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Abstract
Synthesis and characterization of tris(alkylisocyanide)bis(triarylphosphine)cobalt(I) complexes, [Co(CNR)3(PR3)2]ClO4, has been extended to include CNR = CNCMe3, PR3 = P(C6H4CF3-p)3, P(C6H4F-p)3, P(C6H4Me-p)3; PR3 = PPh3, CNR = CNC6H11, CNCH2Ph, CNC4H9-n. Reduction of [Co(CNCMe3)4H2O](ClO4)2 did not occur with PR3 = P(C6H4NMe2-p)3 and P(C6F5)3. Solution and solid state structures of the new [Co(CNR)3(PR3)2]ClO4 complexes appear to be distorted disubstituted trigonal bipyramidal examples with the phosphine ligands in axial positions, but the extent of distortion from rigorous D 3h symmetry in solution appears to vary for different complexes. Oxidation potentials, E1/2, measured in CH3CN, for the oxidation of Co(I) to Co(II), are essentially independent of the RNC ligands, remaining within the narrow range of +0.20 to +0.25V for [Co(CNR)3(PPh3)2]ClO4, R = CMe3, CHMe2, C6H11, CH2Ph, C4H9-n. Variation of the tertiary phosphine ligands has a pronounced effect on the E1/2 values, which range from +0.03 to +0.75V, increasing steadily as PR3 (in [Co(CNCMe3)3(PR3)ClO4) is varied: P(NMe2)3 < P(C4H9-n)3 ∼ P(C6H13-n)3 < P(C6H4OMe-p)3 < P(C6H4Me-p)3 < PPh3 < P(CH2Ph)3 < P(C6H4Cl-p)3 ∼ P(C6H4F-p)3 < P(CH2CH2CN)3 < P(C6H4CF3-p)3. Trends in E1/2 values are better explained in terms of σ-donation than π∗-acceptance. The voltammograms are in general quasi-reversible, but the highest E1/2 values show irreversible behaviour. It is proposed that the Co(I) complex, with approximately trigonal bipyramidal structure in solution, is oxidized to the Co(II) complex, [Co(CNR)3(PR3)2]2+, with a regular trigonal bipyramidal structure.