We have previously shown that Rh(II) centers site-isolated in the porphyrin-containing MOFs PCN-224 (Rh(II)-3) or PCN-222 (Rh(II)-4) are efficient hydrosilylation catalysts. Comparison with homogeneous model systems established that heterogenization substantially benefited both the catalyst's activity and selectivity. In this work, we set out to elucidate the origin of the reactivity differences that emerge when Rh(II) porphyrin centers are placed in a MOFenvironment. Prior to in-depth mechanistic studies, we excluded that hydrosilylation proceeded via a radical chain reaction initiated by Rh(II) based on the following evidence: