J. Phys. Chem. 1993,97, 565-568
565
Wavelength Dependent Photochemical Decarbonylation of H*u(CO)1, To Form Coordinatively Unsaturated H~Ru(C0)12in Solid Matrices at 77 K A Model for the Photoreaction Postulated To Form HzR~(C0)12on the Surface of Silica Sadaaki Yamamoto,'lt Kiyotaka Asakura,* Kudo MacbidaJ Atsuhiko Nitta,? and Haruo KIM&# Central Research Institute, Mitsui Toatsu Chemicals Inc., 1190 Kasama-cho, Sakae-ku, Yokohama 247, Japan, Department of Chemistry, Faculty of Science, Gakushuuin University, Mejiro, Toshima-ku. Tokyo 171, Japan, and Department of Chemistry, Faculty of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku. Tokyo 113, Japan Received: June 30, 1992; In Final Form: October 6. 1992
The photochemical reaction of dihydrotetraruthenium tridecacarbonyl, HzRu4(CO)l3, was investigated by UV-visible and l T - I R absorption spectroscopies at 77 K in both a 3-methylpentane matrix and a polystyrene film. Upon irradiation at 313 nm, HzRu((CO)13 undergoes clean C O dissociation yielding selectively a single product. One CO is released from each parent carbonyl cluster that reacts. Upon warming to 298 K,the efficient recombination of CO trapped in nearby sites with the photoproduct completely regenerates the parent cluster HzRu(CO)l,. From these and other results, the photoproduct is identified as the coordinatively unsaturated cluster H Z R U ~ ( C O ) ~The Z . photoinduced C O dissociation is dependent on the excitation wavelength, which can be interpreted in terms of the electronic structure of the corresponding excited states. At 298 K, UV-visible light irradiation does not produce any changes in the absorption spectra irrespective of the excitation wavelength. This can be attributed to geminate recombination in which released CO efficiently recombines with H~Ru((C0)12. The UV-visible and I R spectra of matrix isolated H ~ R u ~ ( C Oagree ) ~ Z with those of the photoproduct formed when HzRy(C0) 13adsorbed on the surface of silica is irradiated. This spectral agreement allows the photoproduct on silica to be assigned as HzRuq(CO)lz, which supports our previous postulate about the photochemical process occurring when HzRu4(C0)13 absorbed on the surface of silica is irradiated.
Thermal treatments have traditionally been used in the preparation of catalytically active metal carbonyl specieson oxide supports.' As an alternative, a photochemical approach is expected to have several advantages over traditional thermal treatment in the preparation of such surface species because of selective bond excitation. Motivated by this expectation,we have been investigating the photochemical transformation of polynuclear metal carbonyls to catalytically active species on oxide supports.~+ Recently we investigated the photochemical reaction of H2Rud(C0) 13 adsorbed on silica and found that UV-visible light irradiationcauses H~Rw(C0)13toundergoclean CO dissociation to yield a single surface subcarbonyl specie~.~ On the basis of IR, UV-visible, and EXAFS spectroscopic analyses, as well as chemical analysis of the evolved gas, we concluded that the coordinatively unsaturated cluster H ~ R U ~ ( Cwas O ) produced ~~ on the surface of silica. We ascribed the selective formation and stabilization of the unstable coordinatively unsaturated H2Ru4(C0)12 to the efficient release of dissociated CO and interaction of the photoproduct with the surface hydroxy groups of silica. However, the structure of the photoproduct has not been determined beyond doubt to be H2Ry(C0)12 because it has not yet been isolated in a stable crystalline form nor has its IR and UV-visible spectra yet been reported. For confirmation of our hypothesis that the unsaturated cluster is formed on the surface of silica, the spectroscopic characterization and elucidation of the chemical reactivity of the coordinatively unsaturated H2Rw(C0)lz are required. The matrix isolation technique is a well-established method for determining the structure of unstable intermediates. In this technique, an unstable intermediate is formed by photolysis of t Mitsui Toatsu Chemicals Inc. t The
University of Tokyo. Gakushuuin University.
