Rhenium, the third-row congener of technetium, is often used to develop the macroscopic chemistry of potential Tc-99m diagnostic radiopharmaceuticals. The rhenium analogues to Tc-99m-furifosmin are being developed for potential radiotherapy of multidrug-resistant tumors. Complexes of the form trans-[M-III(PR3)(2)(N2O2-Schiff base)](+) are of interest for the potential imaging and treatment of multidrug-resistant tumors. Reaction of the tetradentate Schiff ligand 4,4'-[(1E,l 'E)[ethane-1,2-diylbis(azanylylidene)]bis(methanylylidene)]bis(2,2,5,5-tetramethy1-2,5-dihydrofuran-3-ol) (tmf(2)enH(2)) with the M(V) starting materials (nBu(4)N)[TcOCl4] and (nBu(4)N)[ReOCl4] gave the monomeric products trans-[TcOC1(tmf(2)en)] and trans-[ReOCl(tmf(2)en)], respectively. Reduction of in situ formed trans-[ReOCI(tmf(2)en)] by various tertiary phosphines yielded disubstitued Re(III) products of the general type trans-[Re-III(PR3)(2)(tmf(2)en)r. The rhenium(III) compounds were found to be water-soluble and stable in aqueous solution. Reversible Re-III/Re-IV and Re-III/Re-II redox processes were observed at about 0.8-0.9 and -0.65 to -0.8 V, respectively, for each of the rhenium(III) species. Reaction of in situ formed trans-TcOCl(tmf(2)en) with triethylphosphine yielded the reduced, disubstituted trans-[Tc(PEt3)(2)(tmf(2)en)PF6. A reversible Tc-III/Tc-II redox couple was observed for the technetium(III) species, about 200 mV less negative than their rhenium(III) analogues, in addition to an irreversible Tc-III/Tc-IV process. All compounds were characterized using conventional spectroscopic techniques, single-crystal Xray crystallography, and cyclic voltammetry.