J. Phys. Chem. 1986, 90, 3927-3930
3927
Spectroscopic Study of the Parent and Reduction Products of Some Substituted Bipyridine Complexes of Iron( II)and Osmium( II). 2. Substitution at the 4,4' Positions R. J. Donohoe,+ C. D. Tait, M. K. DeArmond, and D. W. Wertz* Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 (Received: January 13, 1986; In Final Form: March 14, 1986)
The spectroscopic results for some tris(4,4'-substituted bipyridine) complexes of iron and osmium indicate that these systems depart from "model" behavior and provide insight into the metal-ligand interaction. The presence of low-lying ?r* orbitals in the 4,4'-systems is established and related to many of the unusual results for these substituted complexes. These low-lying x* levels complicate both the electronic and vibrational spectra by leading to charge-transfer transitions and back-bonding interactions that are absent in the 5 3 ' - and unsubstituted complexes.
Introduction The R R studies of the redox series of the 4,4'- and 5,5'-diphenyl substituted ruthenium complexes as well as the six stable reduction products of the 4,4'- and 5,5'-diester substituted species yielded contrasting absorption and resonance Raman (RR) behavior. Whereas the only extension from the 'model" behavior of the unsubstituted bpy complexes1 needed in the interpretation of the 5,5'-substituted ruthenium? iron, and osmium' compounds was some degree of back-bonding, the behavior of the 4,4'-substituted complexes appears to be more complicated. To extend the investigation of back-bonding in these 4,4'-complexes, the R R and UV-vis spectra of the iron and osmium analogues have been obtained and are presented here. Experimental Section Acetonitrile and D M F were purified by previously described proced~res.4.~The supporting electrolyte (tetrabutylammonium hexafluorophosphateTBAH) was prepared by metathesis of the chloride salt (Kodak), recrystallized from absolute ethanol 3 times, and dried in a vacuum oven. Synthesis. Ligands. The 4,4'-diester ligand was prepared by a previously published procedure6 and recrystallized twice from absolute ethanol. The 4,4'-diphenyl ligand was purchased from Alfa Products and recrystallized twice from benzene. Complexes. Both iron complexes were prepared by the procedure used by Elliot and Hershenhart in the synthesis of the iron 4,4'-diester complex.6 Attempts to purify these complexes by column chromatography on Sephadex LH-20 in both acetone and acetonitrile were unsuccessful, with the presence of free ligand clearly indicated. Overall, the best results were obtained by simple rotary evaporation/recrystallizationof the iron complexes from a mixture of acetonitrile and tert-butyl alcohol. The osmium complexes were prepared by the same method used to prepare the 5,5'-complexes described in the previous paper.3 Cyclic voltammetry (Table I) and luminescence results for the osmium 4,4'-diester (a dark green solid) indicated a highly purified product. The cyclic voltammetry of the diphenyl product (a brown solid) indicated a small amount of impurity (not ligand) that further recrystallization and chromatography did not remove. The spectroscopic results indicate that this small amount of impurity is not significant for these measurements. Procedure. The experimental procedures for obtaining the absorption spectra and the resonance Raman spectra have been described in the previous paper.3 Although not as unstable as the 5,5'-complexes, the iron 4,4'-complexes also decomposed in D M F and were therefore studied in acetonitrile. Attempts to study the osmium complexes in acetonitrile were hampered by the low solubility of some of the redox products in that solvent. Thus, R a n t address: Department of Chemistry, Carnegie-Mellon University,
Pittsburgh, PA 15213.
0022-3654/86/2090-3927!§01.50/0
TABLE I: El12'lSaValues of Substituted Iron, Ruthenium, and Osmium Complexes (V vs. r/s Fe4COOEt' Ru4COOEt" Os4COOEt -0.89 -0.79 -0.70 21 1 -1.06 -0.93 -0.84 110 01-1 -11-2 -21-3 -31-4
rls
211 110 01-1
-1.27' -1.71'
Fe4PhCJ -1.28 -1.45 -1.66
-11-2
-21-3
-1.13
-1.11+
-1.58 -1.80 -2.12' Ru4Ph -1.15
-1.53 -1.82 -2.07e Os4Ph
-1.31
-1.25 -1.55' -2.03
-1.55' -2.08 -2.29'
-1.08
"Scan rate = 100 mV/s. bThe last stable reduction product in the series is indicated by a dagger. 'Values obtained in CH,CN, all others in DMF. dSee also ref 6. 'Irreversible wave. fNo stable reduction products in CH,CN. it was necessary to study the substituted iron and osmium complexes in different solvents. The maximum number of redox electrons that could be added to the complexes is indicated in Table I, along with the reduction EII2values.
