Substituent and Solvent Effects on Square-Planar–Tetrahedral Equilibria of Bis[4-(arylimino)pentan-2-onato]nickel(II) and Bis[1-aryl-3-(phenylimino)butan-1-onato]nickel(II) Complexes

Nickel(II) bis(Schiff base) complexes, NiL2 (1a–1f) and NiL′2 (2a–2e) (where L = 4-(arylimino)pentan-2-onate and L′ = 1-aryl-3-(phenylimino)butan-1-onate), exist in an equilibrium of square-planar (Sp) and tetrahedral (Td) forms in non-coordinating solvents. The Td form is more stabilized in polar solvents such as CH2Cl2 than in non-polar solvents such as toluene. Thermodynamic data were determined from temperature-dependent 1H NMR contact shift. It is suggested for 1a–1f that the oxygen atoms in Td form interact with solvent such as chloroform via hydrogen bonding. With increase in electron-donating ability of the substituents on aryl groups, the Td form becomes more favored. The free energy change roughly correlates with Hammet’s σp value. The UV–vis–NIR spectra indicated that the ligand field strength increases with the electron-donating ability of the substituents in both the Sp and Td forms. Results of DFT calculation suggest that the observed substituent effect may result from small perturbation of the metal–ligand bonding character.