DONOR NUMBERS OF ANIONS IN SOLUTION - THE USE OF SOLVATOCHROMIC LEWIS-ACID BASE INDICATORS

The shift of the visible d-d absorption bands of [Cu(acac)(tmen)]+ (ac ac = acetylacetonate, tmen = N,N,N',N'-tetramethylethylenediamine) in various solvents resulting from the co-ordination of solvent molecules and anionic species X has been investigated. The results have been us ed to establish donor numbers (DN(X,dce)) of anions dissolved in 1,2-d ichloroethane (dce) on the same scale as that already established for solvents. Apparent donor numbers for anions dissolved in various solve nts other than dce (DN(X,solv)) are also given and used to show that t he donor properties of anions are directly related to the acceptor num ber (AN(solv)) of the solvent. This arises from competition between so lvent and acceptor ion for the anion leading to a steady decrease in t he effective donor number of the anion as AN(solv) increases, culminat ing in an effective donor number of zero for all anions in the strong acceptor solvent water (ion pairing is at a minimum in water). The val ues of the anion donor numbers are related to some experimental result s such as spectral, equilibrium, kinetic and thermodynamic data for se lected systems. This method of establishing donor numbers for anions i s shown to be reliable and easy to apply.