5968
J . Am. Chem. SOC.1988, 110, 5968-5972
Infrared, Resonance Raman, and Excitation Profile Studies of Os2(02CCH3),C12 and Os2(02CCD3),C12. The Assignment of the Osmium-Osmium Stretching Vibration for a Complex Involving an Osmium-Osmium Multiple Bond Robin J. H. Clark,* Andrew J. Hempleman, and Derek A. Tocher Contribution from the Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WClH OAJ, U.K.Received January 25, 1988
Abstract: Extensive Raman studies (1525-40 cm-I) of Os2(02CCH3)4C12have led to the identification of the three strong bands, vl, v2, and v3, at 229, 393, and 292 cm-l to the key skeletal stretching modes, v(OsOs),v(OsO),and v(OsCl), respectively. = 383 nm lead to the development of Raman spectra of the complex at resonance with the intense electronic band at A, a six-membered overtone progression in u1 as well as combination band progressions in vI based upon one quantum of either v2 or v3. This indicates that the principal structural change attendant upon excitation to the resonant state is along the OsOs coordinate. Fourier transform infrared spectra (3500-40 cm-l) have also been obtained. Acetate deuteriation provides conclusive evidence for many of the infrared and Raman band assignments. The study provides the first firm identification of v ( 0 s O s ) for a multiply bonded species.
Following the X-ray determination of the structure of Re2(02CC6H5),C12in 1968l the chemistry of third-row dimetal tetracarboxylates began to be developed. However, there were no reports on possible diosmium complexes until that by Wilkinson et aL2 regarding a brown osmium(II1) chloroacetate of stoicheiometry [Os(02CCH3)2C1],. Shortly afterwards the X-ray crystal structure of the analogous butyrate complex was determined3q4and shown to possess the familiar dimeric tetracarboxylate framework
(OsO), owing to the fact that the angles OsOsO are near 90°, significant coupling between the coaxial modes v(0sOs) and v(OsC1) may be expected. Thus in order to place the band assignments on a firm basis it is essential to study both Os2(O2CCH3),C12 as well as Os2(02CCD3)&12. This has now been accomplished, and the results are reported herein. Experimental Section
Preparations of Complexes. The complex O S , ( O ~ C C H ~ ) ~was CI~ prepared by the established procedure.) The deuteriate was prepared from [Ph,N]2[Os,C18]11*12 by use of acetic acid 100.0 atom% D and acetic anhydride 99+ atom% D (Aldrich). Elemental analyses for C, H, and CI were satisfactory. Attempts to isolate an analogous bromide for comparative studies were not successful, unsatisfactory analyses being invariably obtained. Instrumental. Raman spectra were recorded by use of a Spex 14018 Since this time the crystal structures of O S ~ ( O ~ C C H and ~ ) ~ C ~ (R6) ~ spectrometer in conjunction with Coherent CR 3000 K and CR 12 O S ~ ( O ~ C C ~ H have ~ ) ~ Calso I ~ been determir~ed,~the tetralasers. Samples were held as pressed KCI discs at ca. 80 K in a liquid nitrogen cell. carboxylate framework being virtually identical for all three Infrared spectra were recorded at ca. 80 K as KCI discs (3500-500 complexes. The average OsOs distance in these complexes is 2.3 10 cm-I) and as pressed wax discs (660-40 cm-') at a spectral resolution of A, consistent with the formal OsOs triple bond that should result 1 cm-' with a Bruker 113 V interferometer (for details of disc preparafrom their d5-ds configuration. All such complexes are air stable tions see ref 8). Overlap between mid- and far-infrared regions allowed and, surprisingly, paramagnetic, with p = 2.0-1.6 pugat room matching of band intensities, which are quoted on an arbitrary intensity temperature; this implies either3 that there is an equilibrium scale of vw < 0.02, w 0.02-0.2, m 0.2-0.6, s 0.6-0.9, vs > 0.9; br = broad, between the two possible ground-state configurations, 6*2 and 6 * ~ * sh = shoulder. or, more probably, that the moment represents the expected value The 20 K solid-state transmission spectrum (700-230 nm) of Os2for the 3E, (6* T * ) term split by first-order spin-orbit coupling.6 (O2CCH3),CI2was obtained, as a pressed disc of the complex dispersed As an extension of earlier vibrational studies of d i r h ~ d i u m ~ - ~ in KCl, by use of a Cary 14 spectrometer in conjunction with an Air Products Displex system: band maxima/nm, 246 s, 274 s, and 383 s. and diruthenium tetracarboxylates,1° we have embarked on a systemat,ic vibrational study of the diosmium analogues with the Results and Discussion view, in particular, of assigning the key skeletal modes v(OsOs), Raman Spectra. Unlike dirhodium tetracarboxylates for which v(OsO), and v(OsC1). This is difficult, since all three types of Raman spectra are obtainable with any available excitation line, mode are expected to occur in the 400-2OO-cm-' region. Moreover, diosmium tetracarboxylates yield only poor Raman spectra unless although there may be little coupling between v(0sOs) and vrecorded at or near resonance with a fully allowed transition. The electronic spectrum of Os2(02CCH3),C12as a KCl disc at ca. 20 (1) Bennett, M. J.; Bratton, W. K.; Cotton, F. A.; Robinson, W. R.; Inorg. = 383 nm. ExK is characterized by an intense band at ,,A Chem. 1968, 7 , 1570-1575. citation within the contour of this band, viz. with A,, = 406.7 nm (2) Moore, D . S.; Alves, A. S.; Wilkinson, G . J . Chem. Soc., Chem. (Figure l ) , yields a resonance Raman spectrum which is domiCommun. 1981, 1164-1 165. nated by a very intense band at 229 cm-I denoted vl, by an ( 3 ) Behling, T.; Wilkinson, G.; Stephenson, T. A.; Tocher, D. A,; Walkinshaw, M. D. J . Chem. SOC.,Dalton Trans. 1983, 2109-2116. associated overtone progression (reaching 6vl), and by two com(4) Stephenson,T. A.; Tocher, D. A.; Walkinshaw, M. D. J . Organomet. bination band progressions (reaching 3vl + u2 and 4vl + v3, where Chem. 1982, 232, C51-C54. v2 and v3 are Raman-active fundamentals a t 393 and 292 cm-', (5) Cotton, F. A.; Chakravarty, A. R.; Tocher, D. A,; Stephenson, T. A. Inorg. Chim. Acto 1984, 87, 115-119. respectively). Detailed band listings both for the proteo as well (6) Miskowski, V. M.; Gray, H. B., to be published. (7) Clark, R. J. H.; Hempleman, A. J.; Flint, C. D. J . Am. Chem. SOC. 1986, 108, 518-520. (8) Clark, R. J. H.; Hempleman, A. J. Inorg. Chem., in press. (9) Clark, R. J. H.; Hempleman, A. J. Croar. Chim. Acta, in press. (10) Clark, R. J. H.; Ferris, L. T. H. Inorg. Chem. 1981,20, 2759-2766.
0002-7863/88/1510-5968$01.50/0
( 1 1 ) Fanwick, P. E.; King, M. K.; Tetrick, S. M.; Walton, R. A. J . Am. Chem. SOC.1985, 107, 5009-501 1. ( 1 2) Agaskar, P. A,; Cotton, F. A,; Dunbar, K. R.; Falvello, L.R.; Tetrick, S. M.; Walton, R. A. J . Am. Chem. SOC.1986, 108, 4850-4855.
0 1988 American Chemical Society
J . Am. Chem. SOC.,Vol. 110, No. 18, 1988 5969
IR and RR Studies of O S ~ ( O ~ C C Hand ~ ) Os2(02CCD3)+C12 ~C~~
I kM
I W
imo
Ka Mo Yovenumber/cm-l
un
W
Figure 1. Resonance Raman spectrum (1 525-40 cm-I) of Os2(02CCH3)4C12as a KCI disc at ca. 80 K with 406.7-nm excitation. Resolution ca. 2 cm-I. 25
30
Table I. Wavenumbers (cm-I) of Bands Observed in the Resonance at ca. 80 K Raman Spectrum" of [OS~(O~CCH,)~CI~] t assignment t assignment 161 vw 688 w, br 190 vw 702 vw, br 229 vs w I , ~ ( 0 s - 0 ~ ) 723 vw 237 m, sh 754 vw, br 776 vw, sh 249 w, sh 273 vw 820 vw, br 292 s w,, v(OSCI) 826 VW, sh 849 vw, br 296 m, sh v(0s-CI) 305 VW, sh ~(OS-CI) 917 vw, br 346 vw u(0s-0) 965 vw u(Os-0) 982 vw, br 386 W , sh 390 w, sh u(Os-0) 1048 vw, br 1079 vw, br w2, w(Os-0) 393 m 458 w 2Vl 1115 vw 468 W , sh YI 237 1148 vw, br 479 w, sh uI 249 1214 vw, br vI v, 525 w, br 1348 vw 2w3 592 vw, br 1379 vw V I + Y2 619 w 1460 vw PdCOO) 631 vw 1472 vw " = 406.7-nm excitation.
