Solvation and complex formation in strongly solvating solvents

Earlier proposed concepts for estimation of donor properties of solvents are briefly discussed and a comparison between some of the concepts is made for 53 solvents. The kind of solvents which can be regarded as strongly solvating are proposed for the further discussion. A comparison is also made of the solvation of typically soft and hard acceptors for a number of solvents. The solvation of univalent and some divalent ions in methanol, acetonitrile, dimethylsulfoxide, pyridine, tetrahydrothiophene and liquid ammonia have been studied by means of transfer thermodynamics from water. Oxygen donor solvents and nitriles solvate in general hard acceptors well and soft ones poorly. Amines, sulfur and phosphorous donor solvents solvate soft acceptors strongly while on the other hand they solvate hard acceptors poorly. The stability of a complex is in general inversely proportional to the solvation of the metal ion or complex and the ligand. The complex formation will there— fore be weaker in solvents where the acceptor is strongly solvated. When the dielectric constant is lower than 10 the tendency to neutralization of charge through ion pair formation becomes important and the stabilities of neutral complexes will increase dramatically. CLASSIFICATION OF SOLVENTS Several auhtors have proposed concepts for a general systematizing of the donor properties of solvents. It is, however, doubtful if such a general systematizing is possible. It is plausible that several donor scales for estimation of solvation ability of solvents are necessary because of the very different acceptor properties of metal ions and complexes. The first concepts was originated by Gutman et.al. who introduced the donor numbers, DN, for the coordinating property of a solvent (ref. 1—4). The donor number is defined asthe —AH° value, in kcal mo11, of the formation of the 1:1 adduct between the donor solvent and the chosen reference electron acceptor antimony(V) chloride in dilute l,2—dichloroethane solution. Antimony(V) chloride is regarded as an acceptor on the border—line between hard and soft. In recent years alternative concepts to the donor numbers have been proposed (ref. 5—12) in order to simplify the measurements and to extend the number of solvents. The donor number can not be determined for all solvents by the original procedure since other chemical reactions take place beside the adduct formation (ref. 5,6,11,13). No donor numbers have been reported for sulfur and phosphorous donor solvents. These will certainly react immediately with antimony(V) chloride and direct measurements of donor numbers are therefore not possible. Indirect measurements of donor numbers are not always reliable (ref. 11) and such will not be further discussed in this paper. It has therefore been important to find a simple approach from which it is possible to estimate the donor properties of especially soft donor solvents. Mercury in mercuric bromide is regarded as a fairly soft electron acceptor and has been chosen as probe in the concept donor strength. Donor strength, , is defined as the difference between the symmetric Hg—Br stretching frequencies of the neutral mercuric bromide complex in gaseous phase and in a saturated solution of the studied solvent (ref. 6). Mercuric bromide is soluble and stable in all solvents studied except in liquid ammonia where it dissociates (ref. 14) and in isocyanates where it decomposes (ref. 6,15). Maria and Gal have used an approach very similar to the definition of the donor numbers (ref. 5). They have used boron trifluoride as acceptor instead of antimony(V) chloride and dichloromethane as solvent instead of 1,2—dichioroethane in order to reduce the number of side reactions. The values are given in kJmol-. There is of course a very good correlation between the and LHBF scales.