J . Phys. Chem. 1986, 90, 5567-5570 reported values a t 30 OC are p A M = 3.90 D (Bats and Hobbs") and P A M = 3.92 D (Kumler18). Using the averages of the values of PAM together with those of p, we calculated the corresponding values of 6 and then, with the above-mentioned valence angles, the resulting 0 shown in Table 11.
Conclusions The average of 6 for the four compounds here studied is 114 f 2O, in good agreement with the value of 116 f 4 O found for the diamides previously studied.l The angle obtained for Nethyl-p-chlorobenzamide is somewhat lower than those of the rest (16) Purcell, W. P.; Singer, J. A. J . Phys. Chem. 1967, 71, 4316. (17) Bates, W. W.; Hobbs, M. E. J. Am. Chem. SOC.1951, 73, 2151. (18) Kumler, W. D. J. Am. Chem. SOC.1952, 74, 261.
5567
of amides. It is noteworthly to point out that Flory et aL2 found a similar difference for para-halogenated benzoate esters whose structure is comparable to N-ethyl-p-chlorobenzamidewith substitution of the ester by amide groups. They concluded that if this difference was real it should be attributed to possible mesomeric effects peculiar to compounds of this kind. The comparison of the values obtained for the angle 6 for N-ethyl-pchlorobenzamide (109.8O) and for N,"-dimethyl-terephthalamide' (120°), in which the chlorine atom has been substituted by a second amide group whose mesomeric effects should roughly compensate those of the first group, seems to corroborate this qualitative idea. Registry No. 4-BrC6H,NHCOCH3,103-88-8; 4-CIC,H,NHCOCH3, 539-03-7; 4-CIC6H4N(CH2CH3)COCH3, 74283-46-8; succinimide, 123-56-8.
Transient Spectroscopy of the Lowest Exclted States of Binuclear Rhodium(I ) I socyanldes Steven J. Milder,* David S. Kliger, Department of Chemistry, University of California, Santa Cruz, California 95064
Leslie G. Butler, Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803
and Harry B. Gray Arthur Amos Noyes Laboratory,t California Institute of Technology, Pasadena, California 91 125 (Received: February 12, 1986)
The binuclear complexes Rh2b(+ (b = 1,3-diisocyanopropane)and Rh2(TMB)2+ (TMB = 2,5-dimethyl-2,5-diisocyanohexane) both exhibit aljsorptionfrom their lowest excited singlet state (lAh) and their lowest triplet state (3Ah). The excited-triplet-state absorption spectra are similar for the two complexes, with strong bands at 420 and 470 nm in Rh2b:+ and at 400 and 490 nm in Rh2(TMB)2+. The two bands are oppositely polarized (x-y, 420 and 400 nm; z, 470 and 490 nm) and are assigned to d r pa (le,, 2alg) and da da* ( l a l g la2,,) transitions, respectively. Both complexes have weak, structureless absorptions in the red (A > 600 nm) that are attributed to pa d6 (2alg d,2-,,2) transitions. The excited singlet spectrum of each complex exhibits a strong band in the blue (450 nm, Rh2b:+; 440 nm, Rh2(TMB)42+)that is primarily z-polarized in Rh2(TMB)42+and is assigned to da* pa (lazu 2al,).
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Introduction Excited-state spectra of mononuclear transition-metal complexes have been investigated by several re~earchers-l-~ Binuclear complexes also have received much attenti~n,"~particularly d8-d8 lAZuand species, where the lowest energy transitions are IAl, 3A2,,(d$da*2 d$da*pa).15,1620.21Because these d8-d8 IA,, transitions involve promotion of an electron from da* to a bonding p orbital, the metal-metal bond strengthens and the metal-metal distance contracts in the lowest excited states.'5*20~zz~23 Here we report the excited-state spectra of two binuclear d8-d8 complexes, Rh2b42+ ( b = 1,3-diisocyanopropane) and Rhz(TMB)42+(TMB = 2,5-dimethyl-2,5-diisocyanohexane).We use the ground-state absorption spectra of these complexes and related d7-d7,d7-d8,and ds-dspl species as guides in making assignments.
