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a preconditioned all-glass ampoule equipped with a break-seal, in accordance with the sort of procedures that were adopted for the synthesis and isolation of the gallane (q.v.); subsequent manipulation and analysis of the reaction mixture could usually be accomplished using a more conventional vacuum line having a distillation train and greased ground-glass or, if necessary, greaseless stopcocks as well as ground-glass joints lubricated typically with Apiezon L grease. In none of the experiments was there any evidence that unchanged gallane survived the encounter with an excess of the reagent (HC1, NMe3, NH3, PH3, or C2H4) under the conditions outlined in the preceding section.All noncondensable gases-Ar, Kr, and N2 (research grade)-were used as received from the British Oxygen Co. (B.O.C.). The following reagents, from the commercial sources indicated, were purified before use by fractionation in vacuo: HC1 (Air Products), NH3 (B.O.C.), and C2H4 (B.O.C.). Toluene-

Single-crystal polarized absorption, magnetic circular dichroism (MCD), and EPR spectroscopies have been employed to elucidate the electronic structure of the oxidized rubredoxin model complex Fe(SR)i [R = 2,3,5,6-(Me),c6H], These studies have led to a definitive assignment of the S4 symmetry-split components of the 6Al -4Tla, 4T3, and Ea spin-forbidden ligand field transitions, which are analyzed to give an experimental determination of the Fe d orbital axial splitting diagram. The transitions are correlated with the ground-state zero-field splitting (ZFS) to examine the origin of the large ZFS in ferric tetrathiolate complexes. As in our earlier studies3' on a Du distorted Td Fe(C1); complex, it is found that anisotropic covalency must be included to account for the observed ZFS. From comparisons to the data on DZd Fe(CI),-the orientation of the R group is found to determine the electronic structure of the iron tetrathiolate complex. A definitive assignment of the spin-allowed charge-transfer spectrum is presented. This is based on the polarized absorption data, and the sign and magnitude of the excited-state spin-orbit splittings as determined by single-crystal MCD spectroscopy. It is found that the charge-transfer spectrum is dominated by the thiolate S-pu to Fe-du transitions (between 16000 and 28000 cm-' with c = 5000-10000 M-I cm-I), with the S-ps to Fe-dn set of transitions at 13000 cm-' being weak ( t = 300 M-I cm-I). This observation, along with the d orbital splitting pattern, and the lack of measurable in-state spin-orbit splitting of the S, -Fe, CT band indicate that the S, bonding to the iron is not significant. In contrast, the dominance of the thiolate to iron u bonding interaction is suggested by the dd and CT data. This interaction is very dependent on the orientation of the thiolate R group. This thiolate S pu bonding model is used to evaluate existing electronic structure calculations on ferric thiolates and is used to analyze the spectroscopic features of the oxidized rubredoxin active site. (21) Ueyama, N.; Sugawara, T.; Tatsumi, K.; Nakamura, A. Inorg. Chem. 0002-7863/90/ 15 12-22 17$02.50/0 0 1990 American Chemical Society 2218 , \ h T E V \ \ \ \ \ / support of this research. M.S.G. and J.C.D. acknowledge NSF for graduate research fellowships.be small for the rubredoxin active site. In addition, in the oxidized site the d z and dXLy2 orbitals should be substantially split. If these orbitals were close to degenerate, reduction to give the ferrous 5E ground state would result in a Jahn-Teller distortion; however, this splitting of these orbitals removes need for this distortion.

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