Nonlinear optical properties of novel crystalline complexes of cobalt (II) and nickel (II) with 4-bromo-3-methyl-1H-pyrazole: combined experimental and computational study

Two new isostructural cobalt [Co(II)(C₄H₅BrN₂)₄(Cl)₂], I and nickel [Ni(II)(C₄H₅BrN₂)₄(Cl)₂], II complexes were synthesized for linear and nonlinear optical (NLO) responses using the finite field method. The synthesized complexes were characterized using elemental analysis, infrared and H-NMR spectroscopy, TG/DSC analysis, and X-ray crystallography. The crystal analysis indicates that both complexes I and II are mononuclear, and crystallize in the monoclinic space group P2₁/c. The metal ion in each complex is six coordinated with four nitrogen atoms (from 4-bromo-3-methyl-1H-pyrazole ligand) and two chloride ligands adopting octahedral coordination geometry. Furthermore, the computational predictions for molecular geometries and average third-order nonlinear polarizabilities of I and II are made in current study. The average theoretically predicted third-order nonlinear polarizabilities 〈γ〉 of I and II are 95.50 × 10⁻³⁶ and 91.25 × 10⁻³⁶ esu, respectively. The 〈γ〉 amplitudes values of complexes I and II are ∼5.26 and ∼5.0 times greater than the prototype NLO molecule of para-nitroaniline (p-NA) at same level of theory, which indicates their decent potential as efficient NLO materials.