Results Absorption Spectra. Effect of Variation of n. The M4COOEt2-" and M4Ph"" 'I complexes show variation in their absorption spectra as n changes (Figures 1 and 2). The absorption maxima and approximate extinction coefficients for the spectra of the redox series of the iron and osmium 4,4'-diester substituted complexes as well as those for the n = 0 ... 3 products of the analogous ruthenium complex are presented in Table 11. The spectra of Figures 1 and 2, along with those of the Ru4COOEt2* redox series? display a band near 32 X lo3 cm-' that, like the (1) Heath, G. A,; Yellowlees, L. J.; Braterman, P. S . J . Chem. Soc., Commun. 1981,281. (2) Donohoe, R. J.; Tait, C. D.; DeArmond, M. K.; Wertz, D. W. Spectrochim. Acta, Part A 1986,42A, 233. (3) Donohoe, R. J.; Tait, C. D.; DeArmond, M. K.; Wertz, D. W. J . Phys. Chem., preceding paper in this issue. (4) Angel, S . M.; DeArmond, M. K.;Donohoc, R. J.; Wertz, D. W.J. Phys. Chem. 1985,89,282. (5) Ohsawa, Y.;Hanck, K. W.; DeArmond, M. K. J. Electroanal. Chem.
1984,175,229. ( 6 ) Elliot, C. M.; Hershenhart, E. J . Am. Chem. Soc. 1982, 104, 7519. (7) Consistent with previous notation, the complexes of interest will be
referred to in the following manner: the initial symbol will be the atomic symbol of the central metal atom followed by a number designating the position of the substituent (e.g., 4 = 4.4'-) followed by a code for the s u b stituent (COOEt = diester, Ph = diphenyl). Thus, the osmium 4,4'-diester complex will be written as Os4COOEtZ-"(where n is the number of redox electrons) and the iron 4,4'-diphenyl complex is labeled Fe4Ph". M will be used to designate a general metal ctnter (Fe, Ru, Os).
0 1986 American Chemical Society
3928
Donohoe et al.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 7o
60
t
$2-n Fe(4,4'(COOEt), bpy),
In
30
20
10
kK Figure 1. Absorption spectra of Fe(4,4'(COOEt)2bpy)3z-n;n = 0, 1, 2, 3 in CH3CN.
70
-
60
-
50
-
TABLE I 1 Absorption Maxima, Approximate Extinction Coefficients, and Assignments for Bands of Redox Series of M(4,4'(COOEt),bpy)B*-n(M = Fe, Ru, Os; n = 0, 1, 2, 3)
n Fe(4,4'(COOEt)2bpy)32-n 0 [31.5 (65000), 32.7 (sh)] = IL [18.4 (16000), 20 (sh), 25.6 (1 2 OOO)] = MLCT 1 [6.S (4000), 8.0 (2SOO), 18.6 (llOOO), 20.1 (SOOO), 26.4 (lSOOO), 27.9 (sh)] = IL- [31.3 (40000)] = IL [23.0 (12000), 16.2 (lOOOO)] = MLCT 2 16.0 (9000), 7.0 (7000), 18.5 (lSOOO), 19.8 (17000), 26.9 (32000)l = IL- [31.2 (23000)] = IL [21.3 (sh), 14.5 (9000)l = MLCT 3 [5.2 (14000), 7.0 (7000), 18.2 (19000), 19.6 (19000), 26.7 (46000), 35.5 (3SOOO)] = IL- [21 (sh), 13.5 (9200)l = MLCT n Ru(4,4'(COOEt),b~y)~~-" 0 [32.1 (72000)l = IL [21.3 (24000), 22.5 (20000), 28.2 (19000)] = MLCT 1 [6.6 (SSOO), 8.0 (7100), 18.7 (21000), 19.7 (22000)l = IL- [31.7 (6SOOO)] = IL [26.7 (28000), 21.1 (19000)] = MLCT 2 [6.4 (lOOOO), 7.6 (8400), 18.5 (19000), 19.8 (18000), 28.7 (35000)l = IL- 131.7 (43000)] = IL [23.3 (22000)l = MLCT 3 [6.3 (14000), 18.4 (sh), 19.6 (sh), 28.2 (50000)] = IL- [21.S (26000)l = MLCT n Os(4.4'(COOEt~,buvl12-" 0 [32.0 (64000)l = IL [14.3 (4700), 14.9 (SZOO), 16.4 (S4OO)l = 'MLCT [20.0 (19000), 22.0 (26000), 27.3 (17000)] = IMLCT 1 [7.0 (3000), 29.3 (sh)] = IL- [31.7 (S2000)] = IL [13.1 (4600), 20.4 (23 000), 26.4 (21 OOO)] = MLCT 2 [7.0 (8600), 29 (sh)] = IL- [31.3 (38000)l = IL [12.0 (4300), 19.7 (22000), 23.5 (22000)l = MLCT 3 [7.1 (14000). 