Individual Solvation Numbers around the Nickel(II) Ion in an N,N-Dimethylformamide and N,N-Dimethylacetamide Mixture Determined by Raman Spectrophotometry

Individual solvation numbers around the nickel(II) ion have been determined by titration Raman spectroscopy in N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) mixtures at 298 K. The in-plane bending vibration (δ(O=C-N)) of DMF and the stretching vibration (ν(N-CH3)) of DMA were used in the present analysis. These Raman bands of solvent molecules shift to higher frequencies upon coordination of the solvent molecules to the metal ion. By analyzing the band intensities of free and bound solvent molecules with increasing concentration of the metal ion, the solvation number around the metal ion can be evaluated. Because the individual solvation numbers of DMF and DMA around the nickel(II) ion in the mixture are determined independently, the total solvation number is obtained as their sum. It was found that the total solvation number remains 6 in all mixtures of the DMA mole fraction x = 0 - 1. Although DMF and DMA have practically the same electron-pair donor capacities, the nickel(II) ion prefers DMF to DMA, and an equal solvation number is attained at x = 0.75. This is ascribed to the solvation steric effect of DMA.