The oxidative dehydrodimerization of the phenylvinylidene manganese complexes (η5-C5R5)(CO)(L)Mn=C=CHPh (5: R=Me, L=CO; 6: R=H, L=PPh3) into the corresponding bis-vinylidene compounds (η5-C5R5)(CO)(L)Mn=C=CPh-CPh=C=Mn(CO)(L)(η5-C5R5) (7: R=Me, L=CO; 8: R=H, L=PPh3) 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 [(η5-C5R5)(CO)(L)Mn≡C-CPhH-CPhH-C≡Mn(CO)(L)(η5-C5R5)]2+ (10 ++: R=Me, L=CO; 11 ++: R=H, L=PPh3), which can be converted into bis-vinylidene complexes 7 and 8 respectively by reduction with (η-C6H6)2Cr. 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 C-H bonds in 19-electron transition metal carbyne complexes.