Oxidative dehydrodimerization of some phenylvinylidene complexes of manganese is studied by cyclic voltammetry. In the case of (η5-C5H5)(CO)2Mn=C=C(H)Ph, the process occurs as the homolysis of the Cβ–H bond in the radical cation of {(η5-C5H5)(CO)2Mn=C=C(H)Ph}+· and the dimerization of intermediate σ-phenylethinyl cation [(η5-C5H5)(CO)2Mn–C≡C–Ph]+ to a binuclear dication of bis-carbine type (η5-C5H5)(CO)2Mn+≡C– C(Ph)=C(Ph)–C≡Mn+(CO)2(η5-C5H5). The reduction of the latter leads to binuclear bis-vinylidene complex (η5-C5H5)(CO)2Mn=C=C(Ph)–C(Ph)=C=Mn(CO)2(η5-C5H5). Oxidative dehydrodimerization of complexes (η5-C5R5)(CO)(L)Mn=C=C(H)Ph (R = H, L = PPh3; R = Me, L = CO) occurs through the immediate Cβ–Cβ coupling of radical cations {(η5-C5R5)(CO)(L)Mn=C=C(H)Ph}+· and yields binuclear dication bis-carbine complexes (η5-C5R5)(CO)(L)Mn+≡C–C(H)(Ph)–C(H)(Ph)–C≡Mn+(CO)(L)(η5-C5R5), whose reduction leads to neutral compounds (η5-C5H5)(CO)2Mn=C=C(Ph)–C(Ph)=C=Mn(CO)(L)(η5-C5H5). Complex (η5-C5H5)(CO)2Mn=C=C(Ph)–C(Ph)=C=Mn(CO)2(η5-C5H5) undergoes the oxidation-induced nucleophilic addition of water, forming cyclic bis-carbene product with a bridge heterocyclic ligand (μ-3,4-diphenyl-2,5-dihydro-2,5-diylidene)-bis-(η5-cyclopentadienyldicarbonyl manganese).