Mixed-chelate complexes of ruthenium have been synthesized using tridentate Schiff-base ligands (TDLs) derived from condensation of 2-aminophenol or 2-aminobenzoic acid with aldehydes (salicyldehyde, 2-pyridinecarboxaldehyde), and tmeda (tetramethylethylenediamine). [Ru-III(hpsd)(tmeda)(H2O)](+) (1), [Ru-III(hppc)(tmeda)(H2O)](2+) (2), [Ru-III(cpsd)(tmeda)(H2O)](+) (3) and [Ru-III(cppc)(tmeda)(H2O)](2+) (4) complexes (where hpsd(2-)=N-(hydroxyphenyl)salicylaldiminato): hppc(-)=N-(2-hydroxyphenyl-pyridine-2-carboxaldiminato); cpsd(2-)=(N-(2-carboxyphenyl)salicylaldiminato): cppc(-)=N-2-carboxyphenylpyridine-2-carboxaldiminato) were characterized by microanalysis, spectral (IR and UV-vis), conductance, magnetic moment and electrochemical studies. Complexes 1-4 catalyzed the epoxidation of cyclohexene, styrene, 4-chlorostyrene, 4-methylstyrene, 4-methoxystyrene, 4-nitrostyrene, cis- and trans-stilbenes effectively at ambient temperature using tert-butylhydroperoxide (t-BuOOH) as terminal oxidant. On the basis of Hammett correlation (log k(rel) vs. sigma(+)) and product analysis, a mechanism involving intermediacy of a [Ru-O-OBut](center dot) radicaloid species is proposed for the catalytic epoxidation process. (C) 2009 Elsevier B.V. All rights reserved.