Anodic Reaction of Co(η5-C5H5)(CO)(PPh3): An Oxidatively Induced Ligand Exchange Involving a 17 e−/18 e− Redox Pair

Unlike a number of other monocarbonyl-substituted complexes CoCp(CO)L (Cp = (η 5 -C 5 H 5 )), chemical oxidation of the L = triphenylphosphine derivative CoCp(CO)(PPh 3 ), 1, is known to give, in the absence of added PPh 3, the di substituted phosphine complex [CoCp(PPh 3 ) 2 ] + rather than the simple 17-electron radical cation 1 + . Electrochemistry of 1 in CH 2 Cl 2 /[NBu 4 ][B(C 6 F 5 ) 4 ] ( E 1/2 ( 1 / 1 + ) = −0.38 V vs ferrocene) shows that the favored anodic products depend on the concentration of 1 . At low concentrations (e.g., 10 −4 M) and on the cyclic voltammetry time scale, the radical cation 1 + dominates, but at higher concentrations and longer reaction times, a quantitative mixture of CoCp(CO) 2 and [CoCp(PPh 3 ) 2 ] + is produced in an overall “half-electron” process. On the basis of cyclic voltammetry, chronoamperometry, bulk electrolysis, and IR spectroelectrochemistry, a “radical−substrate” mechanism is proposed involving the reaction of 1 + with 1 to give transient [Co 2 Cp 2 (CO) 2 (PPh 3 ) 2 ] + . This putative dimer radical cation intermediate may be viewed as an odd-electron analogue of the intermediates that have previously been invoked by Kochi, Atwood, and others to explain ligand-transfer reactions between cation/anion pairs of organometallic complexes.