17-ELECTRON AND 19-ELECTRON COMPLEXES [FE-III(ETA(5)-C(5)R(5))(S(2)CNME(2))L](N- ELECTRONIC-STRUCTURE AND SUBSTITUTION AND REDOX CHEMISTRY - FORMATION OF [FE-IV(ETA(5)-C(5)R(5))(DTC)(2)] AND CHARACTERIZATION OF BOTH 17E AND 19E STATES OF A TRANSITION-METAL COMPLEX() (N=1,0) )

Oxidation of [FeCp(eta(1)-dtc)(CO)(2)], 1 (Cp* = eta(5)-C(5)Me(5), dt c = S(2)CNMe(2)), or [FeCp(eta(2)-dtc)(CO)], 2, using [Fe(III)Cp(2)]( +)X(-) (X(-) = PF6- or BF4-, Cp = eta(5)-C5H5) in THF cleanly gives [F e(III)Cp(eta(2)-dtc)(CO)](+)X(-), 2(+)X(-), as microcrystalline green , thermally stable, but substitution labile, salts. The substitution o f CO in 2(+)PF(6)(-) by various solvents (CH2Cl2, THF, CH3COCH3, CH3CN ) (visible spectroscopy) follows pseudo-first-order kinetics but shows clearly the influence of the incoming solvent ligand on the substitut ion rate and, hence, is in good agreement with an associative mechanis m. Displacement of the labile solvent ligand in these complexes by a p hosphine results in the 17-electron (17e) cations [Fe(III)Cp(eta(2)-d tc)(L)](PF6-)-P-+, L = PPh(3) (7(+)PF(6)(-)) or eta(1)-dppe (8(+)PF(6) (-)). The same reaction in the presence of the anionic ligands CN-, SC N-, and Cl- affords the corresponding neutral 17e Fe-III complexes (re spectively compounds 11, 13, and 14). All these 17e complexes were cha racterized by IR, ESR, and Mossbauer spectroscopies and elemental anal ysis. The cations were reduced to isostructural neutral Fe-II complexe s using 1 equiv of [Fe(I)Cp(C(6)Me(6))] in THF or oxidized to the robu st green 18e Fe-IV complex eta(5)-C(5)Me(5))(eta(2)-S(2)CNMe(2))(2)]()-PF6-, 9(+)PF(6)(-), using Na(+)dtc(-) . 2H(2)O. -5(CH2C6H5)(5))(eta( 2)-S(2)CNMe(2))(2)](PF6-)-P-+, 16(+)PF(6)(-), was structurally charact erized, and the dihapto mode of coordination of both dtc ligands was e stablished. The 19e Fe-III species 9 was shown to be an intermediate w hich further reduced H2O. It could be alternatively synthesized by red uction of the 18e precursor 9(+)PF(6)(-) using 1 equiv of [Fe(I)Cp(C(6 )Me(6))] or by addition of anhydrous Na(+)dtc(-) to 3(+)PF(6)(-) in Me CN at -40 degrees C. The 19e complex 9 showed an ESR spectrum indicati ng an axial symmetry (two g values) in contrast with the ESR spectra o f all the 17e species (2(+)-14) which show three g values characterist ic of a rhombic distortion (for instance, the very close model 13). Th e Mossbauer doublet of 9 very slowly evolved to the new doublet of the thermally stable 17e complex 9'. In MeCN solution, the transformation of the blue complex 9 to the purple 17e complex 9' was much more rapi d (above -40 degrees C) as indicated by the rhombic spectrum of 9' in frozen solution and by low-temperature C-13 NMR. In toluene, however, the 19e complex 9 showed a remarkable stability up to room temperature , which allowed recording of the C-13 spectrum in d(8)-toluene. MO cal culations have been performed on models for the 17e and 19e bis-dtc Fe -III complexes. They suggest that the 17e species should have some sig nificant sulfur spin density. The 19e species is found to have its odd electron occupying an antibonding metal-centered orbital. The cyclic voltammogram of 9(+)PF(6)(-) under continuous scanning for the monoele ctronic reduction and the two monoelectronic reductions showed the dec rease of the waves of 9(+)PF(6)(-) and the concomitant increase of tho se due to the partially decoordinated dtc complexes formed upon reduct ion. This permits an interpretation of the CV in terms of a triple-squ are scheme involving 9(+/0/-), 9'(+/0/-), and solvent (DMF) adducts in 18- and 19e states.