J. Phys. Chem. 1982, 86, 1043-1044
simultaneously to increase the electron densities of the Cl, and C1, ligands more than that of the Cl, ligand. As becomes clear above, the 2B2state has four Ru-C1 bonds strengthened by the Ru(d,)-Cl(p,) interaction, while the 2E(%1)state has three such bonds; that is, the 2B2state receives more stabilization from the Ru-Cl bonds than the 2E(2B1)state. It also becomes clear why the strength of the Ru-Cl, bond does not depend so much on the electron configuration; i.e., the Ru(d,)-Cl(p,) interaction is formed in both Q2 and states, resulting in a strong Ru-Cl, bond in both states. A similar but more simpler discussion is possible for t ~ a n s - R u C l ~ ( P H ~In ) ~the + . 2B2state, the half-occupied d, orbital can form the Ru(dJ-Cl(p,) interaction with four Cfligands, whereas the half-occupied d,, orbital can form the Ru(d,)-Cl(p,) interaction with two C1 ligands in the 2E state. This bonding nature reflects in the electron distribution shown in Table 111; in the 2E(2Bl)state, two C1 ligands have 1.96 e p, electron densities and the other two C1 ligands have 1.99 e p, electron densities, but in the 2B2 state four C1 ligands have 1.96 e p, electron densities. These electron distributions suggest that four C1 ligands are responsible for the formation of the Ru(d,)-Cl(p,) interaction in the 2B2state, while two C1 ligands are responsible in the %(%') state. Thus, the 2Bzstate receives more stabilization from the Ru(&)-Cl(p,) interaction than does the %(%J state. In this complex, the PH, coordinate bond does not become stronger in the B2state than that in the 2E(2B1)state, perhaps due to the less a-accepting ability of the PH, ligand than that of the CO ligand. In t r ~ n s - R u X ~ ( N H ~the ) ~situation +, is quite different from the above two complexes; i.e., these complexes have a-donating halogen ligands on the z axis. In the 2E(2B1) state, the half-occupied d,, orbital forms the Ru(d,)-X(p,) interaction with two halogen ligands, whereas the halfoccupied d,, orbital cannot form such interaction with any halogen ligand in the 2B2state. Simultaneously, this Ru-
1043
(d,)-X(p,) interaction reduces the halogen p, electron density and consequently reduces the halogen's negative charge in the 2E(2B1)state (see Table 111),weakening the electrostatic repulsion between the negatively charged halogen ligand and the nitrogen atom of the ammine ligand in this state, as shown in Table 11. Thus, the 2E(%1)state has stronger Ru-X bonds and less electrostatic repulsion between the halogen ligand and the nitrogen atom than the 2B2state, whereas the 2B2state has stronger Ru-N bonds due to the hyperconjugate type interaction between the a-type orbital of the ammine ligand and the half-occupied d, orbital of ruthenium. As a result, the 2E(2Bl) state becomes more stable than the 2B2 state and the Ru(d,)-X(p,) interaction plays important roles in stabilizing the 2E(2Bl)state. In summary, a half-occupied d, orbital tends to interact with as many a-donor ligands as possible, but a doubly occupied d, orbital tends to interact with the a-acceptor ligand. Consequently, the d, orbital becomes a half-occupied orbital in RuCl,(CO)'- and truns-RuCl4(PH3),-, because four a-donating halogen ligands are placed on the xy plane and the a-accepting CO ligand is placed on the z axis; their ground state becomes the 2Bzstate. In contrast with this, either the d,, or the dyz orbital becomes a half-occupied d, orbital to interact with axial halogen ligands in trans-RuX2(NH3),+, tr~ns-RuCl,(en)~+, and trans-RuC12[(Me2P),C2H4I2+; i.e., the ground state is the 2E one when the Jahn-Teller effect is neglected. I t can be said that the types of d,,-orbital splittings are determined by the a-donating and a-accepting ability of the ligand. Acknowledgment. We thank Dr. S. Imamura (Kyoto Institute of Technology) and JEOL Co. Ltd. for kindly allowing us to use their ESR instrument. Calculations were performed with the FACOM M-200 computer of the Data Processing Center at Kyushu University.
COMMENTS Comment on "Quantum Yield and Electron-Transfer Reactlon of the Lowest ExcRed State of the Uranyl Ion"'
Sir: Both the uranyl ion and polypyridine complexes of ruthenium(I1) share the properties of having a relatively long-lived excited state with redox properties very different from the corresponding ground states. These characteristics make these ions suitable comDonents for solar enerm conversion systems, and such systems have previously be& proposed which involve redox reactions either of excited ruthenium(I1) complexes2b or of the uranyl ion excited state.@ The excited uranyl ion, however, is a strong (1) Roeenfeld-Gmwdd,T.;Rabani, J. J. Phys. Chem. ig80,84,2981.
