Competition between One- and Two-Electron Unimolecular Reactions of Late 3d-Metal Complexes [(Me₃SiCH₂)_nM]^–(M = Fe, Co, Ni, and Cu; n = 2–4)

Although organometallic complexes of the late 3d elements are known to undergo both one-and two-electron reactions, their relative propensities to do so remain poorly understood. To gain direct insight into the competition between these different pathways, we have analyzed the unimolecular gas-phase reactivity of a series of well-defined model complexes [(Me₃SiCH₂)_nM]⁻ (M = Fe, Co, Ni, and Cu; n = 2–4). Applying a combination of tandem-mass spectrometry, quantum-chemical computations, and statistical rate-theory calculations, we find several different fragmentation reactions, among which the homolytic cleavage of metal-carbon bonds and radical dissociations are particularly prominent. In all cases, these one-electron reactions are entropically favored. For the ferrate and cobaltate complexes, they are also energetically preferred, which explains their predominance in the corresponding fragmentation experiments. For [(Me₃SiCH₂)₄Ni]⁻ and, even more so, for [(Me₃SiCH₂)₄Cu]⁻, a concerted reductive elimination as a prototypical two-electron reaction is energetically more favorable and gains in importance. [(Me₃SiCH₂)₃Ni]⁻ is special in that it has two nearly degenerate spin states, both of which react in different ways. A simple thermochemical analysis shows that the relative order of the first and second bond-dissociation energies is of key importance in controlling the competition between radical dissociations and concerted reductive eliminations.