+ + +
3wl 2wl
+
uI
'
+ 2u3
2u, 4wl
v(C-C)
+ w, p(CH,) 3wl + w 2 w 2 + S(OC0) 3vl
5wl 4w1
/ I I 1
A.406.7 nm
/I
+ v2
II
-t,
i ;
I
I / I I
- - ro~o~c~'cl~1
KCI disc
ca.80 K
U
U a
[ 1 I I I I
I I
I 1 I I ! 1
+ w,
6ACH3) 6Vl
L(CH1) %a(COO)
P,(CO0)/2~, VI
+ w2
Y2
+ w3
2~2 2Vl + w, Y]
+ w2 + +
UII
m
3y1
M
Wovenumber/cm-l
Figure 3. Resonance Raman spectra (410-210 cm-l) of Os2(02CCH3)4C12(-) and Os2(O2CCD,),Cl2 (--) (ul and u, bands truncated) as KCI discs at ca. 80 K with 406.7-nm excitation. Resolution ca. 2 cm-I.
Excitation profiles (EP's) have been constructed for the bands attributed to vl, v2, vj, 2vl, v 1 v2, and v1 vj (Figure 2). These demonstrate that all of the Raman bands reach their maximum intensities within the contour of the 383-nm electronic band, although the exact wavenumber of the EP maxima could not be determined owing to the lack of suitable exciting lines at all desired wavenumbers. Clearly, vl, v2, and vj are all strongly coupled to the 383-nm band, which appears with axial polarization and is .rr*(Osz)transition;6 moreover, all three assigned to the r(C1) modes are likely to be totally symmetric, owing to their involvement in resonance Raman progressions. The assignment of the three bands in the 400-200 cm-l region is difficult owing to the expectation that all three key skeletal stretching fundamentals, viz. v(OsOs), v(OsO),and v(OsCl), could have similar wavenumbers-at least to within a factor of two. Thus it was considered essential to synthesize and study the perdeuterio analogue since this has provided a means for the unambiguous identification of v(Mo0) for Mo2(O2CCH3),,.l3 Assignments of Raman-Active Fundamentals. v(0sOs). The band at 229 cm-', denoted vl, is assigned to v ( 0 s O s ) on the grounds that (a) it is the most likely candidate on the basis of mass considerations; (b) it forms the dominant progression-forming
+
+
-
3u1
a(oc0)
Y,
4C4)/4YI(?) 3wl Y,
UCDd
+
4wl w, ~ACOO) COO)
as for the deuterio forms (vide infra) are given in Tables I and 11, respectively. No Raman spectra were detected at resonance with the weak transitions (e.g., 6 6* at 850 nm)6 originating from the 'EU(6*7r*) ground state.
-
- lO+(OJCH,1i,CIzl
Y,
Table 11. Wavenumbers (cm-I) of Bands Observed in the Resonance Raman Spectrum" of [OS~(O~CCD,)~CI~] at ca. 80 K t assignment t assignment 183 vw 592 vw 223 m, sh 602 vw 230 vs Y I , ~ ( 0 s - 0 ~ ) 651 vw 249 w, sh 672 vw 272 vw 684 vw 280 vw 690 vw 293 s Y,, w(OS-CI) 699 vw w(0s-CI) 296 m, sh 747 VW, sh 302 W, sh ~(OS-CI) 755 vw 327 vw w(0s-0) 827 vw 353 vw, br 899 vw 367 vw 913 vw 370 W , sh ~(0s-0) 981 vw, br 374 w, sh w(Os-0) 1053 vw,br Y2, v(Os-0) 1100 vw 375 m 459 w 2Wl 1140 vw, br 471 w, sh 1210 vw, br 478 W, sh V I + 249 1410 vw 527 w, br v I w, 1467 vw 577 vw = 406.7-nm excitation.
