The design rationale, synthesis, and preliminary radiolabeling evaluation of new N,N,O-type pyridyl-tert-nitrogen-phenol ligands for the [M(CO)(3)](+) core, where M = Tc-99m or Re, are described. The capability of the ligands to bind this technetium core is initially demonstrated by using the cold surrogate [Re(CO)(3)](+). NMR studies of the relevant rhenium tricarbonyl complexes indicate the formation of either a monomeric or a possible dimeric complex with each phenolic O atom bridging between two metal centers. Labeling with [Tc-99m(CO)(3)](+) provided further insight into the differences in complex formation on the dilute, no carrier added, level compared to the macroscopic scale at which the Re-1 counterparts were made. These new tridentate; monoanionic ligands are competent chelates in binding the [Tc-99m(CO)(3)](+) core because radiolabeling yields ranged from 85 to 99% and the resulting complexes were stable to cysteine and histidine challenges for as long as 24 h.