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Experimental Section
Materials. [Rh2b4][BPh4]$4 and [Rh,(TMB),] [CF3S0,]$5 were prepared by published procedures. Propionitrile and isopentane were Aldrich reagent grade and acetonitrile was Burdick and Jackson spectrograde. 2-Methyltetrahydrofuran (2MeTHF) and 2-propanol were Aldrich reagent grade and were distilled +Contribution No. 7352 from the California Institute of Technology.
0022-3654/86/2090-5567$01.50/0
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before use. All solvents were dried with 4A molecular sieves. Solutions of the rhodium complexes were prepared for study at (1) Bensasson, R.; Salet, C.; Balzani, V. J. Am. Chem. Soc. 1976,98, 3722. (2) Pyke, S. C.; Ogasawara, M.; Kevan, L.; Endicott, J. F. J . Phys. Chem. 1978, 82, 302. (3) Maestri, M.; Bolletta, F.; Moggi, L.; Balzani, V.; Henry, M. S.; Hoffman, M. Z . J. Am. Chem. SOC.1978, 100, 2694. (4) Viaene, L.; D'Olieslager, A.; Ceulemans, A.; Vanquickenborne, L. G. J. Am. Chem. SOC.1979, 101, 1405. (5) Serpone, N.; Jamieson, M. A,; Henry, M. S.; M. Z. Hoffman, M. Z.; Bolletta, F.; Maestri, M. J . Am. Chem. SOC.1979, 101, 2907. (6) Fleming, R. H.; Geoffroy, G. L.; Gray, H. B.; Gupta, A.; Hammond, G. S.; Kliger, D. S.; Miskowski, V. M. J. Am. Chem. SOC.1976, 98, 48. (7) Miskowski, V. M.; Nobinger, G. L.; Kliger, D. S.; Hammond, G. S.; Lewis, N. S.;Mann, K. R.; Gray, H. B. J . Am. Chem. SOC.1978, 100,488. (8) Miskowski, V. M.; Twarowski, A. J.; Fleming, R. H.; Hammond, G. S.; Kliger, D. S. Inorg. Chem. 1978, 17, 1056. (9) Miskowski, V. M.; Goldbeck, R. A.; Kliger, D. S.; Gray, H. B. Inorg. Chem. 1979, 18, 86. (10) Che, C.-M.; Butler, L. G.; Gray, H. B. J . Am. Chem. SOC.1980,103, 7796. (11) Milder, S.J.; Goldbeck, R. A,; Kliger, D. S.;Gray, H. B. J. Am. Chem. SOC.1980. 102. 6761. (12) Miskowski, V.'M.; Smith, T. P.; Loehr, T. M.; Gray, H. B. J. Am. Chem. SOC.1985, 107, 7925. (13) Levenson, R. A,; Gray, H. B. J . Am. Chem. SOC.1975, 97, 6042.
0 1986 American Chemical Society
5568 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
room temperature by dissolving the solid complex in acetonitrile and diluting the solution until an appropriate optical density was obtained. The solutions were deoxygenated by bubbling Ar through them for 15 min. For studies of Rh2b4’+ and Rh2(TMB)42+ at 77 K, samples were dissolved in a minimum of propionitrile and then diluted with 2MeTHF to give a solvent composition (v/v) of approximately 1:15. For some studies of Rh,(TMB),*+ at 77 K, the complex was dissolved in 2-propanol and diluted with isopentane to give a solvent composition (v/v) of approximately 1:l. Equipment. Ground-state absorption spectra were taken on either a Cary 219 or an IBM 9420 recording spectrophotometer. Excited-state spectra were recorded at both U.C. Santa Cruz and Caltech. The apparatus at Santa Cruz has been described previously.26 It consists of a frequency-doubled Quanta Ray Nd:YAG laser (A = 532 nm, r = 8 ns) synchronized with an EG&G short-arc pulsed xenon lamp (1000 V, 2 gF, 4 gs). The laser was fired at the time of maximum intensity of the probe pulse. The beams crossed at about 15’. The probe pulse was directed into a 0.45-m Pacific Precision Instruments monochromator and was detected by an EM1 D279 photomultiplier tube. The photomultiplier output was digitized by a Biomation 6500 transient waveform recorder and transferred to a Z-80-based microcomputer for signal averaging and data analysis.2’ The apparatus at Caltech also consisted of a Quanta Ray Nd:YAG laser synchronized with a pulsed probe beam. The probe beam source was either a xenon flash lamp or, for probing long-duration transients, a shuttered tungsten lamp. The probe beam was focused onto a 0.35-m McPherson monochromator and was detected with a Hamamatsu R928 photomultiplier. The anode current of the photomultiplier was amplified with either a highspeed LeCroy VV 101ATB amplifier or was dropped across a load resistance (500 ohms, 5 kohms, or 50 kohms) and amplified with a Tektronix P6201 FET probe. With either amplification method, the photomultiplier anode current was limited to no more than 0.1 mA. Digitization of the signal was carried out by a Biomation 6500, and the data were then transferred to a PDP- 1 1 /03 for signal averaging and analysis. Data Analysis. The excited singlet (s-s) and triplet (t-t) transient absorption spectra were determined point by point by manually varying the monochromator and successively collecting data at each wavelength. Polarization measurements were made by placing a sheet polarizer in front of the monochromator slit. The transient signals were obtained with the transmitted light polarization both parallel and perpendicular to the polarization axis of the exciting laser pulse. For the unpolarized spectra the laser energy was usually between 5 and 10 mJ/pulse and the beam diameter was approximately 0.3 cm. For determination of the polarized spectra the laser energy was kept below 0.4 mJ/pulse and the beam diameter was approximately 0.5 cm. While this low fluence decreases the size of the transients, it was necessary to ensure minimum absorption of the laser by molecules with transition moments oriented off axis to the laser polarization. Analysis of the data for each complex at each temperature and (14) 303 1. (15) (16) (17)
Trogler, W. C.; Solomon, E. I.; Gray, H. B. Inorg. Chem. 1977, 16,
Rice, S. F.; Gray, H. B. J . Am. Chem. SOC.1981, 103, 1593. Rice, S. F.; Gray, H. B. J . Am. Chem. SOC.1983, 105, 4571. Milder, S. J. Inorg. Chem. 1975, 24, 3376. (18) Milder, S. J.; Kliger, D. S. J . Phys. Chem. 1985, 89, 4170. (19) Marshall, J. L.: Stobart, S . R.; Gray, H. B. J . Am. Chem. SOC.1984, 106, 3027. (20) Mann, K. R.; Lewis, N. S.; Williams, R. M.; Gray, H. B.; Gordon, J. G., I1 Inorg. Chem. 1978, 17, 828. (21) Mann, K. R.; Gray, H. B. Adu. Chem. Ser. 1979, 173, 225. (22) Dallinger, R. F.; Miskowski, V. M.; Gray, H. B.; Woodruff, W. H. J . Am. Chem. Soc. 1981. 103, 1595. (23) Fordyce, W. A,; Brummer, J. G.: Crosby, G. A. J . Am. Chem. Soc. 1981, 103, 7061. (24) Lewis, N. S.; Mann, K. R.; Gordon, J. G., 11. J . Am. Chem. SOC. 1976, 98, 146. (25) Sigal, I. S.; Gray, H. B. J . Am. Chem. SOC.1981, 103, 2220. (26) Milder, S. J.; Kliger, D. S. J. Am. Chem. SOC.1985, 107, 7365. (27) Horwitz. J. S. Ph.D. Dissertation, University of California, Santa Cruz. 1983
Milder et al. 9
0 D
8
0 0
200
7 (ns) Figure 1. Transient optical density vs. time data for R h , ( T M B ) p in propionitrile/ZMeTHF (1:15) at 77 K. Each trace represents the average of 16 shots. Each data point represents one 2 4 s data bin. (a) 440 nm; the fast rise and decay show the strong s-s absorption that decays to the long-lived t-t absorption. (b) 490 nm; the absorption rises to a metastable value nearly coincident with the laser pulse, indicating that the ss component is much smaller than the large t-t absorption a t this wavelength.