+
+ 249
+
I cm-'
Figure 2. Excitation profiles of uI (A), u2 (W), us (e),2vl (A), V I + v2 ( O ) , and w I v, (0)for OS~(O~CCH,)~CI~ at ca.80 K together with the transmission electronic spectrum as a KCI disc at ca. 20 K.
S(OC0) 2w1
15
20
Wavenumber
(13) Hempleman, A. J.; Clark, R. J. H.; Flint, C. D. Inorg. Chem. 1986, 25,2915-2916. Clark, R. J. H.; Hempleman, A. J.; Kurmoo, M.; J . G e m . SOC.,Dalton Trans. 1988, 973-981.
5970 J . Am. Chem. SOC.,Vol. 110, No. 18, 1988
Clark et al.
Table 111. Vibrational Structural Data for Related Dinuclear Cage Structures u(MM) r(MM) v(MCI) r(MC1) complex (cm-l) (A) (cm-') (A) 305' 2.407* 158" 2.695' [Pt2(P205H2)4C121e 2.44Sd 292c 2.314d O S ~ ( O ~ C C H , ) ~ C I ~ 229c 24Se 2.489/ 295e 2.235' Re2(02CC6H5)4C12 326g 2.577"' 2.287hJ 1598 Ru2(02CCH3)&I "Kurmoo, M.; Clark, R. J. H. Inorg. Chem. 1985, 24, 4420-4425. 'Ch6, C.-M.; Herbstein, F. H.; Schaefer, W. P.; Marsh, R. E.; Gray, H. B. J . A m . Chem. SOC. 1983, 105, 4604-4607. CThis work. dReference 5. eOldham, C.; Davies, J. E. D.; Ketteringham, A. P. J . Chem. SOC.,Chem. Commun. 1971, 572-573. /Reference 1. #Reference 22. Reference 16. ' A more obvious ruthenium analogue would be [Ru2(O2CCH3),CI2]-;however, although structural data exist for this ion (reference 16, r(MM) = 2.286 A,r(MC1) = 2.521 A), no Raman data are yet available.
mode, this feature being a characteristic of the Raman spectra of metal-metal multiply bonded species taken at resonance with electronic transitions involving substantial change of metal character;I4J5 and (c) it suffers no shift (to within 1 cm-l) on deuteriation, as expected for a mode which is virtually orthogonal to the v(Os0) modes (Figure 3). From the wavenumbers of the bands in the ulul progression (uI = vibrational quantum number), it is evident that v 1 is almost exactly harmonic, the anharmonicity constant, x1I , being 0.0 f 0.1 cm-'. Although O S ~ ( O ~ C C His~ insufficiently )~C~~ soluble for measurements of depolarization ratios, both O S ~ ( O ~ C C ~ Hand ~)~CI~ a00 sm 4 a XXI 80 O S ~ ( O ~ C C ~ Hare. ~ ) ~AtC resonance I~ the p values (0.38 and 0.37, respectively) imply not only that v I is polarized and therefore Wovonumbor/cm-l Figure 4. FTIR spectra (660-140 cm-l) of Os2(02CCH3),CI2 and totally symmetric but that the resonant electronic transition must O S ~ ( O ~ C C D ~ as ) ~ wax C I ~discs at ca. 80 K. be axially polarized. v ( O s 0 ) . The band at 393 cm-I, denoted u2, is assigned to structurally related complexes (Table 111). Thus Os2(02CCv(Os0) on the basis that it and it alone (of vl, u2, and u3) is shifted H3)4C12 is similar to the diphosphite ion [Pt2(P20sH2)4C12]4in to lower wavenumber by a substantial amount (18 cm-I) on that v(MM) < v(MCl), where M = metal; the coupling of these deuteriation. The ratio of the v2 values for the deuterio and proteo coordinates has the effect of lowering v(MM) but raising v(MC1) forms (0.95) is comparable with that found for the analogous from their expected wavenumbers as uncoupled oscillators. The modes of M o ~ ( O ~ C C H ~ ) ~ and Rh2(02CCH3)4(PPh3)2 (0.97)13 converse is true for Re2(02CC6H5)4C12 and Ru2(02CCH3),C1 (the (0.96).