Development of new ligands for fac-M(OH2)(3)(CO)(3)(+) (M = Re, Tc-99m) led the investigation with S-(pyridin-2-ylmethyl)L-cysteine, 1. The ligand 1 has potential to coordinate with the metal through three different tridentate modes: tripodal through cysteine (O,N,S) and two linear involving the S-pyridyl and cysteine (O,S,N-Py, N,S,N-Py). From the reaction with 1, two species were observed in the H-1 NMR, where the primary product was the linear fac-Re(N,S,N-Py-1)(CO)(3)(+), 2a, complex. To identify the coordination mode of the minor product, functionalized analogues of 1 were prepared from S -(pyridin-2-ylmethyl)-BOC-L-cysteine-methyl ester, 3, with orthogonal protecting groups on the C terminus (methyl ester) in S-(pyridin-2-ylmethyl)-L-cysteine methyl ester, 4, or N terminus (Boc) in S-(pyridin-2-ylmethyl)-BOC-L-cysteine, 6, that specifically directed the coordination mode of fac-M(H2O)(3)(CO)(3)(+) to either N,S, N-Py or O,S,N-Py, respectively. Two diastereomers [fac-Re(CO)(3)(N,S,N-Py-4)](+), 5a and 5b, were observed and independently characterized by X-ray structure analysis and NMR in high yield with 4. Surprisingly, the O,S,N-Py Re complex with ligand 6 was not observed and simplified versions, 3-(pyridin-2-ylmethylthio) propanoic acid, 7, and 2-(pyridin-2-ylmethylthio)acetic acid, 8, were investigated. Ligand 7 did not yield the desired linear tridentate O,S,N-Py product. However, the shorter ligand 8 formed fac-Re(CO)(3)(O,S,N-Py-8), 9, in high yield. Tc-99m labeling studies were conducted and yielded similar results to the rhenium complex and effective (>99%) at 10(-5) M ligand concentration.