wavelength varied depending on whether the lifeime of the lowest triplet excited state was less than or more than 1 pus. When the triplet lived longer than 1 ps, the optical density change was taken to be the average value for the 25 data points corresponding to probe times 250-300 ns after the laser pulse. For Rh2b2+at 295 K and both of the complexes at 77 K, the triplet lifetimes are significantly longer than 1 p s and this procedure was used. However, for Rh2(TMB)42+at 295 K the triplet lifetime is less than 100 ns and this procedure could not be used. Instead, the t-t spectrum was found by determining the maximum optical density change obtained directly after the laser pulse. For Rh2(TMB),’+ this caused distortion in the t-t spectrum due to the overlap of the s-s spectrum. For the two Rh(1) complexes, the transient absorption spectrum of the lowest singlet could also be obtained. This was done by using a modified version of the method of Kliger and Albre~ht.~’,’~To use this method it is necessary that the triplet state live much longer than the singlet state. Thus, for Rh2(TMB)42+we determined the s-s spectrum at 77 K, where the t-t absorption lasts significantly longer than the s-s absorption.
Results We will discuss the spectroscopy of both Rh2b4’+ and Rh2(TMB),*+ assuming Dlh ~ymmetry.’~Figure 1 shows examples of the data obtained for Rh,(TMB),*+ at 77 K in propionitrile/ 2MeTHF. At 440 nm, shown in Figure la, both the s-s and t-t absorptions are intense. The s s absorption rises and falls quickly, nearly in coincidence with t h e profile of the laser pulse. This is consistent with the 1.8-11s lifetime of the lAzu state of Rh,(TMB),*+ at 77 K.)O The t-t absorption appears directly after the decay of the s-s absorption and does not decay on the time scale of the figure (300 ns). Figure l b shows the time profile of the transient absorption taken at 490 nm. At this wavelength there is no s-s absorption, while there is strong t-t absorption. Figure 2a shows the 77 K transient absorption spectra of Rh2(TMB)42+in propionitrile/2MeTHF. In the t-t spectrum there are strong bands with maxima at 400 and 490 nm and a (28) Kliger, D. S.; Albrecht, A. C. J . Chem. Phys. 1970, 53, 4059 (29) Mann, K. R.; Thich, J. A.; Bell, R. A,; Coyle, C. L.; Gray, H. B. Inorg. Chem. 1980, 19, 2462. (30) Milder, S. J., unpublished results.