* Although one might not expect there to be a very close latter contains linear chlorine bridges)l6Pz2for which v(MM) > correlation between u ( M 0 ) and r ( M 0 ) for dimetal tetrau(MC1); coupling of these coordinates in this case has the opposite carboxylates, nevertheless a moderate correlation is apparent: thus, for the series O S ~ ( O ~ C C H ~ R) ~UC~ ~( O ~ ~, C C H ~ Rhz) ~ C ~ , effect, viz. it leads to a raising of u(MM) and a lowering of v(MCI) from their expected wavenumbers as uncoupled oscillators. The (02CCH3)4(PPh3)2,and M o ~ ( O ~ C C Hv(M0) ~ ) ~ , decreases in the expected reciprocal relationship between u(MC1) and r( MC1) is order 393, 372,'O 338,8 and 32313 cm-', respectively, in parallel apparent from the data listed in Table 111; the relationship is less with the increase in r ( M 0 ) in the order 2.009,52.018,162.045,17 clear cut for u(MM) and r(MM), owing to the complication that and 2.1 19 &I8 respectively. much of the metal-metal restoring force has, as its basis, not the v(0sCI). The third intense band (at 292 cm-') in the 400-200 intrinsic metal-metal bond order and thus force constant but the cm-I region, denoted v3, is assigned to v(OsC1) on the grounds four OCO bending force constants. It is also obvious from Table that (a) this band occurs at a wavenumber similar to that found I11 that there is a reciprocal relationship between v(MM) and for v(OsC1) for other octahedrally coordinated osmium complexes u(MC1). containing OsCl bonds, viz. 0s(CO),Cl2 (298, 328 cm-I)I9 and The two main bands attributed to u2 and v3 show evidence of Os(C0)4(HgCl)CI (320 cm-')20 and (b) it is unshifted (to within structure and/or shoulders which may be related to site and/or 1 cm-I) on deuteriation of the complex, as expected for a mode correlation effects (there are two molecules per unit which should be virtually uncoupled from v(CH) modes and as Shoulders on the low wavenumber side of u2 are assigned (Table distinct from v(Os0) modes. I) to other (non-a',) u(Os0) fundamentals on the basis of their Considerable coupling between OsOs and OsCl symmetry cosimilar shifts to v2 on deuteriation. ordinates is expected since the latter are coaxial and share a Other Modes. Other band assignments are included in Tables common atom-a situation2I which is found for many other I and 11. Infrared Spectra v ( O s 0 ) . The infrared spectra (660-140 cm-I) (14) Clark, R. J. H.; Stead, M. J. Inorganic Chemistry: Towards the 21st Cenrury; Chisholm, M . H . , ed.; ACS Symposium Series 211; American of O S ~ ( O ~ C C H ~ )and ~ C IOs2(02CCD3),C12 , as wax discs at ca. Chemical Society: Washington, DC, 1983; pp 235-240. 80 K are shown in Figure 4. Bands for the former a t 403, 395, (15) Clark, R. J. H.; Stead, M. J. Inorg. Chem. 1983, 22, 1214-1220. and 346 cm-I which shift on deuteriation to 387, 379, and 326 (16) Bino, A.; Cotton, F. A.; Felthouse, T. R. Inorg. Chem. 1979, 18, cm-' (average shift 17 cm-') are assigned to v(OsO),cf. the near 2599-2604. ( 1 7 ) Christoph, G . G.; Halpem, J.; Khare, G. P.; Koh, Y . B.; Romanowski, identical shift of the Raman-active v ( O s 0 ) fundamentals (18 cm-', C. Inorg. Chem. 1981, 20, 3029-3037. vide supra) on deuteriation. Only two such fundamentals (a2u (18) Cotton, F. A.; Mester, 2.C.; Webb, T. R. Acta Crystallogr., Sect. + e,) should be infrared active if the molecule could truly be B Struct. Crystallogr. Cryst. Chem. 1974, 30, 2768-2770. approximated to a M2(02CR),L2 species of D4hsymmetry.I3 The (19) Bennett, M. A.; Clark, R. J. H. J . Chem. SOC.1964, 5560-5568. (20) Bradford, C. W.; van Bronswyk, W.; Clark, R. J. H.; Nyholm, R. S.
