 cm-1 (A,). Therefore, this can be assigned to the C-C stretching vibration (A,).

(3) A doublet baud observed between 1260 and 1300 om-1 is considered to correspond to the 1310~cm-1 (B,) band of the oxalate ion of the potassium salt and to the 1166-cm-1 (B,) band of the cis ester. This is the C-C-O stretohing vibration in the oxalate ion. In the c&3 ester the C-O bond order is less than the C=O bond order and therefore this vibration arises mainly from the C-0 vibration. However, there is a contribution of the C=O stretching vibration in the same phase as the C-O vibration, and therefore we shall denote this vibration of the oxalato complex as Y, (C-C-O).

Table 7. Adgnment of Borne observed frequenaiea of dimethyl oxdate in the cis form

observed fiwqutmcy I

Tspe of vibr8tion

404 861 882 958 987

1166 1326 1770 1778

(4) A strong absorption observed at about 1400 cm-l corresponds to the 1489~om-l band (a,) of the oxalate ion and to the 1326-cm-1 band (A,) of the tie ester. Therefore, this band can be assigned to the C-C-O symmetrio stretching vibration (A,). This band does not show a doublet splitting.

(6) Strong doublet bands were observed in the 1700~cm-l region. Here we may expect A, and B, type C-C-O stretching vibrations. As shown below, the B,- type vibrations appear as double bands, of which the higher frequenoy band is considered to overlap the A,-type band.

(6) The band observed at about 3460 cm-l can undoubtedly be assigned to the stretching vibrations of the water of crystallization.

The following explanation is proposed for the observed doublet structure of the

66

B,-type vibrations. In the preceding discussion we have been dealing with the vibrations of oxalato complexes as a seven-body problem, taking into account one oxalato group and the central metal atom. Strictly speaking, however, the vibra- tions should be treated as a nineteen-body problem. In other words, in addition to the frequencies just considered, we have to take into account the coupling of three such systems.

The trioxalato metal complex ions have a symmetry of D,. Consequently, the B,-type vibrations of a single oxalato group are classed as symmetric to the trigonal axis (A,) or degenerate (B), both of which are ix&a-red-active. On the other hand, of the AI-type vibrations of a single oxalato group those symmetric to the trigonal axis (A,) are inactive and those degenerate are active. Thus it is understandable that the BI vibrations of the oxalato group are observed as double bands, whereas the A, vibrations of the group are single .

The carboxylate frequencies in the spectra of the oxalato metal complexes am intermediate between those of the oxalates of the alkali metals (in which the metal-to-oxygen bonds are essentially ionic) and those of dimethyl oxalate,

Table 8. Fraoticnal shifta of oerboxylofe etretohing fbqllenoiefJ in trioxlbl8to ccxnplex~

!Cype~ of vibration Fe Cr Co Al

suggesting a partial covalent character for the oxygen-to-metal bonds in these complexes. Using the carboxylate frequencies of sodium oxalate as the standard of reference, the ratio of the shift in these frequencies in a complex to that in the ester, which we shall term the fractional shift, may be taken as a measure of the covalent character of the M-O bonds. The relative positions of the absorption bands in K,C,O,*H,O and KJAl(C,04)&3H,O are shown in Pig. 1.

For this comparison of frequencies we have chosen the O-C-O antisymmetric stretching vibration (B,) and the O-C-O symmetric vibration (A,), thereby avoiding the difficulty arising from coupling with the M-O stretching vibration, the CHgO stretching vibration, and the CH, rocking vibration. The result is shown in Table 8, where simple oxalate salts are assigned a value of zero and di- methyl ester a value of unity. The mean value of the fractional shift is about O-6, which indicates that the M-O bonds in these complexes have about half the co- valent character present in the C-O bond of the dimethyl ester.

Acknowledgement--This investigation was supported in part by a research grant H-2218 (C) from the Department of Health, Education and Welfare, Public Health Service.

57

M. J~DITE i!hxwsz St at.: Infkw0d spehxb of oxal& complexes

Wave numbers em-*

0 2 3 4 5 6 7 Q 10 11 12 13 14 15

Wove length micron

Fig. 1. Potsssium oxalete monohydrmti, K&,O,*H,O.

- - - - - - Potiium trioxalatoal uminak trihydrsti, K&u(C,O,),~3H,O.

[l] DO~IIJ& F., DWAL C., snd LECOMP’XE J. BUU. So& CM?& Fvavace 1942 9 548. [2] SEN D. N. Ph.D. thesk,.Utivereity of Notre Dame, 1964. [3] WYBBXER A. Ann. 1914 408 261. [4] R-Y I% L. J. Okem, Sot. 1929 1307. [6] LAXD~EN G. 2. Anmg. Clbcna. 1926 154 432. [6] MANX F. G,, CisoWroo% D., GA-B D. C., and WOOB~ N. J. Cliern. Sot. 1936 184% [7] MIZUS~ S., NAKAGAWA I., snd QOA~LXAS~O’ 3. V. J. Chess. Z%y8. 1966 RB; 1867. [S] JEEFItBY C. A. and PARRY G. S. J. Cbra. Sot. 1962 4864. [9] Priveti communication of Dr. T. Stiouchi.

[lo) &ffYAZAWA T. J. ChRnr. Sot. Jcqxxn 1986 77 381. [I l] MrznS~ s., s= OUUIXI T., and NAIIAB~UF~A K. p&a& oommunioetion. [12] EDSAX& J. T. J. Churn. P&p. 1937 & 608. [13] MntazhWA T. J. C?te~%. Sot. Japan 1954 745 540.