We report a systematic investigation of ethylene oligomerization using new modified nickel and zirconium acetylacetonate-based catalytic systems in various solvents, combining experimental studies with density functional theory (DFT) calculations. Catalytic performance was strongly influenced by the nature of the activator, solvent, and added modifiers. The use of ethylaluminum sesquichloride as an activator provided the highest catalytic activity and enhanced the selectivity toward C6–C10 olefins by 15–20%. Electron-donating modifiers, including tetrahydrothiophene, di-n-pentyl ether, and triethylamine, shifted the product distribution toward higher oligomers (C6–C10 up to 77.9%), whereas triphenylphosphine favored C4 formation (up to 61%). Aromatic solvents significantly increased catalytic activity compared with aliphatic media, consistent with π-electron stabilization of the active metal center. DFT calculations reveal similar ethylene binding strengths for Ni(acac)2 and Zr(acac)4, indicating that the superior activity of the nickel catalyst is unlikely to arise solely from differences in ethylene coordination thermodynamics. These results establish clear relationships between catalyst electronic structure and oligomerization performance, offering guidance for the rational design of acetylacetonate-based systems for selective ethylene conversion.