Polyaminopolycarboxylate (pac) ligands and the imidazole-based chelate HAPH compensate iron triad metal centers for π-electron withdrawal by additional ligands. This stabilizes Fe-O and Ru-O oxenoid species in the oxidation of Fe^II/III or Ru^II/III by O₂, H₂O₂ or t-BuOOH. It has been commonly thought that O₂ and H₂O₂ are reduced by Fe^II(pacs) by one-electron reduction sequences which contribute to Fenton chemistry and produce free HO∙. Current evidence implicates HO∙ formation via a minor side reaction to the main pathway in which H₂O₂ is heterolytically cleaved, forming LFe^IV(:Ö:^2-) ferryl intermediates. Ferryl intermediates at one unit higher oxidation level, formally Fe^V(:Ö:^2-) ↔ Fe^III(:Ö:), are the active DNA cleavage agents in O₂-activated DNA cleavage by the antitumor drugs bleomycin and by the small molecule [Fe^II(HAPH)]⁺. Development of other synthetic small molecule complexes which offer controlled DNA cleavage in the absence of free HO∙ is an important arena of research. Current studies with ruthenium systems are examined. Bleomycin epoxidizes stilbenes via its ferryl form. The epoxidation of stilbenes by a ruthenium-O-atom complex, [Ru^III O(hedta)], which is at a parallel oxidation state level of Ru^V(:Ö:^2-) ↔ Ru^III(:Ö:) is discussed in comparison with 16 other published ruthenyl oxygen epoxidation catalysts. An intermediate of radicaloid character rather than concerted O-atom transfer accounts for the main epoxidation pathway. The enhanced π-donation of Ru^II(pacs) toward π-acceptors such as olefins, pyridines, CO and pyrazines is well-known. However, the coordination of [Ru^II (hedta)]^- to the C-5-C-6 bond of pyrimidine nucleobases offers a new mode of DNA metallation of potential importance to chemotherapy. The recent observations with Ru^II(pacs) in forming η² complexes with simple olefins and nucleobase ligands are described.