A fluorinated, air-stable cobalt(ii) complex serves as a turn-on 19F magnetic resonance imaging (MRI) tracer for reactive oxygen species including H2O2. Upon oxidation with H2O2, the complex converts from paramagnetic high spin CoII to diamagnetic low spin CoIII resulting in a chemical shift change and enhancement in 19F NMR signal. Further, the oxidation can be reversed in the presence of reductant Na2S2O4. The turn-on response is demonstrated by 19F MRI, characterized by a ∼2-3 fold enhancement in signal.