29.8 (39000)l = IL- 18.2 (sh), 21.8 (28000)l = MLCT
"Maxima reported in IO' cm-l and extinction coefficients in M-' cm-'. IL indicates unreduced ligand transition; IL-, singly reduced ligand transition. MLCT indicates metal-to-ligand charge-transfer.
E X 10-3
lo3cm-I in the spectrum of Fe4COOEt2+,a band found at 25.6 lo3cm-' appears to red-shift upon reduction to 23.0 X lo3 cm-I. Any intensity change as n is increased is difficult to gauge due to overlapping of this band with other transitions, but it appears that the intensity remains roughly unchanged. Effect of Variation of M . The absorption spectra of both the unreduced and reduced 4,4'-complexes show a marked dependence on the metal center. A comparison of the spectra of the n = 0 species (see Figures 1 and 2) indicates that, in addition to the MLCT band near 20 X lo3 cm-', another feature shows a significant variation in its absorption maximum as M is varied. This is the same peak that is observed to red-shift from 25.6 to 23.0 X lo3 cm-I with the addition of the first electron to Fe4COOEt2+. The Os4COOEt2+analogue is found at 27.3 X lo3 cm-I and shifts to 26.4 X lo3 cm-' upon addition of the first electron. In the ligand localized model, the n = 3 product contains three singly reduced ligands. Thus, the n = 3 absorption spectra might be expected to contain only IL- bands which should not vary significantly as M is changed. This is not the case in the spectra of Fe4COOEt- (Figure 1, n = 3) or Os4COOEt- (Figure 2, n = 3), where comparison of the visible regions indicates that the relative intensity and peak maxima differ. The absorption features at 18.2 and 19.6 X lo3 cm-I in the spectrum of Fe4COOEt- are not apparent in the spectrum of Os4COOEt-. An examination of the spectrum of Elliot and Hershenhad for Ru4COOEt- reveals that the ruthenium complex represents an intermediate case in which the two bands are visible but are obscured by a broader feature that is comparable in energy and intensity to the 21.8 X lo3 cm-I band, which dominates the visible portion of Os4COOEt-. The 22 X lo3 cm-' shoulder in the spectrum of Fe4COOEt- may be the analogue of this feature. While the visible portions of the n = 3 spectra differ due to the changing intensities of several unresolved transitions, the other regions differ as M is varied even though the observed maxima appear to be well-resolved IL- transition^.^-^ As an example, the strong band in the UV spectrum of Fe4COOEt- is found at 26.7 X
X
10 kK Figure 2. Absorption spectra of 0~(4,4'(COOEt)~bpy),~-"; n = 0, 1, 2, 3 in DMF.