0022-3654158/ 2097-0565$04DO / O
a parent molecule isolated in a soli matrix. Severalcoordinatively unsaturated metal carbonyl species have been trapped and characterized by this technique using frozen rare gases4 and hydrocarbons5 as well as polymer films6 as the solid matrix. Therefore, it is expected that this technique will allow the coordinatively unsaturated HzRw(C0)12 to be isolated and characterized by UV-visible and IR spectroscopy. We conducted this study on the photochemical reaction of H ~ R U ~ ( C by O )using ~ ~ a 3-methylpentane matrix and a polystyrene film at 77 K. In this study, we observed for the first time the UV-visible and FT-IR absorption spectra of coordinatively unsaturated HzRu4(C0)12 and found that its formation from H2Ru((CO)13is temperature and excitation wavelength dependent. ExperiwnW Section
The metal carbonyl cluster H2Rw(CO)l, was prepared according to the method reported in the literature.' The 3-methylpentane(Cica-reagent, Kanto Chemical) was dried over CaClz and then distilled from Na metal and LiAI& powder prior to use. Benzene (Uvasol, Merck) was used without further purification. The carbonyl cluster was dissolved in 3-methylpentane. The concentration was limited to about 0.15 mM to avoid aggregation of cluster molecules when the samplewascooled to 77 K. The solution was deoxygenated by bubbling helium through it for about 30 min. The solution was then loaded into an IR-cell with sapphire windows (2.0-mm path length) or a quartz cell (10 mm X 10 mm) under an argon atmosphere in a drybox (DRI-LAB equipped with HE493 DRI-TRAIN, Vacuum Atmospheres Co.). Often 3-methylpentane has been used as a solvent for matrix isolation of coordinativelyunsaturated carbonyl clusters. However, 3-methylpentane has an IR absorption band at 2132 cm-1 that overlaps with the weak stretching absorption arising from trapped CO. Since the intensity of the 3-methylpentane absorption increases drastically with decreasing temperature, it is difficult to observe the weak absorption resulting (8
1993 American Chemical Society
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Yamamoto et al.
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
from the dissociated CO trapped in the matrix. As an alternative system, we used a polystyrene film as the solid matrix for isolating the carbonyl cluster. Polystyrene films are transparent in the UV-visible region above 300 nm, and they have no IR absorption bands at 2132 cm-I. Moreover, it is possible to prepare homogeneous films with high concentrations of the carbonyl cluster that do not suffer from aggregation of the molecules when cooled. This makes it possible to more precisely observethe weak IR absorption band arising from dissociated CO trapped in the matrix. Polystyrene films of the metal carbonyl were prepared by mixing a benzene solution (4 mL) of the metal carbonyl with a benzene solution (4 mL) of polystyrene (20 wt Q TOPOREX 555-57, MW = 29 300, Mitsui Toatsu Chemicals, Inc.) The solution was then transferred to a glass plate, and the benzene was allowed to evaporate in an atmosphere at room temperature for about 1 day. The partially dried films were then transferred to an evacuable vessel, and the remaining benzene was removed by evacuation (1 X Torr) at room temperature. The film thickness was about 200 bm. Irradiation was done by mounting the sample in a cryostat (DN1704, Oxford) and irradiating the sample (in a helium atmosphere) with a ultrahigh pressure Hg lamp (BLM-SOOD, 500 W,WACOM) through one of three different filter combinations that transmitted light at 3 13,366, and 405-578 nm. The light passing through the samples was reflected back through the samples to increase the efficiency. The filter combination providing the 3 13-nm light was made with aqueous solutions of K2Cr04 (0.27 g/L) and Na2COp (1 g/L)" and a glass filter (UVD33S, Toshiba). The 366-nmlight was obtained by using a water filter in combination with a two glass filters (UV-D33S and UV35, Toshiba). The 405-578-nm light was obtained by using an aqueous solution of CuSO4 (transmittance at 366 nm = 0.82, optical path length = 5 cm)" and a cutoff filter (L39, Toshiba). The optical path length of the quartz cell for the aqueous filter solutions was 5 cm. The relative quantum yield for CO dissociationwas determined under irradiation conditions where the excitation photons are completely absorbed by the sample. Here the IR absorbance changeof the product at 2100cm-1,A(2100cm-1), with timecan be expressed by eq 1. Here &(A) is the quantum yield for CO
