Existing examples in which the assembly of silanes and substrates by a transition metal center leads to transformations with a low activation barrier can be found for the formal transfer of silylium cations (Figure ). A mechanism put forth originally by Crabtree, and extensively refined by Brookhart and Oestreich invokes a 3-center-2-electron (3c-2e) interaction between a silane and an empty transition metal orbital as a crucial reaction intermediate. Binding of the silane to the metal center enables a low energy barrier transfer of a silylium equivalent to polar substrates such as ketones, alcohols, 14 CO2, and even weakly nucleophilic alkyl halides and dialkylethers. Inspired by the role of the 3c-2e interaction in providing a low energy pathway for the formal transfer of silylium, we aimed to access a corresponding 3c-3e silane σ-adduct to facilitate silyl radical transfer (Figure ). While many metal complexes featuring 3c-2e silane adducts have been studied and structurally characterized, to the best of our knowledge, there has been no report of a 3c-3e silane adduct. A substantial challenge in the stabilization of 3c-3e interactions is that one of the two species participating in the interaction is a radical, which can undergo further reaction with the 3c-3e adduct to generate two strong 2c-2e bonds (Figure ). Kinetic barriers for the reactions of radicals tend to be low, so that the low prevalence of 3c-3e interactions is readily accounted for by a decomposition pathway that is thermodynamically favorable, and, in the absence of steric or geometric restrictions, extremely facile. Ribas and coworkers reported a rare example of a 3c-3e adduct between a macrocycle-supported Cu(II) and a C-H bond on the ligand, which decomposes readily via a bimolecular pathway. Wayland previously demonstrated that the extreme steric bulk surrounding Rh(II) in 2 permitted EPR observation of 3c-3e interactions with ethylene (Figure ). Addition of ethylene to less sterically encumbered 1 leads to rapid addition of the metalloradicals across the ethylene double bond, likely via the intermediacy of a 3c-3e ethylene adduct of 1 (Figure ). Even for bulky 2, the 3c-3e adduct only proved stable at low temperature, and irreversible insertion of two ethylene molecules between a pair of Rh(II) porphyrin complexes was observed upon heating (Figure ). Prior work on Rh(II) porphyrin systems highlights four salient points: i) spin density from the rhodium center is partially shifted to the carbon center of the ethylene 3c-3e adduct; ii) this spin density can be used to form substrate-substrate bonds (Figure ) but iii) if sterically accessible, formation of substrate-catalyst bonds is preferred (Figure ) and iv) even if substrate-substrate bond formation occurs, two Rh(II) metalloradicals are converted into unreactive carbon ligated Rh(III) centers (Figure ). To stabilize a 3c-3e σ-adduct of a metalloradical, follow-up reactions with another metalloradical thus need to be prevented. Here we show that thermally stable 3c-3e adducts can be generated via geometric frustration. Rh(II) porphyrins that are siteisolated in a MOF matrix are not sterically encumbered and thus able to form σ-complexes with weakly donating silanes. The geometric frustration of Rh(II) sites in the MOF ensures that the σ-adduct cannot be approached by another metalloradical to furnish undesired closed-shell dimers (Figure ). The resulting stability of the 3c-3e adduct permits the assembly of tri-component transition state for direct silyl radical transfer with an activation barrier of 20 kcal•mol -1 (Figure ). An alternative pathway that proceeds via free silyl radicals is associated with a minimum energy barrier of ~35 kcal•mol -1 and was realized experimentally with the assistance of 390 nm photons, which carry an energy of 73 kcal•mol -1 . Both direct silyl transfer and release of free silyl radicals can be achieved from the 3c-3e σ-adduct, so that the relative facility of both pathways could be directly compared.