Oxidative dehydrodimerization of manganese vinylidene complexes (η 5 ‐C 5 R 5 )(CO)(L)MnCCHPh (R = Me, L = CO; R = H, L = PPh 3 )

The oxidative dehydrodimerization of the phenylvinylidene manganese complexes (η 5 ‐C 5 R 5 )(CO)(L)MnCCHPh ( 5 : R=Me, L=CO; 6 : R=H, L=PPh 3 ) into the corresponding bis‐vinylidene compounds (η 5 ‐C 5 R 5 )(CO)(L)MnCCPhCPhCMn(CO)(L)(η 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 (β) C (β) coupling of the radical cations 5 +· and 6 +· to give dicationic bis‐carbyne complexes [(η 5 ‐C 5 R 5 )(CO)(L)Mn≡CCPhHCPhHC≡Mn(CO)(L)(η 5 ‐C 5 R 5 )] 2+ ( 10 ++ : R=Me, L=CO; 11 ++ : R=H, L=PPh 3 ), which can be converted into bis‐vinylidene complexes 7 and 8 respectively by reduction with (η‐C 6 H 6 ) 2 Cr. In this process, binuclear 19‐electron bis‐carbyne compounds 10 ·· and 11 ·· undergo further transformation into the dimers 7 and 8 via homolysis of C (β) H bonds of the bis‐carbyne ligand, thus exhibiting rare examples of reductive activation of CH bonds in 19‐electron transition metal carbyne complexes. Copyright © 2002 John Wiley & Sons, Ltd.