Manganese coordination complexes with utility as magnetic resonance probes and as biological reductant sensors are disclosed. In one embodiment, ligands can stabilize both the Mn2+ and Mn3+ oxidation states. In the presence of a reductant such as glutathione, low relaxivity MnIII-HBET is rapidly converted to high relaxivity MnII-HBET with a 3-fold increase in relaxivity, and concomitant increase in magnetic resonance signal. In another embodiment, ligands were designed to chelate Mn(ll) in a thermodynamically stable and kinetically inert fashion while allowing for direct interaction of Mn(ll) with water. In yet another embodiment, high molecular weight multimers containing six Mn(ll) chelators were prepared. The high molecular weight results in slower tumbling of the molecules in solution and can strongly enhance the Mn(ll) relaxivity.