J . Chem. SOC.A 1968, 2456-2463.
(21) Stein, P.; Dickson, M. K.; Roundhill, D.M. J. Am. Chem. Soc. 1983, 105, 3489-3494.
(22) Miskowski, V. M.; Loehr, T.M.; Gray, H. 8. Inorg. Chem. 1987,26, 1098-1108.
J . Am. Chem. SOC..Vol. 110, No. 18, 1988 5971
IR and RR Studies of O S , ( O ~ C C H ~ )and , C ~0s2(O2CCD3)+Cl2 ~
Table IV. Wavenumbers (cm-I) of Bands Observed in the Infrared SDectrum of IOsdOXCH,IAC1,l at ca. 80 K 3 assignment 3 assignment ~~
3026 vw 1 2938 w, sh )U(C-H) 2989 2932 w I 2406 vw, br 2392 vw, br 1708 vw, br 1674 vw, br 1657vw,br 1648 vw, br 1476 m 1456 vs, sh
1043 + 630 1026+630 1029 618
+
v,,(COO) S,,(CH3)
1423 m 1418 m, sh
v,(COO)
i
1359 m 1349 m S,(CH,) 1345 m 1048 m, sh 1045 m 1043 m, sh p(CH3) 1029 m 1026 m, sh
I
1014 w. sh 618 + 395 952 w, sh "(C-C)
1I
S(OC0)
700s 950w 695 s 635 w 630 vw 618 w 616w 403 w 395 m 346 m 272 vw 261 vw 248 m, sh)
out-of-plane COO)
1 }
in-plane p,(coo)
v(Os-0)
239 m ) 228 vw S(0-Os-0) or OS-Os-0) 190 w 146 vw 129 vw 104 vw
Table V. Wavenumbers (cm-') of Bands Observed in the Infrared Spectrum of [ O S ~ ( O ~ C C D ~at) ca. ~ C80 ~ ~K]
I
assignment
3 Wovanumbar/cm-'
2424 w Figure 5. FTIR spectra (1600-600 cm-I) of O S ~ ( O ~ C C Hand ~ ) ~ C ~ ~ 2241 vw Osz(0zCCD3)4C12as KCI discs at ca. 80 K. 2234 vw 2200 vw 2195 vw observation of three bands implies that the four R groups have 2120 vw lowered the symmetry of the molecule with the consequence that 2109 vw the degeneracy of the e, mode has been resolved, presumably into 1468 m, sh the 403 and 395 cm-I bands in the proteo form and into the 387 1463 s and 379 cm-' bands in the deuterio form. 1459 m, sh v(0sCI). The bands at 247 (with shoulders at 248 and 245 1454 m cm-I) and 239 cm-l in the infrared spectrum of O S ~ ( O ~ C C H ~ ) ~ C ~ 1445 ~ m, sh 1438 m, sh are assigned to u(OsC1). They have weak Raman counterparts 1433 m, sh and are virtually unshifted on deuteriation. Only one asymmetric 1426 s OsCl stretching mode should be infrared active (a2, species) for 1410 vs the isolated molecule, and hence the appearance of more than this 1381 w implies the presence of site and/or factor group effects (possibly 1375 w, sh complicated by chlorine isotopic effects). As is not unusual for 1361 vw vibrations of heavy metal complexes, u,,(OsCl), -243 cm-' < 1320 vw v,,,(OsCl), 292 cm-l, cf. the same situation for the complexes 1319 vw 1311 vw [PtZ(P20SH2)4XZ]e, X = C1, Br, or L2' However for Rez(O2C1304 vw CH3)4C12the infrared- and Raman-active v(ReC1) fundamentals 1095 w are virtually coincident, while for the much lighter and chain 1088 polymeric complex R U ~ ( O ~ C C H ~the )~C situation I is reversed, 1074 w, sh with uaSym(RuC1) being 40 cm-' above v,,(RuC~).~~ 1068 w Other Modes. The infrared spectra of O S ~ ( O ~ C C H and ~ ) ~ C ~ ~ 1033 w
1 }
v(C-D)
)
