Figure . A) Direct transfer of a silyl radical equivalent to an acceptor is associated with a prohibitively high entropic barrier, so that radical hydrosilylation instead proceeds in a step-wise manner via the intermediacy of free silyl radicals (B). If a 3c-3e σ-adduct between the catalyst and silane could be accessed, direct silyl radical transfer would be rendered entropically feasible (C). (D) A MOF-based Rh(II) catalyst that can stabilize a 3c-3e adduct is able to promote thermal hydrosilylation with a barrier of ΔG ǂ = 20 kcal•mol -1 . Spheres represent individual molecules that are present. Small red arrows indicate the anticipated entropy contribution to the minimum energy pathway; large red arrows depict the total energy associated with the minimum energy pathway.