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A pair of tridentate ligands incorporating heterocyclic hydrazones of 9,10‐phenanthrenequinone viz,. pyridyl‐hydrazino‐phenathrenequinone HLPy1a and benzothiazolyl‐hydrazino‐phenathrenequinone HLBenz1b have been synthesized and both behave as monoanionic towards nickel(II), forming meridional octahedral complexes of type [Ni(LPy)2] 2a and [Ni(LBenz)2] 2b as evident from single crystal X‐ray diffraction studies. The complexes are electro‐active in solution and nature of redox orbitals has been analysed by theoretical means. They display two oxidative and two reductive responses that have been ascribed to the redox processes of coordinated ligands. The electronic absorption spectral patterns of two complexes are analogous barring the fact that lower energy transition for 2b is marginally bathochromically shifted relative to that of 2a and it has been clarified by TD‐DFT studies. Anti‐bacterial efficacy of the ligands HLPy1a, HLBenz1b and complexes [Ni(LPy)2] 2a, [Ni(LBenz)2] 2b against four Gram‐positive and four Gram‐negative strains has been explored. [Ni(LPy)2] 2a exhibits more pronounced efficacy than [Ni(LBenz)2] 2b and these are greater than those of the corresponding ligands. The mode of action of 2a is essential via DNA damage while protein leakage and membrane lipid damage were observed upon treatment with 2b. [ABSTRACT FROM AUTHOR]

Two types of bivalent ruthenium complexes [RuLPy(CO)Cl(PPh3)3] 3 and [RuLBenz(CO)Cl(PPh3)3] 4 were synthesized starting from [RuH(CO)Cl(PPh3)3] and heterocyclic hydrazone ligands 1 and 2 respectively. X-ray diffraction studies reveal that in both types of complexes, the ligands behave as monoanionic bidentate Nhydrazonyl and Npyridyl/Nbenzothiazolyl donors towards ruthenium(II), thereby furnishing four-membered metallacycles. The multiple transitions in the electronic spectra have been elucidated by time dependent density functional theory (TDDFT). The redox active nature of both 3 and 4 has been substantiated from the well-defined oxidative responses and theoretical scrutiny corroborates that one of them is exclusively ligand centred while the other one is chiefly due to the RuII/RuIII oxidation. Both the types of complexes exhibit a significant antimicrobial activity, although the activity of 4 is more prominent, particularly over Pseudomonas. These are analyzed by measuring ZOI, MIC as well as the extent of membrane damage and protein leakage studies. The complexes probably cause free-radical facilitated oxidative damage to the bacterial cells during the course of their activity. [ABSTRACT FROM AUTHOR]