Spectroscopic and Structural Studies on Azomethine–Transition Metal Complexes: A Valence State Perspective

Abstract

Azomethines (Schiff bases) are versatile ligands that coordinate readily with transition metal ions through their imine nitrogen and other donor atoms, forming complexes with diverse geometries and physicochemical properties. This study investigates the spectroscopic and structural characteristics of a series of azomethine complexes with divalent (Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) and trivalent (Cr³⁺, Fe³⁺) transition metal ions. The primary objective is to elucidate the influence of metal valence state on bonding, geometry, and electronic structure. Complexes were synthesized via condensation reactions followed by metal–ligand coordination under controlled pH and stoichiometric conditions. Characterization was carried out using elemental analysis, FT-IR, UV-Vis, ¹H NMR, ESR, magnetic susceptibility measurements, and powder X-ray diffraction (XRD). The results reveal that divalent complexes generally exhibit higher ligand field stabilization energies (LFSE) in octahedral and square planar environments, while trivalent complexes display shorter metal–nitrogen bond lengths and greater covalent character due to higher charge density. Shifts in ν(C=N) stretching frequencies in the IR spectra, variations in d–d transition energies in UV-Vis spectra, and changes in magnetic moments correlate with the oxidation state of the metal center. ESR spectra for Cu²⁺ complexes indicate axial symmetry, while trivalent complexes exhibit broadened signals due to stronger spin–orbit coupling. Structural insights from XRD suggest distinct crystalline packing influenced by metal valence. The study concludes that oxidation state plays a pivotal role in modulating the spectroscopic and structural parameters of azomethine–metal complexes, offering valuable guidance for their applications in catalysis, materials science, and bioinorganic chemistry.