dissociation, Zo(A) is the incident light intensity at the respective irradiation wavelength A, and t is the time. From the s l o p of eq 1 obtained at excitation wavelengths A and A', the relative value Zo(A) &(A)/Zo(A') &(A') can be obtained. The relative intensities of the light ZO(A')/ZO(A) passing through the filters are determined by using a rhodamine B quantum counter? Combining Zo(A) &(A)/Zo(A') #,(A') with ZO(A')/ZO(A), the relative yield, &(A)/&(A'), can be obtained. The UV-visible spectra were recorded on a Shimadzu UV2200 spectrophotometer, and the IR spectra were recorded on a JASCO FT/IR-8300 FT-IR spectrometer. Results 1. Irrndiatioo with 313-nm Light at 77 I(. Figure 1 shows the UV-visible absorption spectral change of H2Ru4(C0)13 in a 3-methytpentane matrix upon 313-nm irradiation at 77 K. The same spectral change was observed when H2Ru4(C0)13 in polystyrene was irradiated with 313-nm light. The UV-visible spectra of H ~ R % ( C O )at I ~77 K in either the 3-methylpentane matrix or the polystyrene film contain a series of broad absorption bands at 310,345,410, and 540 nm. These bands are sharper and slightly blue shift compared with those obtained from the samples at 298 K. Since this temperature dependence is known to becharacteristicof absorptionsassociated with metal-centered u-u* t r a n s i t i o n ~ , ~these ~ J ~ bands . ~ can be assumed to be due to
0
200
400 600 Wave1e ngt h ( nm )
800
Figure 1. UV-visible absorption spectral changes accompanying 3 13(1.0 ) I ~X 10-4 M)in a 3-methylpentane nm irradiation of H ~ R U ~ ( C O matrix at 77 K. Irradiation time (min): (- -) 0; (--) 10; (- -) 30; (-)
-
60.
mo.0
2100 .o
1000.0
LWO .o
1000 .o
Iavbnu8bbr
Figure 2. (a, top) FT-IR absorption spectral changes accompanying 313-nmirradiationofH2Ru4CO)13(1.1X 1 P M ) ina 3-methylpentane matrix at 77 K. Irradiation time (min): (1) 0; (2) 30; (3) 60; (4) 90. (b, bottom) FT-IR difference spectrum obtained by subtracting spectrum 1 from spectrum 4 shown in Figure 2a. u-us transitions." Upon 3 13-nm irradiation, the absorption spectra change and show clear isosbestic points. The absorption bands become broader, and their intensity decreases. In addition, warming the samples to 298 K leads to disappearance of the weak and broad band at 600 nm and complete regeneration of the UV-visible absorption spectrum of H2R%(CO)13. Figures 2 and 3 show the FT-IR absorption spectral changes of H2Ru,(C0)13 at 77 K in 3-methylpentane and a polystyrene film, respectively. The FT-IR absorption spectrum of the polystyrene film sample and its spectral changes are essentially the same as those observed for the cluster in the 3-methylpentane matrix. There were some differences in the intensity ratio of the absorption peaks of the terminal CO group. This difference can be attributed to solvent effects because the spectrum of H2Ru4(C0)13 in benzene coincides with that in the polystyrene film. The FT-IR absorption spectrum of the 3-methylpentane matrix containing H*Ru4(C0)13 exhibits bands associated with
Coordinatively Unsaturated H2Ru4(C0)12in Solid Matrices I
0 .a000 2150.0
I
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 561
I
8
P100.0
2000.0
1000.0
1m.o
Figure 3. FT-IR absorption spectral changes accompanying 313-nm irradiation of HzRw(C0)13 (1.1 X 1@ M)in a polystyrene film at 77 K. Irradiation time (min): (1) 0; (2) 5; (3) 20; (4) 40. FT-IR absorption spectra (insert) of H~Rw(C0)13adsorbed on silica (- -) and the photoproductfrom HzRU*(C0)13adsorbed on silica (-) are also shown.3
terminal CO groups at 2081 (s), 2066 (s), 2055 (s), 2031 (w), and 2025 (m, br) cm-I and an additional band at 1862 (w) cm-I that is associated with the CO bridging two Ru atoms. Upon 3 13-nm irradiation the FT-IR absorption spectrum changes, showing clear isosbestic points. The intensities of the peaks at 208 1 and 2055 cm-I decrease more drastically than the intensity of the peak at 2066 cm-I. The weak absorption peak at 2132 cm-l, which is characteristic for CO frozen in a 3-methylpentane matrix, can be observed clearly in the difference spectrum obtained by subtracting the spectrum of the initial carbonyl cluster from that of the photoproduct, as shown in Figure 2b. The total amount of released CO was estimated to be the same as the amount of H2Ru4(C0)13 by using the absorbance of 0.07 measured at 2132 cm-l and the value of the extinction coefficient e (350 M-' ~ m - l ) ~ for CO trapped in methylpentane. The absorption peak due to trapped CO can also be observed in the difference spectrum obtained for the sample in the polystyrene film. Warming either the irradiated 3-methylpentane matrix or the polystyrene film samples to about 170 K followed by refreezing to 77 K leads to complete regeneration of the spectrum of H ~ R u ~ ( C O )In~ ~ . addition, the absorption due to released CO can no longer be observed. 