Trinuclear organometallic macrocycles are able to complex amino acids and peptides in aqueous solution. The complexation, which is driven by hydrophobic and π − π interactions, induces a large shift of the 1H NMR signals of the guest. The organometallic host can thus be employed as an NMR shift reagent. Structurally related trinuclear macrocycles selectively bind lithium and fluoride ions in protic as well as non-protic solvents. The binding event can be followed electrochemically or can be transduced via a subsequent chemical reaction into a change of color, which offers the possibility to construct chemosensors for these ions. The organometallic complex [Cp*RhCl2]2 can be combined with the dye azophloxine to build an indicator displacement assay for the sequence-specific detection of histidine- and methionine-containing peptides. The assay allows one to detect peptides with His/Met residues close to the N-terminus down to a concentration of 0.3 μM in water at neutral pH.