Coordinated diimine ligands, oxidation electron transfer

Complexes with pyridine-2,6-diimine ligands, five-coordinate [NiX2(174)] (X = C1, Br) or six-coordinate [Ni(174)2]X2, were usually assumed to have innocent neutral ligands. The X-ray structure and spectroscopic characteristics of [Ni(174)2](PF6) confirm that the complex contains the neutral ligand ([174] ) and a divalent nickel ion.579 The cyclic voltamogram of this complex in CH2C12 shows three reversible one-electron-transfer processes at = 1.19 V, —1.30 V, and — 1.82V (vs. Fc+/Fc), of which the first is a one-electron oxidation, while the other two correspond to two successive one-electron reductions. The spectroscopic data allow one to assign these processes as follows ([174]1 is a one-electron reduced radical form of [174] ) [Nini(174)°2]3+ [NiII(174)02]21 - " [NiI(174)°2]+ = " [NiI(174)°(174)1 ]°. 

The rate constant for the redox step, kr, is unlikely to reflect a simple electron transfer from the monodentate diimine ligand to the metal center because replacement of coordinated water with coordinated hydroxide would be expected to decrease the oxidizing power of the metal-(III) center. This is well documented by the standard reduction potentials of the aqua and hydroxo complexes in Table IV, and it would seem... 

Unlike the CN- ion, replacement of H20 in the coordination sphere of the hexaaqua Fe2+ or Fe3+ by bipy or phen raises the standard redox potential (from 0.77 V to 1.12 V in the case of phen). This stabilization of the lower oxidation state is attributed to extensive back-coordination of the (L electrons into the vacant p orbitals of the a-diimine ligands. The intense red colour of the Fe" complexes is likewise due to this metal-to-ligand charge transfer. The reversibility of the redox reaction and the easily detectable colour change make these complexes very useful redox indicators.61,62... 

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