Formation of metal-metal bonds by ion-pair annihilation. Dimanganese carbonyls from manganate(-I) anions and manganese(I) cations

Article abstract of DOI:10.1021/om00152a010

The coupling of the anionic Mn(CO)₅^- and the cationic Mn(CO)₆⁺ occurs upon mixing to afford the dimeric Mn₂(CO)₁₀ in essentially quantitative yields. Dimanganese decacarbonyl is formed with equal facility from the coupling of Mn(CO)₅^- with Mn(CO)₅(py)⁺ and Mn(CO)₅(NCMe)⁺. By way of contrast, the annihilation of Mn(CO)₄PPh₃^- with Mn(CO)₆⁺ yields a pair of homo dimers Mn₂(CO)¹⁰ and Mn₂(CO)₈(PPh₃)₂ together with the cross dimer Mn₂(CO)₉PPh₃. Extensive scrambling of the carbonylmanganese moieties also obtains with Mn(CO)₄P(OPh)₃^- and Mn(CO)₅PPh₃⁺, as indicated by the production of Mn₂(CO)₈[P(OPh)₃]₂, Mn₂(CO)₈[P(OPh)₃](PPh₃), and Mn₂(CO)₈(PPh₃)₂ in more or less statistical amounts. These diverse Mn-Mn couplings can be accounted for by a generalized formulation (Scheme VI), in which the carbonylmanganese anions Mn(CO)₄P^- and the cations Mn(CO)₅L⁺ undergo an initial electron transfer to produce Mn(CO)₄P^? and Mn(CO)₅L^?, respectively. The behaviors of these 17- and 19-electron radicals coincide with those independently generated in a previous study of the anodic oxidation of Mn(CO)₄P^- and the cathodic reduction of Mn(CO)₅L⁺, respectively. The facile associative ligand substitution of 17-electron carbonylmanganese radicals by added phosphines provides compelling evidence for the interception of Mn(CO)₄P^? and its interconversion with 19-electron species in the course of ion-pair annihilation. The reactivity trend for the various ion pairs qualitatively parallels the driving force for electron transfer based on the oxidation and reduction potentials of Mn(CO)₄P^- and Mn(CO)₅L⁺, respectively, in accord with the radical-pair mechanism in Scheme VI.