2. Wavelength Dependence. The 405-578-nm broad band irradiation into the first absorption bands did not induce any changes in the UV-visible and FT-IR absorption spectra of either sample of H ~ R u ~ ( C O On ) ~ ~the . other hand, irradiation at 366 nm produced UV-visible and FT-IR absorption spectral changes similar to those seen for the irradiation done at 313 nm. This indicates that the reaction resulting from 366-nm irradiation is the same as that resulting from 313-nm irradiation. Under the irradiation conditions where the light transmitted through the sample was reflected by a metal window and reabsorbed by the sample, the 313- and 366-nm excitation light was almost completely absorbed by the samples. Thus, according to the method described in the experimental section, the relative yield 1$,(366 nm)/gr(3 13 nm) was determined. As shown in Figure 4, the plot of A(2100 cm-') versus time, t , gives a straight line. Since 10(366 nm)/I0(313 nm) was determined to be 2.6, the relative yield, &(366 nm)/&(313 nm) was determined to be 0.03. The plots of the relative reaction quantum yield versus excitation wavelength are summarized in Figure 5. These results indicate that the relative reaction quantum yields at 77 K are strongly dependent on the irradiation photon energy. 3. TemperatureDepeadeace. When H2Ru4(CO)13 was irradiated in 3-methylpentane or the polystyrene film at 298 K,no UV-visible nor FT-IR absorptionspectral changeswere observed, irrespective of the excitation wavelength (from 313 to 578 nm). Discussion The spectral analyses provide great insight into the nature of the photochemical processes that occur when H ~ R u ~ ( C Ois) I ~
t(min)xlrj-' Figure 4. Plots of 4 2 1 0 0 cm-l) versus time, 1. Irradiation wavelength (nm): (- 0 -) 313; (- -) 366.
C
a
~
Wavele ngthhm) Figure 5. Plots of the relative reaction quantum yield, #r(X)/&(313 nm), versus excitation wavelength. irradiated. The UV-visible and FT-IR spectral changes showed clear isosbestic points when the samples were irradiated at 313 nm. Such spectral changes imply that the reaction is clean and that there are no secondary products. Furthermore, when the photoproduct in the 3-methylpentane matrix or polystyrene film was warmed, IR spectroscopy showed that the parent cluster H ~ R U ~ ( C Owas ) I ~completely regenerated as the released CO disappeared. The recombinationreaction was also demonstrated by the UV-visible absorption spectral changes which showed that the parent cluster H2Ru4(CO)ls was regenerated upon warming the samples and the weak absorption band around 600 nm due to the photoproduct disappeared. The complete regeneration of the parent HzRQ(CO)I~indicates that the RQ metal framework is still intact in the photoproduct. The fact that the UV-visible absorption spectrum of the photoproduct shows the same broad bands considered to be due to the metal-centered u-us transition also suggests that the original RQ metal framework is retained in the photoproduct. The fact that the total amount of CO formed is equal to the initial amount of H~Ruq(C0)13implies that one CO is released from each HzRud(C0) 13 molecule. From these findings, it can beconcluded that the photoproduct from H ~ R Q ( C O ) Iis~the coordinatively unsaturated cluster H2Ruq(C0)12. A weak and broad band centered around 600 nm for the product supports this assignment since a similar broad band has also been reported for other coordinatively unsaturated metal carbonyls clusters.5bJ0.* From these results, one can conclude that the primary photochemical event is the selective dissociation of a single CO ligand from the parent carbonyl cluster. This results in the formation of thecoordinativelyunsaturated cluster HzRQ(CO)IZ as shown in eq 2. Both the coordinatively unsaturated H ~ R U ~ ( C Oand ) I Z CO are trapped near each other in the 3-methylpentane matrix or
Yamamoto et al.
