Dynamic Speciation as a Hidden Driver of Reactivity: Lessons from Organozinc Chemistry.

Organometallic reactivity is traditionally rationalized in terms of well-defined molecular structures. In this Account, we argue that for organozinc compounds, this picture is incomplete: their reactivity is governed not by isolated molecules but by dynamic speciation landscapes─equilibrating Lewis acid-base adducts, interconverting aggregates, species generated by Schlenk redistribution, transient intermediates, and phase-confined assemblies whose composition evolves throughout a reaction. Drawing on more than two decades of our group's work, we show that reaction outcomes emerge from the identity and evolution of these reactive ensembles rather than from any single species that can be crystallized and assigned a formula. Moving from static structures to reactive landscapes, we first consider molecular complexes as programmable reactive platforms, in which aggregation and donor coordination generate families of interconverting species that can be deliberately tuned. We then recast Schlenk equilibria, long regarded as a passive redistribution nuisance, as an active determinant of reactivity: by continuously reshaping the pool of available reagents, they define which species are present when the reaction occurs and thereby redirect its course. Asymmetric catalysis puts the ensemble picture to its sharpest test. For example, chiral amplification in the dialkylzinc addition to aldehydes is classically explained by an equilibrium between catalytically active and dormant aggregates, a two-level picture of the reactive ensemble. Our work on bifunctional alcohols indicates that this ensemble is richer: we isolated an unprecedented tetranuclear zinc aggregate, pointing to higher-nuclearity species as possible sources of the catalytically competent centers. The unexpected incorporation of the solvent into one such species further showed that even the reaction medium participates in shaping the ensemble. Conversely, where the ensemble proved simple enough to define─as in the epoxidation of enones by zinc alkylperoxides─it became a design tool: systematic ligand modification delivered enantioselectivities reaching 91% ee. Speciation control overturns an even longer-standing assumption. The reaction of organozinc compounds with O2, long treated as an uncontrolled radical process, emerges instead as a sequence of elementary steps governed by latent Lewis acidity, inner-sphere electron transfer (ISET), and product inhibition─steps that allow reactive intermediates to be intercepted, giving controlled access to alkylzinc alkylperoxides and related oxygenated species. Beyond homogeneous solution, we show how phase-dependent speciation modulates reactivity in the solid state: mechanochemical and phase-confined environments, together with supramolecular self-assembly, stabilize ensembles and open pathways─and give access to functional materials─inaccessible in solution, establishing a continuum linking molecular speciation to materials formation. Across these systems, one principle recurs: the reactive species is often not an isolated molecule but a transient member of an evolving ensemble. We close with three challenges we consider decisive: dynamic speciation must be monitored directly under operating conditions, combining in situ spectroscopy, diffraction, computation, and kinetics; aggregation, donor coordination, Schlenk equilibria, and confinement should be treated as design variables rather than complications; and mastering organozinc speciation offers a route to more predictable reactivity and to rational molecular-to-material transformations.