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, 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.