CYCLIC VOLTAMMETRIC STUDIES OF TRIS(ALKYLISOCYANIDE)BIS(TERTIARY PHOSPHINE)COBALT(I) COMPLEXES

Synthesis and characterization of tris(alkylisocyanide)bis(triarylphosphine)cobalt(I) complexes, [Co(CNR) 3 (PR 3 ) 2 ]ClO 4 , has been extended to include CNR = CNCMe 3 , PR 3 = P(C 6 H 4 CF 3 -p) 3 , P(C 6 H 4 F-p) 3 , P(C 6 H 4 Me-p) 3 ; PR 3 = PPh 3 , CNR = CNC 6 H 11 , CNCH 2 Ph, CNC 4 H 9 -n. Reduction of [Co(CNCMe 3 ) 4 H 2 O](ClO 4 ) 2 did not occur with PR 3 = P(C 6 H 4 NMe 2 -p) 3 and P(C 6 F 5 ) 3 . Solution and solid state structures of the new [Co(CNR) 3 (PR 3 ) 2 ]ClO 4 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, E 1/2 , measured in CH 3 CN, 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 (PPh 3 ) 2 ]ClO 4 , R = CMe 3 , CHMe 2 , C 6 H 11 , CH 2 Ph, C 4 H 9 -n. Variation of the tertiary phosphine ligands has a pronounced effect on the E 1/2 values, which range from +0.03 to +0.75V, increasing steadily as PR 3 (in [Co(CNCMe 3 ) 3 (PR 3 )ClO 4 ) is varied: P(NMe 2 ) 3 < P(C 4 H 9 -n) 3 ∼ P(C 6 H 13 -n) 3 < P(C 6 H 4 OMe-p) 3 < P(C 6 H 4 Me-p) 3 < PPh 3 < P(CH 2 Ph) 3 < P(C 6 H 4 Cl-p) 3 ∼ P(C 6 H 4 F-p) 3 < P(CH 2 CH 2 CN) 3 < P(C 6 H 4 CF 3 -p) 3 . Trends in E 1/2 values are better explained in terms of σ-donation than π*-acceptance. The voltammograms are in general quasi-reversible, but the highest E 1/2 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 (PR 3 ) 2 ] 2+ , with a regular trigonal bipyramidal structure.