Electron transfers (ETs) in mixed-valent ferrocene/ferrocenium materials are ordinarily facile. In contrast, the presence of 1:1 mixed-valent ferrocenated thiolates in the organothiolate ligand shells of <2 nm diameter Au-225, Au-144, and Au-25 monolayer-protected clusters (MPCs) exerts a retarding effect on ET between them at and below room temperature. Near room temperature, in dry samples, bimolecular rate constants for ET between organothiolate-ligated MPCs are diminished by the addition of ferrocenated ligands to their ligand shells. At lower temperatures (down to similar to 77 K), the thermally activated (Arrhenius) ET process dissipates, and the ET rates become temperature-independent. Among the AU(225), AU(144), and Au-25 MPCs, the temperature-independent ET rates fall in the same order as at ambient temperatures: Au-225 > Au-144 > AU(25). The MPC ET activation energy barriers are little changed by the presence of ferrocenated ligands and are primarily determined by the Au nanoparticle core size.