The talk will discuss two projects that feature rational ligand design as a means of controlling the redox chemistry of transition metal ions. In the first project, an iron complex with a bulky tetradentate ligand is used to guide the regioselectivity of catalyzed C-H activation away from the thermodynamically weakest bonds in alkane substrates. An additional benefit provided by the ligand is that it allows the study of reactive intermediates in the catalyzed oxidation of C-H bonds by O2. In the second project, a redox-active ligand is used to prepare a manganese-containing MRI contrast agent that is capable of detecting H2O2 under physiologically relevant conditions. The reaction with H2O2 couples the mononuclear contrast agent into a binuclear species that is less paramagnetic per Mn(II) ion, reducing the overall relaxivity.