The new pentadentate amine thioether thiolate ligand 'N2H2S3'-H-2 (= 2,2'-bis(2-mercaptophenylamino)diethyl sulfide) (3) was synthesized in order to obtain iron and ruthenium complexes with high electron densities at the metal centers. The reaction of 'N2H2S3'(2-) With Fe2+ yielded the dinuclear high-spin complex [Fe('N2H2S3')](2) (5). Complex 5 added CO to give the low-spin complex [Fe(CO)('N2H2S3')] (6) whose low frequency nu(CO) (1932 cm(-1)) indicates a high electron density at the iron center and a strong Fe-CO bond. However, 6 is labile and readily dissociates CO in solution. Treatment of suitable ruthenium precursor complexes with 'N2H2S3'(2-) yielded [Ru(Co)(PCy3)('N2H2S3')] (7), [Ru(PPr3)(2)('N2H2S3')] (8), [Ru(PR3)('N2H2S3')] (R = PT (9), Ph (10)), and [Ru-(NO)('N2HS3')] (13). In complexes 7 and 8,'N2H2S3'(2-) acts as a tetradentate ligand. When heated in solution, complex 8 dissociates one PPr3 ligand to give 9. Complex 13 contains the trisanionic 'N2HS3'(3-) resulting from deprotonation of one amine NH function. All [Ru(L)('N2H2S3')] complexes proved inert toward dissociation of the Ru-L bonds. The NH functions of [M(L)('N2H2S3')] complexes are acidic and show H+/D+ exchange reactions with D2O. Methylation of the thiolate donors in 10 yielded the thioether derivative [Ru(PPh3)('N2H2S3'-Me-2)]I-2 (11) whose PPh3 ligand is as inert to substitution as that of 10. Complex 11 can reversibly be deprotonated to give [Ru(PPh3)('N2HS3'-Me-2)]I (12). NMR spectroscopic investigations showed that the deprotonation/protonation reactions of 11 and 12 are stereoselective. In contrast, protonation of 13 with HBF4 gives two diastereomers of the corresponding [Ru(NO)('N2H2S3')]BF4 salt (14). X-ray structure analyses of 5, 6, 9, and 11 and NMR spectra showed that the 'N2H2S3'(2-) ligand and its derivatives bind to the metal centers in the same fashion which combines fac and mer coordination of the donor atoms. The [MN2S3] cores of all complexes have an analogous C-1 symmetrical structure in which both the two N and the two terminal 5 donors assume cia positions.