Unusual oxidative behavior of (.eta.5-C5R5)M(CO)2 (R = H, CH3; M = Rh, Co) complexes at mercury and platinum electrodes

Abstract

The electrochemical oxidative behavior of a series of cyclopentadienyl and pentamethylcyclopentadienyl carbonyl complexes of rhodium and cobalt is described. The voltammetric, polarographic, and coulometric data from the oxidations of (n^5-C5H5)Rh(CO)2, (n^5-C5H5)Co(CO)2, (n^5-C5(CH3)5)CO(CO)2, and (n^5-C5(CH3)5)Rh(CO)2 are reported. The electrochemical experiments were conducted in methylene chloride at platinum and mercury electrodes. The cyclopentadienyl complexes (n^5-C5H5)Rh(CO)2 and (n^5-C5H5)CO(CO)2 exhibited oxidative behavior dependent on electrode material. At a platinum-bead electrode there was evidence that the organometallic solute adsorbs to the electrode surface, while at a hanging-mercury-drop electrode it was found that mercuric chloride formed at the surface of the electrode at potentials greater than 0.200 V vs. SCE. This HgC12 reacts with the solute to form an insoluble Lewis acid-base adduct, [Cl2Hg]*[M(CO)2(n^5-C5H5)], where M = Rh, Co. These adducts were also formed from the homogeneous oxidation of the organometallic solute with [(n^5-C5H5)2Fe] [PF6], but only when mercury was present in the solution. The pentamethylcyclopentadienyl complexes (n^5-C5(CH3)5)CO(CO)2 and (n^5-C5(CH3)5)Rh(CO)2 were found to undergo chemically reversible one-electron oxidations to relatively stable cation radicals at both platinum and mercury electrodes. There was no evidence of either pentamethylcyclopentadienyl compbx reacting with the mercuric chloride. Extended Huckel calculations were conducted on all four complexes. Correlations were seen between the experimental observations and Huckel predictions of adduct formation and radical stability.