Excited States of Binuclear Rhodium(1) Isocyanides 0.6
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5569
r
1.0 r a
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450 Wavelength (nm)
Figure 2. (a) Transient absorption spectra of the lowest excited singlet, lAzu( O ) , and the lowest triplet, 3A2u(B), obtained upon excitation of Rh2(TMB)42+ in propionitrile/ZMeTHF(1:lS) at 77 K. Excitation was
at 532 nm with 5-mJ, 7-ns pulses. (b) Polarized transient absorption spectrum of the lowest triplet (3A2u)of Rh2(TMB):+ taken with the probe beam polarized parallel ( 0 ) and perpendicular (A) to the laser polarization. Excitation was at 532 nm with 0.3-mJ, 7-ns pulses. The complex was dissolved in a 2-propanol/isopentane (1:l) glass at 77 K. weak, broad band in the red. Attempts to obtain analogous spectra a t room temperature always gave inconclusive results for Rh2(TMB)42+due to its short triplet lifetime at 295 K.I7q3' The transient spectrum of Rh2(TMB)42+at 295 K, taken as the maximum optical density change obtained during the laser pulse, was essentially a sum of the t-t and s-s spectra obtained at 77 K. Figure 2b shows the analogous polarized t-t spectra taken at 77 K in 2-propanol/isopentane with the laser power at 0.3 mJ/ pulse. No broad band in the red was seen here because the absorption was too weak to observe at the laser energy used. As shown, the transition centered at 490 nm is primarily polarized parallel to the laser polarization, as is the ground-state bleaching. The observed polarization ratios for the two transitions are approximately 2:1, smaller than the value of 3:l expected theoreti ~ r ~ l l yPrevious .~~ work on Rh2b42+has shown that the visible spectrum is dominated by an intense z-polarized transition.Is Thus, the 490-nm transient absorption is also primarily z - p ~ l a r i z e d . ~ ~ The lower than theoretically predicted polarization ratios are probably due to experimental factors. These could include imperfect polarization of the laser or the probe beam, partial off-axis absorption of the laser due to high light fluxes, and the fact that the exciting and probe beams are 15' from collinear. Similar arguments lead to the conclusion that the 400-nm t-t absorption band is primarily x-y-polarized. Because of the small signal, it was not possible to obtain a reproducible polarized s s spectrum. However, the s-s absorption at 440 nm was found to be z-polarized. Transient spectra were taken for Rh2b42+in CH3CN at 295 K and are shown in Figure 3a. In this complex the 3A2ulifetime is 8.5 1.1s at 295 K" and it is simple to differentiate s-s and t-t spectra. (We previously reported the t-t spectrum in the 350(31) Rice, S. F.; Milder, S . J.; Gray, H. B.; Goldbeck, R. A,; Kliger, D. S . Coord. Chem. Rev. 1982, 43, 349. (32) Albrecht, A. C. J . Mol. Spectrosc. 1961, 6, 84. (33) The lowest triplet state (3A2u)is split in zero-field (the spin-orbit components are E,, and A,"). However, since the spin-orbit coupling is not large in the 'A2, level, the selection rules for the t-t transitions are fixed by the orbital symmetry.
I
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650
I 850
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Figure 3. (a) Transient absorption spectra of the lowest singlet, (O), and the lowest triplet, 3A2u(B), excited states of Rh2b:+ in acetonitrile at 295 K. Excitation was at 532 nm with 5-mJ, 7 4 s pulses. (b) Polarized transient absorption spectrum of the lowest triplet (3A2u)of Rh2b:+ taken with the probe beam polarization parallel ( 0 )and perpendicular (A)to the exciting laser polarization. Excitation was at 532
nm with 0.4-mJ, 7-ns pulses. The complex was dissolved in a propionitrile/ZMeTHF (1:15) glass at 77 K.
