Electron transfer versus nucleophilic pathways in the ion-pair annihilation of organoborate anions by carbonylmanganese(I) cations

Substituted carbonylmanganese cations [Mn(CO)(5)L](+), where L = py, PPh3 and PPh2Me, readily react with various organoborate anions (tetramethylborate, methyltriphenylborate and tetraphenylborate) in THF solution to afford a mixture of dimanganese carbonyls, hydridomanganese carbonyls and alkylmanganese carbonyls. The formation of the dimanganese carbonyl dimers as well as the hydridomanganese carbonyls suggests the involvement of 19-electron carbonylmanganese radicals that stem from an initial electron transfer. On the other hand, the acetonitrile-substituted analogue [Mn(CO)(5)(CH3CN)](+) reacts with the same berate anions to afford the alkylated RMn(CO)(5), where R = CH3 and C6H5, as the sole carbonylmanganese product. As such, this alkylative annihilation is best formulated as a direct attack on the carbonyl carbon by the berate nucleophile. The two different pathways can be understood in terms of the balance between the electrophilicity of the carbonyl ligand and the electron affinity of the carbonylmanganese cation. (C) 1999 Elsevier Science S.A. All rights reserved.