O S ~ ( O ~ C C D in ~ )the ~ Cregion ~ ~ 1600-600 cm-' (Figure 5) are in general similar to those of related complexes such as Mo2(02CCH3)4and M O ~ ( O ~ C C D ~The ) ~ . OCO '~ bend is clearly observed as a doublet at 700 and 695 cm-I, shifting to 682, 679, and 674 cm-' on deuteriation. The Raman counterparts are a t 723 and 699 cm-' for O S ~ ( O ~ C C H and ~ ) ~Osz(0zCCD3)4C1z, C~~ respectively. Whereas the out-of-plane COO rock, pw, is found a t similar wavenumber (635 and 630 cm-I) to that for M O ~ ( O ~ C C (636 H~)~ and 628 cm-I),l3 the in-plane rock, pr (618 and 616 cm-l) is much higher than that found for M O ~ ( O ~ C C (583 H ~ )and ~ 575 cm-').I3 This is attributed to increased coupling between u(Os0) and p,(COO) as a result of the ca. 30-cm-l increase in v(Os0) compared to v(Mo0). Upon deuteriation pw(COO) shifts by ca. 85 cm-' to 549 and 546 cm-l and lies below p,(COO) which shifts by ca. 20 cm-l to 598 and 593 cm-I. The much larger shift of pw(COO) compared to p,(COO) is consistent with the shifts
i
1018 w
v,(COO) 911 + 549 908 + 546 852 + 593
assignment
3
911 m 908 m[ 899 w 852 w 682 m, sh 679 m 674 m 598 w 593 vw 549 w 546 w 470 vw 467 vw 435 vw 429 vw
P(CD3)
I 1
1
v(C-C)
P(CD3) S(OC0) in-plane p,(COO) out-of-plane p,(COO)
v,(COO) v(0s-0) 682 + 679
6,(CD3)
326 m 317 w, sh 292 vw 237 w 190 vw,br 174 vw 141 vw 133 vw 125 vw 120 vw 101 vw
v(0s-Cl)
observed for the analogous bands for both M o ~ ( O ~ C C D ~and ),'~ Na[02CCD3].z3 There are no strong infrared bands above 1480 cm-l and thus v,(COO) and v,(COO) must lie below this wavenumber; the medium bands at 1476 and 1418 cm-I, respectively, are so assigned, these shifting to 1468 and 1410 cm-', respectively, on deuteriation. The low value for v,,(COO) for this complex, cf. Cu2(OzCCH3)4(H20)z 1605 cm-',% Rh2(OZCCH3)4(PPh3)2 1595 cm-1,8M O ~ ( O ~ C C 1500 H ~ ) cm-',I3 ~ and O S ~ ( O ~ C C H 1476 ~)~C~~ cm-I, is consistent with the relatively long carbon-oxygen distance, (23) Ito, K.; Bernstein, H. J. Can. J . Chem. 1956, 34, 170-178. (24)Mathey, Y.; Greig, D. R.; Shriver, D. F. Inorg. Chem. 1982, 21, 3409-3413.
5972
J. Am. Chem. SOC.1988, 110, 5972-5977
which is 1.260,251.260,l7 1.277,'* and 1.283 A,5respectively, for the above four complexes. Key band assignments are given in Tables IV and V. Conclusion Raman spectra of O S ~ ( O ~ C C H ~taken ) ~ C at ~ , resonance with OS*))^ are the axially polarized 383-nm band (7r(Cl) characterized by long progressions in which the (axial) mode, vir u(0sOs) at 229 cm-' acts as the progression-forming mode. This behavior, which is characteristic of A-term resonance Raman scattering of alg modes,26 implies that the structural change
-
consequent upon excitation to the resonant 7r*(Os2) electronic state is substantial and principally along the Os-Os coordinate. The wavenumber of v1 is only slightly greater than that (-220 cm-1)6 detected as structure to the long wavelength electronic bands of the complex, such as that (6 6*) at ca. 850 nm; clearly, the progression-forming mode in the electronic bands is likewise u(OsOs),although the small ground-state to excited-state wavenumber change for v, in these bands implies that, for such excited states, only very small changes occur to the osmium-osmium bond length.
-
Acknowledgment. We thank Johnson Mathey p.1.c. for the generous loan of platinum metal salts. Registry No. O S ~ ( O ~ C C H ~ ) 8~1519-41-7; CI~, OS,(O~CCD~)~C~~, 115603-76-4.
