The presence of multiple oxidation and spin states of first-row transition-metal complexes facilitates the development of switchable MRI probes. Redox-responsive probes capitalize on a change in the magnetic properties of the different oxidation states of the paramagnetic metal ion center upon exposure to biological oxidants and reductants. Transition-metal complexes that are useful for MRI can be categorized according to whether they accelerate water proton relaxation (T 1 or T 2 agents), induce paramagnetic shifts of 1 H or 19 F resonances (paraSHIFT agents), or are chemical exchange saturation transfer (CEST) agents. The various oxidation state couples and their properties as MRI probes are summarized with a focus on Co(II)/Co(III) or Fe(II)/Fe(III) complexes as small molecules or as liposomal agents. Solution studies of these MRI probes are reviewed with an emphasis on redox changes upon treatment with oxidants or with enzymes that are physiologically important in inflammation and disease. Finally, we outline the challenges of developing these probes further for in vivo MRI applications.

First‐row transition metal (TM) complexes are of interest as members of a relatively new class of coordination complexes that produce MRI contrast through CEST (chemical‐exchange saturation transfer). CEST reduces the intensity of the water protons upon exchange of a selectively magnetically saturated proton with the water proton pool. Paramagnetic TM complexes produce large paramagnetically shifted protons, which facilitates CEST signal intensity by avoiding tissue background signal (paraCEST agents) and also allowing for faster proton‐exchange rates. TM paraCEST agents have certain advantages over lanthanide paraCEST agents including the possibility of tuning oxidation and spin states to allow for the development of responsive agents. Moreover, the covalent bonding character of the TM ions bestows large through‐bond contributions to the hyperfine shifts of protons, which is beneficial for paraCEST agents. New applications include the development of TM water proton shift reagents (SRs) for improvement of sensitivity through incorporation into supramolecular assemblies. Liposomes with encapsulated and amphiphilic SRs produce lipoCEST, and cells loaded with SRs produce cellCEST. This article summarizes TM paraCEST agents and SRs and highlights new research directions.