Steric effect on solvation and complexation of metal ions in solution
N,N-dimcthylformaniide (DMF), N,N-dimethylacctaniidc (DMA) and hexamethylphosphoric triamidc (I-IMPA) arc all oxygen-donor solvcnts, and thcir clectron-pair donating property incrcascs in the ordcr DMF =c DMA c I-IMPA. The coniplcxation is thus cxpcctcd to be generally wcakcncd in thc sanic order orsolvents. However, the halogcno complexation of metal ions in DMA and HMPA is significantly cnhanccd over DMF. Such an unusual complexation cannot be cxplaincd in terms of the difrcrencc in the strcngth olmctal-solvent bonds, as wcll as thc diffcrcncc in lhcstrcngtli of ligand-solvcnt intcraction,sol- vetit-solvent interaction in thc bulk or nonspccific ion-solvcnt interaction. Although transition nictal(11) ions arc six-coordinated in DMA, likc DMF, the mctal-solvcnt interaction niaybc wcakcncd duc to an accthyl mcthyl group ncxt to the coordinating carbonyl oxygcn atom. On thc othcr hand, as IIMPA is a vciy bulky molcculc, thc nictal ions can not acconimodatc six solvcnl niolcculcs but only four in I-IMPA. 'Thus, stcric intcraction bctwecn coordinating solvcnt niolcculcs plays a key rolc in the thcrmodynamic anomaty. IIcrc, lhcrmodynamic and structural aspects of solvation stcric cffccts will bc dcscribcd.