In developing 99mTc complexes as potential brain-imaging agents, we investigated the coordination chemistry of ligands containing sulfur and nitrogen donor atoms with the general formula R-CH2CH2N(CH2CH2SH)2 (R = C2H5S, (C2H5)2N). These ligands act as tridentate SNS chelates to the TcO3+ core, leaving open one coordination site cis to the oxo group. In reactions with the highly reactive [99TcOCl4]- precursor, this vacancy was occupied by a chlorine atom. When the ligands reacted in the presence of 4-methoxythiophenol, using 99Tc(V)-gluconate as precursor, the vacancy was filled with 4-methoxythiophenol, which acted as coligand. Thus neutral mixed ligand complexes of the general formula [TcO((SCH2CH2)2NCH2CH2R)X], where X = Cl or 4-methoxythiophenol, were synthesized. The complexes were characterized by UV-vis, IR, 1H NMR, crystallographic, and elemental analyses. The crystal structures of 3a (R = C2H5S, X = Cl) and 4b (R = (C2H5)2N, X = 4-methoxythiophenol) demonstrated that the coordination geometry is trigonal bipyramidal with the N1 and Cl or S3 occupying the apical positions and the basal plane defined by the S1 and S2 of the tridentate ligand and the oxo group. The complexes 4a(99mTc) (R = C2H5S, X = 4-methoxythiophenol) and 4b(99mTc) were prepared using 99mTc-glucoheptonate as precursor and were purified by HPLC. Biodistribution in mice showed high initial brain uptake (3.68% and 3.56% dose/organ for 4a(99mTc) and 4b(99m-Tc), respectively). Complex 4b(99mTc) displayed significantly higher brain/blood values and prolonged retention in brain as well. The results suggest that structural modifications based on configurations 4a,b may provide novel 99mTc brain-imaging agents with improved biological characteristics.