Ligand Effects on Transition-Metal Ion Reactivity: Primary and Secondary Reactions of Co⁺and Ni⁺ with Alkenes

The primary reactions of Co⁺and Ni⁺ with alkenes exhibit both C-H and C-C bond cleavages as well as direct condensation products. Both Co⁺ and Ni⁺ will dehydrogenate isobutene whereas Fe⁺ is unreactive with isobutene. Structural investigations of MC₄H₆ ⁺, generated from M = Co or Ni and isobutene, by collision-induced dissociation, ion-molecule reactions, and deuterium exchange indicate that this ion most likely is M⁺-butadiene. Evidently, skeletal rearrangement to butadiene is favored over formation of trimethylenemethane. Allylic hydrogen shifts from the alkene to the metal ion are considerably more facile for Co⁺ than for Ni⁺, as evidenced by the amount of dehydrogenation observed in the primary and secondary reactions with alkenes. The secondary reactions of the metal ions are significantly different. Co(alkene)⁺reacts readily with alkenes, generating both ligand displacement and dehydrogenation products. Ligand coupling, presumably in the form of Diels-Alder reactions, is observed for Co⁺and not for Ni⁺. Ni (alkene)⁺is essentially unreactive with alkenes, usually forming only condensation complexes.