Inorg. Chem. zyxwvu 1990, 29, zyxwvu 2067-2074 2067 cyanate has a pronounced effect on the oxygen binding step, and chloride effects mainly the subsequent redox steps. The rate constant for the binding of zyxwvutsr O2 is fairly insensitive to the presence of anions. The values of kf for the three species in Table VI differ only by a factor of -4. This is somewhat sur- prising, since one might have expected that differences in oxygen affinity would be reflected in the kinetics of both forward and reverse reactions. The effect on the release of O2 is much more pronounced. The rate constant k, is the largest for the aquo complex, intermediate for the chloro, and too small to observe for the thiocyanato. The electronic structure of these cobalt-oxygen adducts is probably best considered intermediate between Co11-02 and C O~~~-O~-.~~,~ The latter structure should be stabilized the most by the most strongly coordinated anion, SCN-, and thus the dissociation to O2 and cobalt(I1) should be least favorable for (SCN)L2Co02+, as experimentally observed. The high equilibrium constant for the binding of O2 might be partly responsible for the accelerated autoxidation of L2C02+ in the presence of SCN-. Subsequent redox steps were not investigated, but they are probably affected too, as judged from the zyxwvutsr results obtained in the presence of chloride. The equilibrium constant for the binding of O2 by (Cl)L2Co+, 560 M-l, is not much larger than that for the uncomplexed L2Cd+, 301 M-*. Both the forward and reverse reactions are somewhat faster for L2c02+ than for (Cl)L%o+. Despite that, the irreversible oxidation of L2C02+ is much faster in the presence of C1-, dem- onstrating that the redox steps following reaction 16 must be accelerated by CI-. Similar conclusions have been reached earlier for some other cobalt c~mplexes.~*~ We have also observed that Br- accelerates dramatically the autoxidation of zyx L%02+, although it has no effect on the oxygen binding step. This is also consistent with strong catalysis in the redox steps. A detailed study of these reactions is in progress. Acknowledgment. This research was supported by the US. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division, under Contract W-7405-Eng-82. We are grateful to S. Lee for the synthesis of the macrocyclic ligand and to Dr. Lee Daniels of the Iowa State University Molecular Structure Laboratory for the crystal structure determination. zyxwvutsrqpo [ Co(C-meso-Me,[ 1 4]aneN4)(C104)2], 126543-78-0. Supplementary Material Available: Tables of positional parameters for hydrogen atoms and general displacement parameter expressions (5 pages); a listing of observed and calculated structure factors (8 pages). Ordering information is given on any current masthead page. Regis* NO. (C1-)L2C0+, 126543-76-8;(SCN-)L~CO+, 126543-77-9; Contribution from the Department of Chemistry, Stanford University, Stanford, California 94305 Variable-Energy Photoelectron Spectroscopic Comparison of the Bonding in Ferric Sulfide and Ferric Chloride: An Alternative Description of the Near-IR-Visible Spin-Forbidden Transitions in High-Spin d5 Complexes Kristine D. Butcher, Matthew S. Gebhard, and Edward I. Solomon* Received September 25, I989 Variable photon energy, valence-band, and core-level photoelectron spectroscopy (PES) have been used to determine the electronic structure and bonding in tetrahedral high-spin ds FeS4>-. The valence-band PES spectra over the range 25-100 eV show strong similarities with our previous results on tetrahedral FeCIL. The three-peak pattern and their energy splittingsand intensity ratios all parallel the data on FeCIi. Also, as in ferric chloride, the major resonance enhancement appears in the deepest binding energy portion of the main band, indicating that dominant metal character is present in the bonding levels at deepest binding energy. No off-resonance PES intensity is observed in the satellite, indicating that little relaxation occurs upon ionization. These results demonstrate that the ground-state electronic structure of ferric sulfide parallels that of ferric chloride and is inverted from the normal description for transition-metal complexes, which places the dominant metal character in the antibonding levels at lowest binding energy. This inverted bonding scheme results from the large spin-polarization effects present in high-spin ds complexes. Analysis of the Fe 2p core level PES spectra allows a comparison of the covalency of tetrahedral ferric chloride and sulfide. The difference is relatively small and is due to the lower ionization energy of the sulfide relative to the chloride ligands. Alternatively, there is a large difference observed between the bound-state optical absorption spectra of ferric chloride relative to the sulfide (and thiolate) complex, which is not satisfactorily accounted for by ligand field theory but is explained by spin-unrestricted zy Xa calculations. These studies indicate that the lowest energy spin-forbidden transitions in high-spin dS complexes, which are normally described as d - d transitions in ligand field theory, have extensive ligand-to-metal charge-transfer character. I. Introduction Our previous work has focused on determining the electronic structure and bonding in high-spin tetrahedral ferric chlorides as a first step toward understanding ironsulfur active sites in proteins such as rubredoxin.' Theoretical studies of model iron thiolate complexes indicate that the ground-state bonding description of these ironsulfur systems is inverted, with the HOMO exhibiting mostly ligand character and the metal character contained in the bonding levels at deeper binding energyS2 This inverted ground (a) Deaton, J. C.; Gebhard, M. S.; Koch, S. A.; Millar, M.; Solomon, E. I. J. Am. Chem. Soc. 1988,110,6241. (b) Gebhard, M. S.; Deaton, J. C.; Koch, S. A.; Millar, M.; Solomon, E. I. J. Am. Chem. Soc., in press. (c) Deaton, J. C.; Gebhard, M. S.; Solomon, E. I. Inorg. Chem. 1989, 28, 877. (d) Butcher, K. D.; Didziulis, S. V.; Briat, B.; Solomon, E. 1. J. Am. Chem. SOC., in press. (a) Norman, J. G., Jr.; Jackels, S. C. J. Am. Chem. Soc. 197597,3833. (b) Norman, J. G., Jr.; Ryan, P. B.; Noodleman, L. J. Am. Chem. Soc. 1980, 102, 4279. (c) Bair, R. A.: Goddard, W. A., 111. J. Am. Chem. Soc. 1978, 100, 5669. 0020- 1669/90/ 1329-2067$02.50/0 state is also predicted for FeCI, and was experimentally confirmed by using variable-energy photoelectron spectroscopy (PES).1d The energy level diagram for the high-spin dS system including ex- change can be depicted schematically as in Figure 1. The ground-state spin-unrestricted Xa calculations indicate that the stabilization of the d levels is due to the large exchange interaction in high-spin ds, which lowers the energy of the occupied d t orbitals relative to the empty d l orbital^.^ The exchange splitting between ligand t and 1 levels is small, while the exchange in the Fe 3d levels is large and is present in the free-ion 6S ground state. This exchange splitting is the greatest for the dS configuration and is sufficient to drop the 3df levels below the ligand 3p valence levels. Both the df and d l orbitals interact with the ligand 3p valence orbitals, resulting in a complex bonding scheme that contains dominant ligand character in both the spin-down bonding and (3) The spin-unrestrictedformalism allows different orbitals for different spins, thus splitting the d orbitals into occupied spin-up and unoccupied spin-down. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ 0 1990 American Chemical Society