An H/D kinetic isotope effect (KIE) of 80 is found at -20 degrees C for the oxidation of 9,10-dihydroanthracene by [Fe-IV(O)(TMCS)](+), a complex supported by the tetramethylcyclam (TMC) macrocycle with a tethered thiolate. This KIE value approaches that previously predicted by DFT calculations. Other [Fe-IV(O)(TMC)(anion)] complexes exhibit values of 20, suggesting that the thiolate ligand of [Fe-IV(O)(TMCS)](+) plays a unique role in facilitating tunneling. Calculations show that tunneling is most enhanced (a) when the bond asymmetry between C-H bond breaking and O-H bond formation in the transition state is minimized, and (b) when the electrostatic interactions in the O---H---C moiety are maximal. These two factors which peak for the best electron donor, the thiolate ligand afford a slim and narrow barrier through which the H-atom can tunnel most effectively.