Abstarct: This research presents the design, synthesis, characterization, and crystal structure determination of some first-row transition mextal Schiff baxse complexes and the investigation of their catalytic, magnetic, and biological properties. First, two Schiff baxse ligands HL1 (monoanionic, NNO or N2O3 donor) and L2 (neutral, NNN donor) were prepared. Then, five new Schiff baxse complexes of cobalt, vanadium, and copper were synthesized as follows: [VVO2(L1)]2 (V-1) [CuII2(µ1,1-N3)2(L2)2(N3)2]. 2CHCl3 (Cu-2) [CoIII(L1)2]Cl (Co-1) [CoII2(µ1,1-N3)2(L2)2(N3)2] (Co-2) {[CuII2(L1)(N3)3]2. 2H2O}n (Cu-1) These complexes were characterized using different techniques. Their crystal-structure was determined by the single crystal X-ray diffraction method. The complex V-1 has a centrosymmetric crystal structure in which the vanadium(V) centers are connected by two bridging oxygen atoms. This complex was studied using DFT calculations, and the optimized structure was in agreement with the results of X-ray analysis. The catalytic activity of this complex was investigated in the epoxidation of alkenes in the presence of tert-butyl hydroperoxide (TBHP) as an oxidant. Moreover, the antibacterial activity of the prepared compounds were investigated against three Gram-positive and three Gram-negative bacteria strains. The crystal structure of the complex Co-1 includes a mononuclear cation ([Co(L1)2+]) and a non-coordinating chloride anion (Cl-) in which the cobalt(III) center has a distorted octahedral geometry. This complex was used as a novel precursor to prepare the octahedral cobalt oxide microparticles (Co3O4) using the thermal decomposition method. In addition, the cobalt oxide microparticles were decorated on chitosan polymer (CS) to prepare the chitosan-cobalt oxide composite (Co3O4@CS). The chemical composition, phase structure, morphology, band gap energy, and magnetic properties of the samples were studied using FT-IR, EDX, XRD, FE-SEM, AFM, UV-visible DRS, and VSM techniques. The photocatalytic activity of the Co3O4 compound was investigated in the degradation of methylene blue (MB) dye under visible light irradiation, and the operating conditions were optimized. Then, the photocatalytic activity of the Co3O4@CS composites was studied in the same reaction under optimum experimental conditions. The kinetics, mechanism, and reusability of the photocatalysts were also investigated. Moreover, the antibacterial activity of the prepared compounds was evaluated against Gram-positive and Gram-negative bacteria. The crystal structure of the coordination polymer Cu-1 consists of tetranuclear units [Cu2(L1)(N3)3]2 bearing double asymmetric end-on azido and mixed azido/phenoxido bridges, which are lixnked via two µ1,1,3 azido bridges. The magnetic properties of this complex were also investigated. Utilizing o-aminophenol (OAP) as the substrate, the phenoxazinone synthase-like catalytic activity of this complex was studied. This complex can efficiently catalyze the aerobic oxidation of OAP to APX in the DMF medium. In addition, this complex was used as an active catalyst in the epoxidation reaction of various alkenes in the presence of TBHP in chloroform solvent. The complex Cu-2 has a dinuclear structure in which the two Cu(II) centers are connected by double symmetric μ1,1 (end-on, EO) azido bridges, generating distorted octahedral CuN6 coordination geometry. The magnetic properties of this complex were also investigated. Moreover, the catalytic properties of this complex were evaluated in the one-pot azide-alkyne cycloaddition reaction in water without any additional reducing agents or baxses. Similar to the complex Cu-2, the complex Co-2 has a dinuclear structure in which the two Co(II) centers are connected by double symmetric μ1,1 (end-on, EO) azido bridges, generating distorted octahedral CoN6 coordination geometry. Moreover, the catalytic activity of this complex was studied in the aerobic epoxidation reaction of alkenes in the presence of isobutyraldehyde as a reductant.