The straightforward synthesis of a new unsymmetrical hydroxy-tethered N-heterocyclic carbene (NHC) ligand, HL, is presented. The free ligand exhibits an unusual OH-carbene hydrogen-bonding interaction. This OH-carbene motif was used to yield 1) the first tantalum complex displaying both a Fischer- and Schrock-type carbene ligand and 2) a unique NHC-based early/late heterobimetallic complex. More specifically, the protonolysis chemistry between the ligand's hydroxy group and imido-alkyl or alkylidene-alkyl tantalum precursor complexes yielded the rare monometallic tantalum-NHC complexes [Ta(XtBu)(L)(CH tBu) ] (X=N, CH), in which the alkoxy-carbene ligand acts as a chelate. In contrast, HL only binds to rhodium through the NHC unit in [Rh(HL)(cod)Cl] (cod=cycloocta-1,5-diene), the hydroxy pendant arm remaining unbound. This bifunctional ligand scaffold successfully promoted the assembly of rhodium/tantalum heterobimetallic complexes upon either 1) the insertion of [Rh(cod)Cl] into the Ta-NHC bond in [Ta(NtBu)(L)(CH tBu) ] or 2) protonolysis between the free hydroxy group in [Rh(HL)(cod)Cl] and one alkyl group in [Ta(NtBu)(CH tBu) ].

Recent proposals for molecular switches are reviewed that adopt a different spin state before and after the encounter with an analyte (metabolite) or physical stimulus (light). Spin‐state switching has significant potential to provide molecular MRI with a first line of probes operating by an off/on mode. Past efforts to design MRI sensors relied on strategies other than spin‐switching and thus mostly comprised probes only modulating the signal before and after encounter with the analyte (X%‐on/Y%‐on). The article thus essentially treats small‐molecule metal ion chelates, i.e. coordination compounds. Initially, a comprehensive treatment of design considerations for optimal spin‐state switches is furnished. The article then enters into a treatment of reported examples of spin‐state switches according to three response mechanisms: 1) changes in the redox state of the metal center; 2) changes in coordination geometry; and 3) modification of the surrounding ligand field, either by peripheral interaction with the analyte, or by replacement of a coordinating ligand by another.