(2) Lin, C.-T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. SOC.1976,98,6536. (3) Balzani, V.;Bolletta, F.; Gandolfi, M. T.; Maestri, N. Top. Curr. Chem. 1978, 75,1. (4) Lieblich-Sofer,N.; Reisfeld, R.; Jsrgensen, C. K. Znorg. Chirn. Acta 1978, 30,259. 0022-3654/82/2086-1043$01.25/0
oxidizing agent, while ruthenium(I1)complexes are strongly reducing in their lowest excited states. In a valuable extension to earlier studies in this area, Rosenfeld-Grunwald and Rabani' have reDorted that. on Dhotoexcitation of systems containing tge uranyl ion and'tris(bipyridine)ruthenium(II), electron transfer takes place to yield the same products, irrespective of which ion is excited. The relevant reactions are
-
+ UOZ2++ *Ru(bpy)32+ *UOZ2+ Ru(bpy)32+
-
UOz+ + Ru(bpy),,+
(1)
U02++ R ~ ( b p y ) , ~ +(2)
If the reverse back-transfer between the products is made to occur in an external circuit, a feasible photogalvanic system is obtained. This system has two advantages; the range of wavelengths Of light which can be converted is greater than with the separate components, and light is (5) Reisfeld, R.; Lieblich-Sofer, N., quoted in ref 1. (6) Burrows, H.D.;Formosinho, S. J.; Miguel, M. da G.;Pinto Coelho, F., Mem. Acad. Cienc.,Lisboa 1976, 19,185.
0 1982 American Chemical Society
1044
Comments
The Journal of Physical Chemistry, Vol. 86, No. 6, 1982
possible with short wavelength light, in our experiments not lost by any inner filter effect of the quenching ion. with UO?+ the same transients were observed with light We have been carrying out similar experiments with of wavelength longer than 300 nm (obtained by suraqueous solutions of uranyl nitrate and tris(1,lOrounding the flash tubes with Pyrex). Under these conphenanthroline)ruthenium(II) perchlorate using steadyditions, no transients were observed upon flash photolysis state fluorescence quenching, and find that with aerated of the anions, indicating that biphotonic processes were aqueous solutions (pH 2), the quenching of *Ru(phen)l+ unimportant. (8 X M solution, A e d t 470 nm, ,,,A 580 nm) by UO;+ (ii) Even without the 300-nm filter, the extent of direct follows good Stern-Volmer kinetics, with K,, = (1.89 f photoionization of the anions will be limited by the strong 0.20) x lo2 M-l. Using this and the known lifetime of inner-filter effect of the UOz2+ion. *Ru(phen)? under these conditions (after correcting for (iii) While Mn(I1) does possess a very weak spin-forquenching by dissolved oxygen),2we obtain a rate constant bidden absorption in the visible region of the spectrum,9 of 1.2 x lo9 M-' s-l for the quenching process. This is close no transient absorption was observed upon microsecond to the rate of quenching of *Ru(phen)?+ by Fe3+,2is slightly higher than the rate of quenching of * R ~ ( b p y ) ~ ~ +flash photolysis of solutions of manganese(I1)perchlorate in water, while only a very weak transient absorption was by UO;+,l and is consistent with the same mechanism as observed in the case of Mn(NO& solutions, indicating that in these two cases, i.e., electron transfer. As observed with photoionization of Mn(HzO),2+is not a significant process. the Ru(bpy)?+/UO?+ system,' prolonged photolysis did In contrast, with aqueous solutions of UO,2+ and Mn2+, not result in any permanent change in the absorption microsecond flash photolysis gave a relatively strong abspectrum of the system, demonstrating a high overall sorption with spectrum similar to that of Mn3+.6,7 photostability. Because of the intense absorption of the We believe the above reasons justify the interpretation Ru(phen)?+ species, our steady-state experiments do not previously given'J' in terms of electron transfer between permit demonstration of the reverse quenching of *UO?+ substrate and some excited uranyl species on photoby Ru(phen),2+. However, studies on the fluorescence oxidation. The failure of these authors' to observe the excitation spectra are consistent with the occurrence of transients in their nanosecond flash photolysis experiments such a process. may either reflect the relatively lower light intensity used In the paper by Rosenfeld-Grunwald and Rabani,' the in their Nz laser or suggest that both inter- and intramoauthors refer to our earlier microsecond flash photolysis lecular processes are important in our systems. studies on uranyl in the presence of Mn2+,I-, SCN-, Br-, and C032- in aqueous solution. In these, we observed absorptions assigned to transients formed by (9) Jsrgensen, C. K. "Absorption Spectra and Chemical Bonding in Complexes"; Pergamon, Oxford, 1962; p 117. electron-transfer photooxidation of the substrate by *UO,2+. They suggest that the transients arise, instead, Chemistry Department Hugh D. Burrows' by direct photolysis of the substrates. We think that this University of Ife is unlikely and that our experiments do demonstrate Ile- Ife, Nigeria electron transfer for the following reasons: (i) While direct photolysis of halide ions or C032- is Sebastlao J. Formoslnho Chemical Laboratory (7) Burrows, H. D.; Formminho, S.J.; Miguel, M. da G.; Pinto Coelho, F., J. Chem. SOC.,Faraday Trans. I 1976, 72,163. (8) Burrows, H. D.; Pedroea de Jesus, J. D.,J. Photochem. 1976,5,265.
University of Coimbra 3OOO-Colmbra, Portugal
Received: September 23, 798 7