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TABLE I: Electronic Swctral Data for Rh,La"+ ComDlexes A,., nm complex L = b L = TMB assignment Rh2L4+ 740 740 2al, dx2-y2, pu a 570 la2u 2a1,, do* 430 =400 lelu 2al,, dn Rh2L:+ 553 515 lazu 2al,, du* 318 313 le, 2al,, dn Rh2L2+ 700 a a 440 450 lai, la2,, d u Rh2L4C1? 420 410 le, la2,, dn* 335 333 lal, la2,, d u Rh2L:+*(3A2,) =750 =750 2a,, dxxy2,pu 470 490 lal, la2u,du 420 400 lelu 2ai,, dn Rh2L42+*(iA2u) 450 440 lazu 2a1,, du*
-- --- -- --- ---- --- --
d6 PO
pu pa pu do* da* do* db du* pu pu
'Not assigned. 650-nm r e g i ~ n . ~In) the t-t spectrum there is a weak, broad, and apparently unstructured absorption in the red that is similar to the red t-t absorption of Rh2(TMB)2+ (Figure 2a). For Rh2b42+, a s s absorption is also observed with a maximum at 450 nm. This feature is analogous to the s-s absorption of Rh2(TMB)42+,although it is somewhat weaker. The low relative strength may be due in part to the shorter lifetime (7 = 1.3 ns) of the singlet probed in Rh2b42+at 295 K. We also have obtained the polarized t-t spectrum of Rh2b42+ between 350 and 600 nm (Figure 3b). The sample was dissolved in propionitrile/2MeTHF at 77 K and was probed with a laser energy of 0.4 mJ/pulse. As shown, the ground-state bleaching centered at 550 nm and the transient absorption centered at 470 nm are polarized parallel to the laser polarization. Thus, these transitions are z-polarized. The transient absorption centered at 420 nm is weakly polarized perpendicular to the laser polarization, and is thus x-y polarized. Here, the tail of the stronger, z-polarized absorption centered a t 470 nm overlaps this band. The absorption due to the s-s transition was so weak under the experimental conditions that reproducible polarization information could not be obtained.
5570 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 202~ (PO')
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Milder et al.
The da* pa transition is the prominent feature in the visible spectrum of the ds-d8 rhodium complexes (its symmetry desig'Azu). The visible spectrum of Rhzbd2+has a nation is 'A,, maximum a t 553 nm (6 = 14500 in CH,CN), while that of Rh2(TMB)t+has a maximum at 515 nm (e = 11 700 in CH,CN). Both complexes exhibit an intense band at approximately 315 nm ]E, ( d a pa). that has been assigned to lA,, The other oxidation state that has been reported is the reduced ds-d8p1 species, Rhz+, which has been produced by flash photolysis1'and pulse radiolysi~.~~ The spectrum shows weak absorption in the red with a maximum a t approximately 740 nm for both Rh2b,+ and Rhz(TMB)4+complexes. For Rh2(TMB)4+there also is an intense absorption at approximately 570 nm,38along with a moderately strong band between 350 and 450 nm." Two intense absorptions are also seen in the spectrum of Rhzb4+,peaking at 520 and 430 nm." The weak, red absorption in the Rhz+ comda* transition, since the da* plexes cannot be due to a dn* orbital is occupied, but it might be a transition to a d,2+ level (pa d6): The band at approximately 570 nm in Rhz(TMB)4+ has been assigned as the da* p a (ZAzu zA,,, lazu 2al,) transiti~n,,~ analogous to the 553- and 515-nm absorptions in the ground-state spectra of Rhzb4z+and Rh2(TMB)4z+.It is likely that in Rh2b4+there is a transition of the same origin at similar energy. It was not observed because of competing strong absorbance in the flash photolysis experiment." The blue absorption 2azu),because the pa* orbital probably is not pa pa* (2al, is too energetic to be populated in this region. Thus, we assign 2al,) the moderately strong blue system to the d r pu (le, transition. The 520-nm band in Rhzb4+remains unassigned. The assignments of the s-s and t-t excited-state spectra can be made by analogy to the various ground-state spectra discussed above. The s-s spectra show one strong, sharp, z-polarized absorption, which appears at 440 nm in Rh2(TMB)4z+.It is reasonable to consider three assignments for the observed s-s absorption: d a da* (lal, lazu),da* pa (lazu 2al,), and 2al,). Of these three, we can eliminate d r d r p a (le, pa, because it does not give the observed z polarization. The two remaining transitions should be strong and z-polarized, and recent work has shown that the one that is consistent with the observed pa.39 transition intensity is da* Of the two strong peaks in the t-t spectrum, one is x-y-polarized while the other is z-polarized (Figures 2b and 3b). The z-polarized transition could be either d a da* or pa pa*. As noted above, the pa pa* transition is not expected to fall in the energy range that we have examined, thereby indicating that the z-polarized da*. The x,y-polarized transition (420 and transition is d a 400 nm in Rhzb42+and Rh2(TMB)42+,respectively) is most likely 2al,). The position of the absorption is similar d r pa (le, to that of the analogous d a pa band in the ground-state spectra of Rhz+. The broad t-t absorption in the red (600-800 nm) region is reminiscent of the pa dh (2al, dX2+) band in Rhzb4+and it is assigned accordingly.