568 The Journal of Physical Chemistry, Vol. 97, No.3, 1993
-
H ~ R u ~ ( C O )H~ ~~ R u ~ ( C O + CO )~~
(2)
polystyrene film at 77 K. Upon warming, the recombination of these two species occurs easily to completelyregenerate the parent cluster H2Rw(C0)13. The fact that the recombination reaction occurs even at 170 K reveals that the coordinatively unsaturated cluster HzRu~(CO)IZ is highly reactive. The drastic increase in the relative quantum yields with irradiation photon energy, as shown in Figure 5, implies that the lowest absorption band responsible for CO dissociation of H2Ru4(CO)13istheforthabsorptionbandwitha peakat 310nm. This wavelength dependence can be interpreted in terms of the difference in the electronic structure of the excited states which are composed of 4d, 5s, and Sp Ru atomic orbitals and 4u, Su, T , and T* CO molecular orbitals, as has been discussed for trinuclear metal carbonyls such as M3(C0)12 (M = Fe, Ru, and OS).IO~*~ Recently, we investigated the photochemical CO dissociationreaction of the isostructural mixed metal tetranuclear clusters HzFeM,(CO)l3 (M = Ru13) in solid matrices at 77 K and found that in contrast to H2Ru4(CO)13, these clusters undergo significant CO dissociation to yield the coordinatively unsaturated H?FeMp(C0)12 even upon irradiation of the first absorption band with visible light (4354111 line of Hg line). Comparison of these results should shed light on the mechanisum of CO dissociation for the tetranuclear metal carbonyls. The difference in the wavelength dependence between the tetranuclear clusters indicates that the electronicstructure of the excited states control the yield of CO dissociation. At present, however, we cannot discuss indetail the mechanismfor COdissociation because the absorption bands for these tetranuclear clusters have not yet been assigned. It is worth noting that HzRw(C0)13 undergoes a photosubstitution reaction in solutions of PPh3 at 298 K. This results in the cluster H2Ru4(C0)12(PPh3).11 The reaction intermediate for this reaction has been proposed to be the coordinatively unsaturated cluster H2Rw(C0)12. Since the coordinatively unsaturated H2Ru4(C0)12 reacts very efficiently with CO in our system, the recombination reaction with CO should occur easily in the 3-methylpentane matrix or polystyrene film at 298 K because of the efficient geminate recombination. This would result in the steady-state concentration of the coordinatively unsaturated HzRuq(C0) 12 remaining low during irradiations at 298 K. This explains why the CO dissociation reaction does not appear to occur at 298 K. The IR spectrum of HzRu4(C0)12 trapped in the matrices, especially in polystyrene film, agrees well with the spectrum of the photoproduct from H2Ru(C0)13 adsorbed on silica as previously reported (see insert in Figure 3).3 The UV-visible spectrum of H2Rw(C0)12 also agrees well with the DRS UVvisible spectrum of the photoproduct on ~ i l i c a .The ~ similarity of the spectra indicatesthat the photoproduct from H2Ru4(C0)13 on silica is the coordinatively unsaturated cluster H2Ru(C0)12. This supports our previous proposal, shown in eq 3, for the H,Ru,(CO),,/SiO,
-
H2Ru4(CO),,/Si0,
+ CO
(3)
photochemical reaction occurring on the surface of silica. It is worth pointing out that this is essentially the same as the reaction shown in eq 2. The initial photochemical process occurring in systems at 298 K, in matrices at 77 K,and on the surface of silica involves the dissociation of one CO ligand. However, there are some important differences between these three systems. In systems at 298 K, the initial photoproduct H2Ru4(C0)12 rapidly reacts with the dissociated CO to regenerate the parent cluster H ~ R u ~ ( C O )In I ~contrast, . the matrix systems allow the photoproduct to be isolated and studied. In a similar manner, on the surface of silica the initial photoproduct H ~ R U ~ ( C Oappears )IZ to be stable and its spectra can be obtained. This stabilization in the silica system can be rationalized as involving the surface
hydroxyl groups which are the only functionality available to act as a stabilizing ligand." This stabilization prevents disproportionation of the tetranuclear photoproduct to smaller clusters. In addition,the dissociatedCO is easily lost from the surfacebecause there is no possibility for a cage effect on the open silica surface. The loss of CO from the surface prevents recombination of the released CO with the coodinatively unsaturated cluster H2Ruq(C0)12. The formation of HZRU~(CO)IZ on the surface of silica and the ability to observe it are in many ways similar to the solid matrix systems at 77 K.