20
30
M5COEt2-" and R ~ ( b p y ) ~ redox ~ - " series, shows the monotonic reduction in intensity that is characteristic of localized systems. This band is assigned to an intraligand (IL) ?r P* transition. Some of the absorption features in the spectra of the redox series of the 4,4'-complexes are similar to those in the 5,5'-complexes3 as relatively constant energies and monotonic intensity increases are observed for n = 1, 2, 3. This type of behavior is exemplified by the bands between 5 and 10 X lo3 cm-I in Figures 1 and 2 (and in the spectra of R U ~ C O O E ~ ~and - " ~by) the band found near 27 X lo3 cm-] in Figure 1. Due to the similarity of this behavior to that of the IL- ?r* P* bands in the 5,5'-complexes~ these bands are also assigned as IL- bands. However, this assignment may be somewhat oversimplified (vide infra). Two other IL- transitions are observed a t approximately 18 and 20 X lo3 cm-I in Figure 1. In contrast to the 5,5'-complexes, the 4,4'-complexes exhibit a shifting of more than one peak as n is varied. In addition to the metal-to-ligand charge-transfer (MLCT) band located at 18.4
-
-
(8) Donohoe, R. J.; Ph.D. Thesis, North Carolina State University, Raleigh, NC, 1985.
4,4'-Substituted Complexes of Fe(I1) and Os(I1)
II JI
L
1000
1200
1400
1688
c m-1
Figure 3. Resonance Raman spectra of Fe(4,4'(COOEt)zbpy)32-", n= 0, 1, from 950 to 1700 cm-' in CH,CN obtained with listed exciting lines. X = solvent mode. X lo3 cm-I, whereas the osmium analogue is at 29.8 X lo3 cm-'. Similarly, the maximum in the near-IR in the Fe4COOEtspectrum is found at 5.2 X lo3 cm-' but at 7.1 X lo3 cm-l in the case of OsCOOEt-. In contrast, the analogue of the IL- transition found at 25.4 X lo3 cm-' in the spectrum of FeSCOOEt- is found a t 25.8 X lo3 cm-' in the osmium c ~ m p l e x . ~ RR Spectra (Effects of Variation of n and M). The R R spectrum of Fe4COOEt+ obtained with 16.1 X lo3cm-I excitation is similar to the spectrum of the parent (n = 0) obtained with 18.9 X lo3 cm-I excitation, Le., probing the MLCT (Figure 3). The reduced ligand vibrations are weakly enhanced compared to the unreduced ligand vibrations. This behavior is also observed in the ruthenium2 and osmium 4,4'-products but to a greater extents9 Thus, the shifts in unreduced ligand vibrational energies are more easily monitored. The magnitude of the vibrational shifts to lower energy is similar for the iron and ruthenium 4,4'-diester comple~es.~ Although the R R peaks associated with a reduced ligand are relatively weak, they can be monitored as n is varied in two of the 4,4'-complexes: Fe4COOEt2" and Os4Ph2-". Surprisingly, the peaks observed in the R R spectra of the Fe4COOEt2-" complex obtained near 20 X lo3 cm-I appear to shift through the n = 1 ... 3 redox series even though the transitions being probed are assigned as IL- bands and the Raman lines therefore due to the reduced ligand. A similar result is obtained for the Os4Ph2" series: a reduced ligand vibration found at 1476 cm-l in the n = 1 product appears to shift to 1470 cm-I in the n = 2 product and to 1464 cm-l in the n = 3 product. This is the first time such a shift has been observed for reduced ligand vibrations.
Discussion The results obtained for the 4,4'-complexes can be best understood by using the 53' results3 as model behavior for localized systems and then indicating areas of difference between the 4,4'and 5,5'-complexes. The 4,4'-substituted complexes show greater variation in their absorption and R R spectra upon reduction than the 5,5'-complexes, but, unlike the 5,5'-complexes, some of the features in the spectra of the reduced products are not easily categorized as intraligand transitions arising either from unreduced, singly reduced, or doubly reduced chromophores. The shifting absorption features and changing vibrational energies of both unreduced and reduced ligands might be taken to indicate that the redox orbitals of the 4,4'-complexes are delocalized and that the description of these systems in terms of reduced and (9) Several rationales for this behavior have precedent in other systems. Experimental causes have been ruled out since strong DMF signals are observed. Antiresonance is also an unlikely explanation since the same weak signal is observed despite the use of several exciting lines. Weak RR signals might be taken to imply that the ground- and excited-state geometries are similar, but the absorption features are fairly broad and indicate significantly displaced excited states. Perhaps the most likely explanation is that some competitive nonradiative p r m is reducing the RR signal. See: Gouterman, M.; Aranowitz, S.;Adar, F. J . Phys. Chem. 1976, 80, 2184.
The Journal of Physical Chemistry, Vol. 90, No. 17, I986 3929 unreduced chromophores is incorrect. However, the monotonic decrease in the UV absorption at 32 X lo3cm-I in the redox series of these 4,4'-complexes is very similar to the behavior of the 5,5'and unsubstituted complexes and is strongly indicative of localized redox orbitals. Also supportive of the localized description is the presence of two distinct sets of vibrational frequencies, which, despite slight shifts as n is changed, suggest two vibrational chromophores. Absorption Spectra. The unusual electronic spectra of the 4,4'-complexes can, for the most part, be rationalized within the localized model as a result of the presence of ligand K* orbitals lying only slightly higher in energy than the redox orbital. The existence of these additional low-lying orbitals is suggested by the low-temperature CV of Ru4COOEt2" lo which exhibits 10 oneelectron waves, four more than the 5,5'- or unsubstituted complexes under comparable conditions, and is corroborated by M O calculations." The presence of an absorption band near 6 X lo3 cm-' in the spectra of the reduced 4,4'-complexes is also suggestive of such orbitals. This feature may consist of two separate transitions as can be most clearly seen in Figure 1, although the higher energy feature may be a vibronic side band. The presence of addition ?r* orbitals leads to the observation of more than one MLCT band in the spectra of the unreduced species. A band near 27 X lo3 cm-' in the spectra of two 4,4'complexes of ruthenium was tentatively assigned as a second MLCT.2 The reason for that assignment was the apparent redshift of the band upon reduction. This red-shift is characteristic of the MLCT transition near 20 X lo3 cm-'. Upon examination of the absorption spectra of the iron and osmium 4,4'-complexes, the assignment of this higher energy band to an additional MLCT transition becomes more certain. As noted in the Results, the absorption maximum of this band shows a marked metal dependence. The assignment of this band to a ligand-to-metal charge-transfer (LMCT) is not consistent with the observed red-shift upon reduction. The electrostatic destabilization of the metal orbitals by the redox electrons4 leads to the prediction of a blue-shift for an LMCT band upon reduction. Finally, the energy difference between the MLCT feature near 20 X lo3 cm-I and the higher energy MLCT band is similar to the energy of the IL- band observed in the near-IR which involves a transition between the redox orbital and the optical orbital of the higher energy MLCT transition. In addition to the MLCT band near 27 X lo3cm-', two MLCT features in the visible spectrum of Os4COOEt2+ (Figure 2) occur at 20.0 and 22.0 X lo3 cm-I (and in Os4Ph2+at 19.5 and 21.8 X lo3cm-I). Since the energy-level separation (2000 cm-I) does not match a vibrational frequency, the higher band is not a vibronic side band and separate electronic transitions must be assignable to each absorption. The absorption spectrum of Os(bpy)?+ also has two maxima in this region, which have been assigned in theoretical treatmentsI3 to MLCT's into different states involving the same orbital. The maxima at 20.0 and 22.0 X lo3 cm-' in the osmium 4,4'-diester complex may also have the same orbital origin rather than involving transitions to different K* orbitals. Furthermore, MO calculations1° do not support MLCT absorption into three separate r* orbitals, as one of the three low-lying r* levels is predicted to be antisymmetric with respect to the ligand C, axis, while charge transfer is, in theory, only allowed into orbitals symmetric with respect to this axis.14 The presence of additional MLCT bands raises some interesting questions concerning these transitions upon reduction. While the lowest energy MLCT entails excitation into the redox orbital, the (10) Ohsawa, Y.; DeArmond, M. K.; Hanck, K. W.; Morris, D. E.; Whitten, D. G.; Neveux, P. E., Jr. J . Am. Chem. SOC.1983, 105, 6522. (11) Ohsawa, Y.; Whangbo, M.-H.; Hanck, K. W.; DeArmond, M. K. Inorg. Chem. 1984, 23, 3426. (12) Angel, S. M.; DeArmond, M. K.; Donohoe, R. J.; Hanck, K. W.; Wertz, D. W. J. Am. Chem. SOC.1984, 106, 3688. (13) (a) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (b) Felix, F.; Ferguson, J.; Gudel, H . U.; Ludi, A. J . Am. Chem. SOC.1980, 102, 4096. (14) VanQuickenborn, L. G.; Beulemans, A. J . Am. Chem. Soc. 1981,103, 2238.
