The carbon-based substituents (listed in blue in Figure ) stand out from their non-carbon counterparts. They enable the lowest transmission for the Si222 and Ge222 systems, and their transmission correlates with the electron withdrawing properties of the substituents as given by the empirical Hammett constants. These correlations are quite strong compared to the correlations reported in other single-molecule conductance studies. Carbon-based substituents thus provide an opportunity for fine-tuning s-interference effects in silicon and germaniumbased compounds.