Conclllsion Using the matrix isolation technique, we were able for the first time to observe the UV-visible and IR absorption spectra of the coordinatively unsaturated cluster HzRw(C0) 12. It was found that H2Ru4(C0)12 is so reactive that it recombines with CO wen at thelow temperatureof 170K. Formationof thecoordinatively unsaturated cluster H2Rw(C0)12 from H2Rw(C0)13 was found to be strongly excitation wavelength and temperature dependent. This wavelength dependence can be interpreted in terms of the electronic structure of the corresponding excited states. The temperature dependence in which the photoreaction does not appear to occur at high temperature can be explained by the rapid recombination of the photoproduct with CO. From the similarity of the UV-visible and IR spectra of the matrix isolated photoproduct with the spectra of the photoproduct formed on silica, we can assign the photoproduct on silica as H2Rw(C0)12. This confirms our previous postulate about the photochemical reaction of H ~ R u ~ ( C Oon ) I the ~ surface of silica. Finally, the natureofthestabilizationof thecoordinatively unsaturatedcluster H ~ R u ~ ( C O by ) I Zthe silica surface, even at room temperature, remains an interesting point that needs to be further clarified.
Acbwledgment. We thank Dr. M. Hoshino of The Institute of Physical and Chemical Research for stimulating discussions and Mr. H. Yao of the Central Research Institute of Mitsui Toatsu Chemicals Inc. for preparation of the polystyrene film sample. References a d Notes (1) Gates, B. C.;Guczi, L.; Knlhinger, H. Metal Clusters in Catalysis; Elswier: Amsterdam, 1986. (2) (a) Yamamoto, S.;Lewis, R. M.;Hotta, H.;Kuroda,H.Inorg. Chem. 1989, 28, 3091. (b) Yamamoto, S.;Lewis, R. M.;Hotta, H.; Kuroda, H. Vacuum 1990,4I, 65. Yamamoto, S.;Lewis, R. M.;Nabata, Y.; Hotta,H.; Kuroda,H. Inorg. Chcm. 1990,29,4342. (d) Yamamoto, S.;Miyamoto, Y.; Koimmi, M.;Lewis, R. M.; Morioka,Y.; Asakura, K.; Kuroda, H. J. Phys. Chcm. 1992, 96, 6367. (3) Yamamoto, S.;Asakura, K.; Mochida, K.; Nitta, A.; Kuroda, H. 1. Phys. Chcm. companion paper in this issue. (4) Burdett, J. K.; Perutz, R. N.; Poliakoff, N.; Turner, J. J. Pure Appl. Chem. 1977,49,271. (5) (a) Hepp,A. F.; Wrighton, M.S.J. Am. Chcm.Soc.1983,105,5934. . (b) Bentsen, J. G.; Wrighton, M. S.J. Am. Chcm. Soc. 1984,IW,4041(c) Bentsen, J. 0.;Wrighton, M.S.J . Am. Chcm. Soc. 1987,109,4518. (6) Hooker, R. H.;Mahmoud, K. A.; Rest, A. J. J. Chcm. Soc., Chcm. Commun. 1983, 1022. (7) Johnson, B. F. G.; Johnaton, R. D.; Lewis, J.; Robinson, B. H.; Wilkinson, G. J . Chcm. Soc. A 1968.2856. ( 8 ) Murov. S. L. Handbook of Photochemistry; Dekker: New York, 1973. (9) Melhuish, W. H.J . Opt. Soc. Am. 1962, 52, 1256. (10) (a) Abrahamson, H. B.; Frazier, C.; Ginlcy, D. S.;Gray, H. B.; Lilienthal, J.; Tyler, D. R.; Wrighton, M.S. Inorg. Chcm. 1977, 16, 1554. (b) Tyler, D. R.; h e n s o n , R. A.; Gray, H.B. J. Am. Chcm. Soc. 1978,100, 7888. Delley, B.; Manning, M.C.; Ellis, D. E.;Bcrkowitz, J.; Trogler, W. C. Inorg. Chcm. 1982, 21, 2247. (11) Foley, H. C.;Geoffroy, G. L. J. Am. Chcm. Soc. 1981,103,7176. (12) Yamamoto, S.;Asakura, K.; Nitta, A,; Kuroda, H.J . Phys. Chcm. 1992, 96,9565. (13) Yamamoto, S.;Asakura, K.; Mochida, K.; Nitta, A.; Kuroda, H. submitted for publication in J. Orgunomct. Chcm. ( 14) Jackson, R. L.;Trusheim, M.R. J. Am. Chcm.Soc. 1982,101,6590.