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(PO
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R h (CNR):
Rh*b:+
Figure 4. Energy level diagram for de and de-d8 complexes in D4hsymmetry. Levels derived from dx2-y2orbitals (expected to be near p,) are not shown.
Discussion In analyzing the excited-state spectra of the R h p species, it is useful first to discuss the ground-state spectra of binuclear rhodium complexes in various oxidation states. Table I summarizes the absorption spectra and assignments of the spectra of RhzL4Clz2+,RhZL4"+( n = 1, 2, 3), and Rh2L4z+*(L = b, TMB) complexes. An energy level diagram for these binuclear metal complexes is shown in Figure 4. Binuclear rhodium(I1) isocyanides exhibit intense absorptions near 300 nm that have been assigned to d a da* transition^.'^*^^ A weak, broad, featureless band to the red of the d o da* band in the spectra of these species has been assigned to d r * da* ?AIg lEU). The d7-ds binuclear rhodium isocyanides can be produced transiently by flash p h o t ~ l y s i s . " * ~The ~ - ~spectrum ~ of Rh2b43+ shows two features, a strong, sharp absorption at 440 nm and a weaker, broad system centered at 700 nm. By analogy to the binuclear Rh(I1) isocyanides, we assign the strong absorption to d a da* (zAzu 2Al,). It is not surprising that the d a da* transition in the Rh(I1/&dimer is red shifted relative to its position in the Rh(I1) species, because in the two-electron oxidized state the metal-metal bond is shorterZ9q3'and s t r ~ n g e r . ' ~ *Thus, ~ * in the one-electron oxidized species we expect that the energy difference between the lal, and lazuorbitals will be less and the d o da* transition will be lower in energy. It is surprising that there are no other strong absorption bands between 800 and 320 nm in the transient difference spectra of 'A,,) Rhzb43+and Rh2(TMB),3+. Thus, the da* pa ('Az, transition, which should be intense, may be obscured in the flash photolysis experiments by the bleaching of the analogous Rhzz+ absorption. The 700-nm band cannot be the do* pa transition, as the observed intensity is too low.
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(34) Lewis, N. S.; Mann, K. R.; Gordon, J. G., 11; Gray, H. B. J . Am. Chem. SOC.1976, 98, 7461. (35) Milder, S . J. Ph.D. Dissertation, University of California, Santa Cruz, 19-18. (36) Miskowski, V. M.; Sigal, I. S.; Mann, K. R.; Gray, H. B.; Milder, S. J.; Hammond, G. S.; Ryason, P. R. J . Am. Chem. SOC.1979, 101, 4383. (37) Mann, K. R.; Bell, R. A,; Gray, H. B. Inorg. Chem. 1979, 18, 2671.
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Acknowledgment. We thank Dr. James Horwitz for help in development of the data collection and analysis programs used at Santa Cruz, Dr. Robert Goldbeck for helpful discussions, and National Science Foundation Grants PMC83-17044 (S.J.M. and D.S.K.) and CHE84-19828 (H.B.G.) for support of this work. Registry No. Rh2b?+, 61 156-14-7; Rh2(TMB)?+, 73367-41-6. (38) Che, C.-M.;Atherton, S. J.; Butler, L. G.; Gray, H. B. J . Am. Chem. SOC.1984, 106, 5143.
(39) Winkler, J. R.; Marshall, J. L.; Netzel, T. L.; Gray, H. B. J . Am. Chem. SOC.1986, 108, 2263.
