Mixed solvents have been widely used in thermodynamic and kinetic studies on ionic interactions in solution with changing solvent properties of the reaction medium. Dielectric constants and viscosities of solvents are changed by altering the solvent composition of the mixtures. Theories proposed for the explanation of the variation of thermodynamic and kinetic data usually assume a homogeneous continuum medium with specific bulk properties, and the theories which are successfully applied to a neat medium usually fail when applied to mixed solvent systems. The disagreement of theoretical values with experimental ones has traditionally been put down to the inadequacy of the simple assumption of continuum of the solvents. The different solvent com- position in the solvation shell from that in the bulk may be another more important factor for causing these discrepancies.
It could reasonably be expected that in a mixed solvent, the solvent with the larger affinity to an ion might be preferentially solvated with it. The solvent composition of the first solvation shell of the main species formed in various mixed AN-DMF mixtures, together with individual bond lengths are summarized in Table 6. While the accuracy of the results may not be so high the variation of the solvent composition of the solvation shell with the composition in the bulk is clearly recognizable.
An attempt to introduce a quantitative measure of preferential solvation of ions concluded that it occurs primarily due to the difference in the Gibbs energies of solvation of the two solvents under study.60 Solvent–solvent interaction is another important factor controlling preferential solvation. This is illustrated by the solvation of CoIIin mixtures of TMU and water.61TMU is a much stronger donor solvent than water and, thus, would be expected to preferentially solvate with cations over water. However in H2O–TMU mixtures with a larger portion of water, the CoII ion preferentially solvated with water. The results can be explained in terms of strong TMU–H2O intermolecular interactions in the bulk, which result in the disappearance of free TMU. In mixtures containing TMU as a major portion, CoII is preferentially solvated with TMU, while spectro- photometric evidence shows that CoIIforms the tetra-solvated [Co(TMU)4]2þin neat TMU.
Yokoyama et al.62 have determined the solvent composition in the solvation shell of Co2þ in various amide–water mixtures by XRD. The Gibb’s free energy of solvation and the Gutmann’s donor number of the amides are not significantly different but they have differing abilities for hydrogen bonding with water molecules due to different numbers of free amino groups in their
Table6Selectedvaluesofion–ligandatomdistances(rM–X)andnumberofsolventmolecules(n)coordinatedtheioninvarioussolvents. IonDMSO r(pm)DMF nDMA r(pm)AN nDMTF r(pm)DMPU nPy r(pm)PDA n Ca2þ 23262296 Sr2þ 25462556 Ba2þ 27662756 Cuþ 20941994233m 4 Agþ 224a 423342264255m 42414 257b Auþ 2194228m 42164 Mn2þ 22062105 Fe2þ 210621462024 Co2þ 209.562136211.6620052176 Ni2þ 206.36206.762005 Cu2þ 1986c 2016c 199d 6c 1924 –284– Zn2þ 21262086236m 41954 Cd2þ 22962306270m 62296 Hg2þ 2396e 257.2m 4 Hg2þ 2224221k 22224267 Sn2þ 4g Ph2þ 2896 Pd2þ 223 Pt2þ 207h 4i 2004i 2304i 221.5 Sc3þ 2096 Fe3þ 2026
Table6continued IonDMSO r(pm)DMF nDMA r(pm)AN nDMTF r(pm)DMPU nPy r(pm)PDA n Y3þ 2368i 2246 Bi3þ 2418j 27962326 La3þ 2508j 248.67.3247.56.52447 Ce3þ 247.48.1245.26.6 Pr3þ 244.77.4242.06.9 Nd3þ 243.87.4240.26.1 Sm3þ 241.68.9236.78.4 Eu3þ 238.57.7234.76.7 Gd3þ 238.57.5233.37.2 Tb3þ 236.97.5231.97.3 Dy3þ 236.07.7230.57.0 Ho3þ 234.67.8229.36.9 Er3þ 233.67.2227.37.6 Tm3þ 232.57.5226.76.8 Yb3þ 230.97.5223.86.3 Lu3þ 229.87.9222.16.0 Ga3þ 196622441925 In3þ 214626162146 DMA:N,N-dimethylacetamide;AN:acetonitrile;DMTF:N,N-dimethylthioformamide;DMPU:1,3-dimethylpropyleneurea;Py:pyridine;TMU:tetramethylurea;PDA:1,3-propanediamine;athreeM bonds;boneMSbond;cJahn–Tellerdistortedoctahedron;dfourequatorialCuObonds;esecond-orderJahn–Tellerdistortedoctahedron;fHgHgbondlength:250pm;gtwobondsarelonger thantheothertwo;htwoMObondsandtwoMSbonds;isquare-planar;jsquareantiprism;kHgHgbondlength:254pm;llinear;mMSbond;nsquarepyramid;obidentateligand.
molecules. Moreover, they have different substituents resulting in different volumes of bulkiness of the molecules, which may cause steric hindrance in the solvation sphere of the ion. When amino protons are substituted with alkyl groups, the amino group in the molecule attracts fewer water molecules in the bulk so that the concentration of free water molecules increases, the result being that Co2þ can be solvated with more water molecules than with the amide. The different volumes of solvent molecules with bulky substituents may be an additional reason for making different degrees of preferential solvation of the amides.
Umebayashi et al.62–66 have applied Raman spectrophotometry to the determination of the total solvation number of various ions together with the composition of the solvent molecules in the first solvation shell. The total number of solvated molecules in the first coordination sphere, as well as the numbers of individual solvent molecules, of MnII, NiII, CuII, and ZnII in DMF–DMA and DMF–TMU mixtures were determined.
Previous results showed that divalent metal ions, with the exception of MnII, are stabilized in DMF better than DMA,67 which is the opposite of what could be expected given the solvation abilities of DMF and DMA.