Tridentate organic ligands obtained by condensation of a hydrazide with aromatic carbonyl compounds is an important class of ligands containing a combination of ONO donor atoms: the phenol and keto/enol oxygen atoms, imine nitrogen atom and additionally another atom (usually N or O) provided by the aldehyde or ketone that forms the Schiff base. The research conducted was pursued the coordination ability of ligands with general formula below (Scheme 1) in reactions with ions VO2 + , MoO2 2+, Co2+ , Co3+ .figureScheme 1. The ligands utilized There have been prepared and characterized by elemental analysis, IR spectroscopy and X-ray study 11 complexes: VO2(HL1 ) (1), VO2(HL2 ) (2), VO2(HL3 ) (3), [MoO2(L3 )(MeOH)] (4), [MoO2(L3 )]4 (5), [MoO2(L4 )(DMSO)] (6), [MoO2(L5)(DMSO)] (7), [CoIII(HL1 )(L1 )] (8) [CoIII(DmgH)2)H2L 1 )Cl]∙4H2O (9), [CoIII(DmgH)2)H2L 2 )Cl]∙2H2O (10),. In compounds 1-3 vanadium(V) atom adopts a square-pyramidal geometry, the tridentate ligand being coordinated in enolic form, and two oxygen atoms in cis position. It also was determined the protonation of the ligand pyridine ring, which was observed earlier in complexes with ions Fe(III) and V(V). The structure of complexes 4, 6 and 7 consist of a dioxomolibden (VI) fragment, a bideprotonated Schiff base ligand, coordinated by tridentate ONO electron donor set and each molecule of solvent occupying the sixth position in coordination polyhedron. In compound 5, the sixth position is occupied by a nitrogen atom of the pyridine ring of another isonicotinic fragment, so four fragments [MoO2(L3 )] form a cyclic tetranuclear structure In compound 8, two molecules of tridentate ligand coordinate meridionally, forming an octahedral complex. The heterociclic pyridine nitrogen atom is protonated, although both ligands are crystallographic independent, i.e. the proton position is disordered between the molecules of both ligands. Compounds 9 and 10 represent a molecular structure built from neutral complex [CoIII(DmgH)2)H2L 1-2 )Cl] and water molecules as solvent. CoIII atom is coordinated four nitrogen atoms arising from two bidentate monoanions of dimethylglyoxime. Octahedral geometry of the central atom is completed by a chloride anion in the trans position - a nitrogen atom of the pyridine ring of izonicotinoilhidrazonic ligand. It is important to note that nitrogen and oxygen atoms of the acilhidrazonice group are not involved in coordination.