3930 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 highest energy MLCT band involves excitation into an orbital other than the redox orbital and may be present in the spectra of the n = 3 products. The identity of this MLCT transition in the n = 3 product would be different from that in the n = 0 product since a charge transfer in the n = 3 product involves promotion of an electron from a nominally +2 center onto a negatively charged ligand. The intermediate (n = 1,2) reduction products would involve charge transfer into analogous orbitals on both reduced and unreduced ligands. These two types of charge transfer are not resolved and both red-shift upon reduction. With Figure 1 as a reference, the higher energy MLCT band red-shifts upon reduction from 25.6 ( n = 0) to 23.0 ( n = 1) to 21.3 X lo3 cm-’ ( n = 2) while the lower MLCT band shifts from 18.4 to 16.2 to 14.5 X lo3c m - I . Note that although both bands appear to redshift by similar amounts, only the lower energy MLCT band loses intensity. The higher energy MLCT bands at 25.6 X lo3 cm-I for n = 0 and at 23.0 X lo3 cm-’ for n = 1 have the same extinction coefficients while the lower MLCT band at 16.2 X lo3 cm-’ (n = 1) is much less intense than the 18.4 X lo3 cm-’ ( n = 0) band. This indicates that the number of chromophores for the 25.6 (n = 0) and 23.0 X lo3 cm-l (n = 1) MLCT transitions is the same even though, in the localized description, one of the n = 1 chromophores is now reduced. Therefore, although there are two distinct types of chromophores in the n = 1 (and n = 2) system(s), the transition energy for promotion of a metal electron into one of the A* orbitals lying above the redox orbital is the same for both types of chromophores (Le., whether or not the redox orbital is occupied). The correlation of the higher energy MLCT band at 27.3 X lo3 cm-‘ in the spectrum of Os4COOEt2+ with its red-shifted counterparts at 26.4 (n = 1) and 23.5 X lo3 cm-l (n = 2) can be continued for the 21.8 X 103 c m - I (n = 3) band, the band that dominates the visible portion of the n = 3 spectrum. Regardless of the correlation of this feature with a band observed in the spectra of the n = 0, 1, and 2 products, it does appear that no analogue of this 21.8 X lo3 cm-’ band is present at the same energy in the spectra of the n = 1 or n = 2 products, typical of MLCT bands.” The two IL- transitions at approximately 18.5 and 20.0 X lo3 cm-’ in the n = 1 ... 3 spectra in Figure 1 (also ref 8) overlap with this MLCT transition in the triply reduced complexes. The surprisingly weak RR spectra observed in the reduced 4,4’-complexes arise when probing this composite feature. The complicated appearance of the absorption spectra of the 4,4’-complexes is therefore largely a result of the presence of more than one MLCT transition, which undergo extensive shifting upon reduction and are responsible for many of the differences between the absorption spectra of the 53’ and 4,4‘ redox series. However, there are other differences that cannot be attributed to the additional MLCT transitions. Even if there are two MLCT bands in the lower energy feature (-20 X lo3 cm-’) of the 4,4’-complexes, it is clear that the individual MLCT transitions carry more intensity in the 4,4’-complexes than in the 5,S-counterparts. Conversely, the IL- bands are much more intense in the $5’products. Perhaps the most significant distinction between the absorption spectra of the 4,4’- and 5,5’-substituted species is the observed dependence on M of the maxima of the IL- bands of the 4,4’-products. As detailed in Results, the maxima of the bands in the vis/near-UV and the near-IR follow the ordering &-(Os) > E,,,(Ru) > E,,,(Fe). Since these transitions originate from the redox orbital, the dependence of these maxima on M may indicate some interaction between the metal and redox orbitals. RR Spectra. The unreduced 4,4’-complexes display a definite shifting of RR frequencies as M is varied. Moreover, the reduced
