Crystallographic, spectroscopic, thermal, optical investigations and density functional theory calculations for novel Co(II) and Mn(II) complexes

Two new picolinate complexes, namely diaquabis(6-chloropyridine-2-carboxylato-N,O)cobalt(II) (1) and diaquabis(6-chloropyridine-2-carboxylato-N,O)manganese(II) (2) were synthesized. These complexes were all characterized using FT-IR and UV–Vis spectra, single crystal X-ray diffraction method and thermal analysis. Both complexes have similar molecular structure characterized as distorted octahedral geometry. B3LYP level was also executed to provide a deeper insight to the structural, electronic and nonlinear optical properties of 1 and 2. The detailed vibrational analysis was performed for 1 and 2 on the basis of potential energy distribution (PED) analysis. The natural bond orbital (NBO) energies between the lone pair electrons of donor N/O atoms and anti-lone pair electrons of Co(II) and Mn(II) ions also proved the distorted octahedral geometry for both complexes. The band gap energy values for the allowed direct transition are found 4.29 and 4.26 eV by using Tauc model for complex 1 and 2, respectively. Finally, the optical conductivity (σopt), electrical conductivity (σelec), the third-order nonlinear optical susceptibility χ(3), the molar polarizability αp and dielectric property are determined for the title complexes in the UV–Vis region. The first-order hyperpolarizability (β) parameter for 2 was obtained as higher than that of 1 due to the high spin electronic configuration of Mn(II) central ions