Electrochemistry of methylidyne and 1,2,3,5-dithiadiazole complexes of iron and nickel and a reinterpretation of the composition of "PhCN₂S₂Fe₂(CO)₆"

The title compounds have been studied in CH2Cl2 by cyclic voltammetry. Nonacarbonyldi(mu(3)-methylidyne)triiron, [Fe-3(CO)(9)(mu(3)-CX)(2)] (X = F, Cl, Br, H), undergo chemically reversible 1e(-) reduction (-0.72 to -0.88 V vs. SCE) and irreversible oxidation (+1.59 to +1.72 V); the compound for which X = F displays a second, irreversible reduction (-1.96 V). Using reduction potentials of comparable mu(3)-E (E = S, Se, NPh, PPh) clusters, mu(3)-methylidyne ligands are shown to be strongly basic, comparable to PPh and NPh and more basic than S or Se. The methylidyne clusters are both more difficult to oxidize and reduce than isomeric alkyne clusters [Fe-3(CO)(9)(C(2)R(2))], indicative of greater thermodynamic stability. The complexes previously formulated as [Fe-2(CO)(6)(S(2)N(2)CC(6)H(4)X)] (X = H, CF3, OCH3) are reversibly reduced (-1.59 to -1.70 V) and irreversibly oxidized (+1.0 to 1.3 V), unlike free dithiadiazole radicals, which are reversibly oxidized around +0.8 V. This behaviour is not consistent with the ring-centered free radical claimed for the case where X = H; instead we find conclusive evidence that these are diamagnetic complexes of the 3-H-1,2,3,5-dithiadiazolines, [Fe-2(CO)(6)(S2N{NH}CC(6)H(4)X)] (nu(NH) = 3376-3381 cm(-1); delta(NH) = 6.55-7.1 ppm). Dicyclopentadienyl-dithiadiazoledinickel, [Ni(2)Cp(2)(S2N2CC6H5)], is both reversibly reduced (-0.79 V) and oxidized (+0.45 V); and irreversibly oxidized at +1.9 V. This is consistent with an unpaired electron, and only the nickel cluster has an ESR signal (g = 2.0410, no resolved hyperfine splitting). Extended Huckel theory locates the free electron in a SOMO restricted to Ni, C, and S atoms.