STERIC EFFECT ON SOLVATION AND COMPLEXATION OF METAL-IONS IN SOLUTION

N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA) and hexamethylphosphoric triamide (HMPA) are all oxygen-donor solvents, and their electron-pair donating property increases in the order DMF almost-equal-to < DMA < HMPA. The complexation is thus expected to be generally weakened in the same order of solvents. However, the halogeno complexation of metal ions in DMA and HMPA is significantly enhanced over DMF. Such an unusual complexation cannot be explained in terms of the difference in the strength of metal-solvent bonds, as well as the difference in the strength of ligand-solvent interaction, solvent-solvent interaction in the bulk or nonspecific ion-solvent interaction. Although transition metal(II) ions are six-coordinated in DMA, like DMF, the metal-solvent interaction may be weakened due to an acethyl methyl group next to the coordinating carbonyl oxygen atom. On the other hand, as HMPA is a very bulky molecule, the metal ions can not accommodate six solvent molecules but only four in HMPA. Thus, steric interaction between coordinating solvent molecules plays a key role in the thermodynamic anomaly. Here, thermodynamic and structural aspects of solvation steric effects will be described.