ABSTRACT Four tridentate phenolic pyridylhydrazones (R'R?C=N-NHpy) designated H(5-R)-bapph were synthesized by the Schiff-base condensation reaction of 2-hydrazinopyridine with 2 hydroxyacetophenone or its para-substituted derivatives (R = Me, OMe or NO2). The polypyridyl ligand dpkph was produced alike with di(2-pyridyl) ketone (dpk) and 2 hydrazinopyridine as starting materials. All five hydrazones were formulated and identified using relevant analytical and spectroscopic techniques. As exemplified by the single-crystal X-ray structure of H(5-NO2)-happh, the phenolic pyridylhydrazones crystallize as dimers whereby the ligand molecules are linked by N-H...Npy hydrogen bonds. In contrast, the hydrazinic N-H group in dpkph is intramolecularly hydrogen-bonded to the nitrogen atom of the pyridyl ring attached to the imine carbon. A striking trend in the 'H-NMR chemical shifts of the phenolic protons of the H(5-R)-happh ligands consistent with the electronic effects of the substituent groups was established. As expected, the hydrazinic proton of dpkph is by far more deshielded than those of H(5-R)-happh. Reaction of H(5-NO2)-happh with Ni(C104)2.6H2O in stoichiometric or equimolar amounts in MeOH afforded the bis(chelate) molecular complex [Ni" (5-NO2)-happh}2]. In sharp contrast, the other phenolic pyridylhydrazones underwent this reaction under the same conditions to yield the bis(chelate) ionic compounds [N;"{H(5-R)-happh} {(5-R)-happh]C104 (R = H, Me or OMe). Evidently, this reaction is substituent-dependent: the electron-withdrawing nitro group imparts greater acidity to the phenolic moiety than do the electron-donating groups (PK, values for 5-R-phenols: 7.15, 9.89, 10.17, and 10.21 when R = NO2, H, Me and OMe, respectively). Treatment of Ni"{H(5-R)-happh} {(5-R)-happh}]C104 with a base converted these ionic compounds to the corresponding molecular complexes [Ni"{(5-R)-happh}2] quantitatively. The electronic effect of the substituents of H(5-R)-happh was established crystallographically with the single-crystal X-ray structures of the monomeric [Ni"{(5-NO2) happh}2] and dimeric [Ni''(Hhapph)(happh)]2(C104)2 compounds. The structure of the latter complex consists of two similar, but not identical, complex cations linked via 0- H O hydrogen bonds. All of these bis(chelate) nickel(II) complexes, namely [Ni" {(5-R)-happh}2] and [Ni" {H(5-R)-happh} {(5-R)-happh}]CIO4, are pseudo-octahedral and paramagnetic with two unpaired electrons. Terary hydrazone complexes of Nill and Cu" of the type [M'{(5-R)-happh}(L-L)]C104 (M= Ni or Cu; R = H, Me, OMe or NO2); L-L = a bidentate dipyridyl ligand) were synthesized by reaction of equimolar amounts of a phenolic pyridylhydrazone ligand, Ni(C104)2-6H20 and a dipyridyl ligands (L-L = bipy, 4,4'-Me-bipy, phen or 2,9-Me2-1,10-phen). Comparative crystallographic analyses of representative Ni" and Cu" ternary complexes were carried out. Whilst these complexes have a distorted square pyramidal coordination sphere in common, a conspicuous difference is in the axial coordinate bond, which is considerably elongated in the case of the Cu" ion in accordance with the tetragonal distortion. The planar tridentate pyridylhydrazone ligand coordinates meridionally to the central metal ion, thus directing the dipyridyl ligand to adopt an axial-equatorial coordination mode. Interestingly, the polypyridyl ligand dpkph forms the six-coordinate complex (Cu'(dpkph)(bipy)(C104)]C104 which is y elongated by the Jahn-Teller effect. Surprisingly, the reaction of H(5-NO2)-happh with Ni(C104)2-6H20 in the presence of bipy or 4,4'-Mez-bipy gave the dinuclear ternary complexes [Ni''2{(5-NO2)-happh)}2(L-L)2](C104)2, ' exhibiting weak antiferromagnetic coupling of spins. Finally, the ternary complexes of Cull were demonstrated to be cytotoxic against HeLa cancer cells in both visible light and in the dark. In contrast, the corresponding Nill complexes showed a photodynamic effect with IC50 values (from the MTT assay) exceeding 100 uM in the dark.