Abstract An unprecedented quantum tunneling effect has been observed in catalytic Si?H bond activations at room temperature. The cationic hydrido‐silyl‐iridium(III) complex, {IrSiMe(o‐C6H4SMe)2(H)(PPh3)(THF)}BArF4, has proven to be a highly efficient catalyst for the hydrolysis and the alcoholysis of organosilanes. When triethylsilane was used as a substrate, the system revealed the largest kinetic isotopic effect (KIESi?H/Si?D=346±4) ever reported for this type of reaction. This unexpectedly high KIE, measured at room temperature, together with the calculated Arrhenius preexponential factor ratio (AH/AD=0.0004) and difference in the observed activation energy (EaD ?EaH )=34.07?kJ?mol?1 are consistent with the participation of quantum tunneling in the catalytic process. DFT calculations have been used to unravel the reaction pathway and identify the rate‐determining step. Aditionally, isotopic effects were considered by different methods, and tunneling effects have been calculated to be crucial in the process.
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