The oxidative dehydrodimerization of the phenylvinylidene manganese complexes (Z 5 -C 5 R 5 )(CO)(L)Mn=C=CHPh (5: R=Me, L=CO; 6: R=H, L=PPh 3 ) into the corresponding bisvinylidene compounds (Z 5 -C 5 R 5 )(CO)(L)Mn=C=CPhÐCPh=C=Mn(CO)(L)(Z 5 -C 5 R 5 ) (7: R=Me, L=CO; 8: R=H, L=PPh 3 ) was studied by cyclic voltammetry and chemical experiments. It was found that dehydrodimerization of 5 and 6 proceeds via direct C (b) ÐC (b) In this paper we report on the oxidative dehydrodimerization of manganese vinylidene complexes (Z 5 -C 5 R 5 )(CO)(L)Mn=C=CHPh (5: R=Me, L=CO; 6: R=H, L=PPh 3 ). Despite 5 and 6 having a close similarity to 1, their dehydrodimerization proceeds according to a different scheme, as will be shown below. (7) was carried out using a procedure similar to that for 1, 9 i.e. 5 was oxidized by AgBF 4 in dichloromethane at À40°C, the solution was kept at this temperature for 2 h and finally reduced by (Z-C 6 H 6 ) 2 Cr. The target complex 7 was obtained in 40% yield and characterized by 1 H and 13 C NMR spectroscopy as well as by mass spectrometry. We found that the yield of 7 can be increased to 85±90% if either potassium t-butylate or triethylamine is used in the final reaction step. In order to determine the mechanism of the dehydrodimerization process, we studied the electrochemical behavior of complexes 5 and 7 by cyclic voltammetry in CH 2 Cl 2 solution (see Table 1).Complex 7 displays two one-electron reversible oxidation waves at 0.17 V (peak A) and 0.58 V (peak B) (see Fig. 1). Complex 5 oxidizes irreversibly at 0.77 V (see Fig. 2, peak C). The reverse scanning of potentials revealed new cathodic peaks at 0.67 V and 0.27 V (peaks D and E respectively). These peaks do not coincide with the reduction peaks A', B' of 7 .Thus, 7 is not an initial product of transformation of 5 Á ,i.e. dehydrodimerization of 5 proceeds by a different mechanism than that for complex 1 (Scheme 1). We believe Scheme 1.Scheme 2. Figure 1. CV of bis-vinylidene complex 7 at 20°C. ). In order to confirm the validity of the mechanism shown in Scheme 3, we studied the oxidative dehydrodimerization of the triphenylphosphine-substituted phenylvinylidene complex (Z 5 -C 5 H 5 )(CO)(PPh 3 )Mn=C=CHPh (6), since we expected the product of direct C (b) ÐC (b) that the isomers differ in the orientation of the phenyl group relative to the cyclopentadienyl ring. Oxidative dehydrodimerization of 6 was carried out under the same conditions as the analogous processes for 1 and 5. The bis-vinylidene complex 8 proved to be rather unstable, and was obtained in 20% yield. Its structure was confirmed by IR-, NMR 1 H, and 31 P spectroscopy and mass spectroscopy. According to the NMR spectra, complex 8 exists as a mixture of two diastereomers in 3:1 ratio.Electrochemical oxidation of 6 and 8 was studied by cyclic voltammetry (CV) in dichloromethane solution. The cyclic voltammogram of the bis-vinylidene complex 8 showed two reversible one-electron oxidation peaks at 0.19 V and 0.61 V (Fig. 3, peaks F and G respectively). The cyclic voltam...

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