Hydrogen Release From Silicon─Hydrogen Systems: From Surface Chemistry to Higher-Coordinate Molecular Mechanisms.

Hydrogen release from silicon-hydrogen systems spans a field ranging from extended silicon materials to discrete molecular compounds. Across these regimes, Si─H activation and H2 evolution arise from the interplay of structure, coordination environment, and access to reactive geometries. In extended silicon systems, hydrogen release is governed by structure-dependent recombinative desorption processes. Zintl phases bridge solid-state and molecular silicon chemistry by enabling transfer of polyanionic silicon units into solution, thereby providing access to discrete Si─H reactivity. In molecular systems, metal-mediated and metal-free pathways contribute to hydrogen release, including oxidative addition, σ-bond metathesis, electrophilic activation, cooperative heterolysis, and donor-assisted Si─H activation. These mechanistic manifolds form a continuum in which variations in coordination environment and electronic structure determine whether dehydrocoupling, hydrogen transfer, or bond-forming processes dominate. A recurring mechanistic feature is the involvement of higher-coordinate silicon species. However, productive H2 evolution depends on their formation, but also on the ability of the silicon framework to access reactive geometries through fluxional behavior and dynamic permutational isomerism. Overall, silicon-based hydrogen-release chemistry illustrates how structure, coordination, and molecular dynamics collectively govern bond activation and hydrogen evolution, providing a conceptual basis for the design of silicon-based hydrogen storage and release systems.