A Symmetry-Based Procedure for the Determination of Molecular Geometry Changes Following Electronic Excitation. 1. Outline of the Qualitative Method V. Baehler* and 0. E. Polansky Contributionfrom the Max-Planck-Institut fur Strahlenchemie, Stiftstrasse 34-36, 0-4330 Mulheim a.d. Ruhr, Federal Republic of Germany. Received June 29, 1987
Abstract: An extension of the Bader-Pearson concept to nondegenerate first excited states is presented which permits a qualitative determination of the relaxation pathway along which the molecular geometry changes following electronic excitation. A symmetry criterion for the relaxation pathway has been developed, according to which the most favored path is that which permits coupling of the first excited state with a large number of higher excited states close to the first excited state. Excited-state geometries of low symmetry are treated in a novel way by considering the pathway symmetry as a superposition of two different irreducible representations; numerical quantities are suggested for estimating the likelihood of particular representations to occur in the superposition. After determining the relevant symmetry species, the pathway is specified in greater detail by inspection of plots of the overlap function between interating orbitals, and a procedure for using such plots is described.
1. Introduction Photochemical and spectroscopic properties of electronically excited molecules are frequently rationalized by referring to the form of the excited state potential energy surface. A photochemical reaction starts along that direction on the excited-state surface which leads to the smallest energy increase or even an energy minimum. Provided the pathway for an energy decrease is identical with a normal mode, a long vibrational progression in that mode may be observed in highly resolved electronic spectra. Large scale configuration interaction methods proved to be reliable for the calculation of excited state potential energy surfaces.' The computational effort, however, would be diminished if a qualitative procedure were available predicting directions on the surface which are energetically favorable. Such a scheme would also be of use for the interpretation of large scale ab initio results, where a qualitative explanation of reliable results is often obscured by a maze of numbers. For ground-state surfaces the Bader-Pearson concept2+ relates explicitly the symmetry of the total wave function and the symmetry of reaction pathways. The essence of this concept is that the direction of a particular nuclear motion is energetically favorable if the electronic charge density
can follow the motion easily. This simple concept led to a comprehensive rationalization of ground-state ~ h e m i s t r y . ~It has recently been incorporated by Bader into his theory of chemical reactivity which is based on a spatial partitioning of the electronic charge d e n ~ i t y . ~ Few attempts have been made to apply the Bader-Pearson concept to electronically excited-state surfaces. A general rule for predicting the geometry of electronically excited molecules has been derived: but for conjugated molecules it had to be e ~ t e n d e d . Devaquet ~ generalized the Bader-Pearson concept to first excited states* and applied it to a variety of electronically excited molecule^.*^^ However, in several instances it seemed difficult to discriminate between alternative directions for relaxation pathways. Nakajima analyzed carbon-carbon bond length changes of conjugated molecules following electronic excitation.' The bond length changes are correctly predicted, but molecular oribtals for a-electrons are not explicitly considered. This restriction limits the procedure to nuclear motions in which planarity is retained. Bond length changes following electronic excitation can also be deduced from the AP-matrix as suggested by Zimmerman.'O This matrix is defined as the difference be-
(1) For a review, see: Hirst, D. M. Adu. Chem. Phys. 1982, 50, 517. Bruna, P. J.; Peyerimhoff, S. D. Adu. Chem. Phys. 1987, 67, 1. (2) Bader, R. F. W. Can. J . Chem. 1%2,40, 1164. Bader, R. F. W. Mol. Phys. 1960, 3, 137.
(5) 6788. (6) (7) 5610. 1022.
( 3 ) Pearson, R. G. Symmetry Rules for Chemical Reactions; Wiley: New York, 1976. (4) Pearson, R. G. J. Mol. Struct., Theochem. 1983, 103, 25.
0002-7863/88/1510-5972$01.50/0
Bader, R. F. W.; MacDougall, P. J. J. Am. Chem. SOC.1985, 107, Pearson, R. G. Chem. Phys. Lett. 1971, 10, 31. Nakajima, T.; Toyota, A,; Kataoka, M. J. Am. Chem. Soc. 1982,104, Nakajima, T.; Toyota, A.; Fujii, S. Bull. Chem. SOC.Jpn. 1972, 45,
(8) Devaquet, A. J. Am. Chem. SOC.1972, 94, 5626. (9) Devaquet, A. J. Am. Chem. SOC.1972, 94, 9012.
0 1988 American Chemical Society
