Solvent-dependent variation of the spin state of bis(salicylaldoximato) nickel(II) in nonaqueous solutions

Magnetic susceptibilities and chemical shifts of salicylaldoxime protons in CDCl3-DMSO-d(6) solutions of bis(salicylaldoximato)Ni(II) were analysed ac cording to the equilibrium between diamagnetic square planar Ni(Saldox)(2) and paramagnetic solvates Ni(Saldox)(2). mDMSO. Magnetic susceptibility dat a yield thermodynamic parameters for inner sphere solvation (i.e., formatio n of the bis-solvate Ni(Saldox)(2). 2DMSO): Delta H=-18.4+/-2.9kJ/mol, Delt a S= -102.1+/-9.2 J/mol K and the magnetic moment of paramagnetic solvate m u(para) = 3.4+/-0.2 mu B. The dependences of chemical shifts of salicylaldo xime protons on solvent composition and temperature require more than two D MSO molecules solvating Ni(Saldox)(2) for an adequate description. Analysis of these data according to the formation of Ni(Saldox)(2). 4DMSO yields th ermodynamic parameters of both inner-sphere and outer-sphere solvation, nam ely Delta H = -56.5+/-1.3 kJ/mol and Delta S = (-249.0+/-5.0) J/mol K. This shows that outer sphere solvation is connected with higher enthalpies than inner-sphere solvation (38.1 and 18.4kJ/mol, respectively) of this complex . This phenomenon is explained by the high contribution of electric dipole- dipole interactions of DMSO molecules in the Ni(Saldox)(2). 4DMSO solvate. Analysis of hyperfine coupling constants of the salicylaldoxime protons sho ws that the outer-sphere solvation of Ni(Saldox)(2) proceeds via hydrogen b onding with oxime groups, which breaks intramolecular hydrogen bonds in Ni( Saldox)2. This model produces a good agreement between measured hyperfine c oupling constants and ZINDO/1 computed spin-densities on protons. The obser ved small equilibrium constants of solvation were found to be due to strong ly negative entropies of solvation (-102.1 and -145.2J/molK, respectively, for inner and outer sphere solvation).