Donohoe et al. ligand frequencies and the unreduced chromophore vibrational energies are dependent upon charge. This dependence of the reduced ligand RR spectra on n for the 4,4’-complexes suggests some degree of interaction between the redox centers. However, the other experimental evidence which supports the localized description of the redox orbitals in these complexes makes it necessary to attempt to account for the reduced ligand frequency shifts within the localized model. One explanation is that the low-lying unoccupied A* orbitals back-bond with the destabilized metal orbitals even if the redox orbital is occupied. That is, the reduced ligands may also help stabilize the metal orbitals. As a consequence, the reduced ligand has increased its antibonding character, even though the redox orbital remains largely isolated from the metal. The frequency shifts observed in the vibrational spectra of the unreduced ligands in the singly reduced complexes of osmium and iron complexes are similar. Since Os is considered a better T back-bonder than Fe, these results seem to be inconsistent with back-bonding. A review of the proposed back-bonding scheme, however, shows that the driving force behind the back-bonding interaction is the destabilization of the metal orbitals. With the use of the unsubstituted complexes (where no unreduced ligand frequency shifts are observed) as a measure of the extent of this destabilization, it is clear that the iron systems exhibit much greater perturbation of the metal orbitals than the ruthenium and osmium follows the trend Fe >> Ru > Os, while the ability to back-bond follows the reverse trend. These two properties may offset each other and result in approximately similar degrees of back-bonding in the reduced complexes.
Summary and Conclusions The 4,4’-substituted bipyridine complexes exhibit metal dependence in their absorption spectra for a given reduction product. Since the UV absorption spectra for these complexes indicate that the redox orbitals are single ring localized, an attempt has been made to rationalize the metal dependence within the localized model. Most of the spectral differences can be explained as a result of the presence of low-lying T* orbitals. These A* orbitals result in additional MLCT transitions in the visible and may explain the shift of reduced ligand vibrations-a result that has not been observed in the RR studies of the unsubstituted or 5,5’-substituted complexes. The 4,4’-complexes exhibit the interaction between the metal ion and unreduced ligands that is observed in the 5,S-cornpiexes but also display behavior indicative of interaction between the metal and reduced ligand orbitals. It is not necessarily true that the redox orbitals are comprised of significant metal character since the interactions between the reduced ligands and the metal center may be solely via back-bonding with unoccupied low-lying s* orbitals. Acknowledgment. We are grateful to Dr. David Bocian for helpful discussions. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. Registry No. Fe4COOEtz+,83605-66-7; FdCOOEt’, 83605-66-7; Fe4COOEt, 83605-67-8; Fe4COOEt-, 83605-68-9; Fe4COOEt2-, 83605-69-0; Os4COOEt2+, 97567-34-5; Os4COOEt+, 102869-32-9; Os4COOEt. 102869-33-0; Os4COOEt-, 102869-34-1; Os4C0OEt2-, 102869-35-2;Os4COOEtS-, 102869-36-3; Os4COOEte, 102869-37-4; FdPh”, 52747-07-6; Fe4Ph+, 71619-81-3; FulPh, 71619-82-4; FdPh-, 71619-83-5; Ru4Ph2+, 67416-18-6; Ru4Ph+, 93462-14-7; Ru4Ph, 93462-22-7; Ru4Ph-, 93461-95-1; Ru4Ph2-, 93462-30-7; Ru4Ph3-, 93461-51-9; Os4Ph2+, 99639-79-9; Os4Ph+, 102869-38-5; Os4Ph, 102869-39-6; 0 ~ 4 P h - ,102869-40-9;Os4PhZ-, 102869-41-0.
