The electrochemistry of the methylidyne tricobalt nonacarbonyl cluster and of its phosphine-substituted derivatives has been reinvestigated. For this purpose six missing members of the series were prepared. The compounds MeCCo3(C0)+,L, for L = PMe,, n = 1-3; L = PMe2Ph, n = 1-3; L = PMePh2, n = 1, 2; L = PPh,, n = 1; L = PEt,Ph, n = 3, were studied by cyclic voltammetry. By varying the ligands, the electrodes, the sweep ranges and rates, the temperatures, the concentrations, and the solvents and by running the experiments in the presence of excess ligands, a consistent set of data was obtained. In contrast to widespread opinion, the radical anion (49-electron complex) of the parent cluster M~CCO,(CO)~ is of limited stability due to the reversible elimination of CO. Analogously, the 49-electron anions of the phosphine-substituted clusters can eliminate one phosphine ligand producing 47-electron monoanions [M~CCO~(CO)~,,L,J. Further reduction of these 47-electron monoanions to their 48-electron dianions occurs more easily than that of the starting clusters M~CCO,(CO)~,,L, (ECE mechanism). Depending on the kind and number of phosphine ligands, [M~CCO,(CO)~-,L,]-can alternatively eliminate CO. The ligand-deficient 47-electron species are the key intermediates in the electron-transfer-induced dissociative ligand substitutions; as such they are capable of either reacting with liberated phosphine or CO or alternatively of being reduced to the dianions. Elimination of CO followed by addition of phosphine occurs over an ETC ligand substitution path. On the reverse electrochemical sweep, the ligand-deficient 48-electron dianions can be reoxidized to the 47-electron monoanions. In some cases further oxidation to the ligand-deficient neutral 46-electron species M~CCO~(CO)+,L,~ occurs, which are converted back to the starting cluster complexes by ligand addition (EEC mechanism). The proposed reaction path including a homogeneous disproportionation is supported by a crossover phenomenon observed in the cyclic voltammograms of the trisphosphine-substituted clusters. Probably for steric reasons both the stability of the 49-electron cluster radical anions and the extent of ETC ligand substitution vs two-electron reduction increase with the number of methyl groups on the PR, ligands, Le., PMe, > ... > PPh,; moreover, they decrease with the number of PR, ligands. The opposite holds true for the stability of the 47-electron intermediates. Their remarkable stability in the case of M~CCO,(CO)~L, has allowed an investigation of their reactions with CO and phosphines from which the dissociative mechanism of the electron-transfer-induced ligand substitutions in the RCCo, clusters could be additionally confirmed. The cyclic voltammograms including the crossover phenomenon can be simulated by using the above-mentioned electrochemical steps and a ligand-dependent set of kinetic parameters. In addition to the well-established ETC-catalyzed substitution of CO by phosphine ligands, the measurements have shown that in the presence of CO the pho...
