Thermal, spectral, DFT and biological activity evaluation of Co(II), Ni(II) and Cu(II) complexes of N,S-chelated benzotriazole ligand

[ML₂]·yC₂H₅OH·zH₂O complexes (M=Co^II (1) and Ni^II (2), y = 4, z = 0; M=Cu^II (3), y = 0, z = 3; HL = N-(2-thiazolyl)-1H-benzotriazole-1-carbothioamide) were prepared, characterized (elemental analysis, TG, FT IR, UV-Vis, EPR, magnetic and conductance measurement) and tested for their antimicrobial activity against Escherichia coli and Staphylococcus aureus. Complexes 1-3 consist of a metal center having a considerable tetrahedral distortion in the xy-plane from the square planar stereochemistry MN₂S₂ formed by the two deprotonated benzotriazole ligands. TD-DFT calculations were carried out at B3LYP/6-31G^∗ level of theory to understand the electronic structure and to explain the related experimental findings. Natural bond orbital analysis was performed to provide details about the electronic arrangement, type of hybridization and the nature of bonding. Coordination of HL to Co^II gave rise to inactive compound, but the development of Ni^II and Cu^II complexes did not clearly change the toxicity of the free HL.