The shift of the visible d-d absorption bands of [Cu(acac) (tmen)] -+ (acac = acetylacetonate, tmen = N,/V,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 used to establish donor numbers (DN,,,) of anions dissolved in 1.2-dichloroethane (dce) on the same scale as that already established for solvents. Apparent donor numbers for anions dissolved in various solvents other than dce (DNx,miv) are also given and used to show that the donor properties of anions are directly related to the acceptor number (AN,,) of the solvent. This arises from competition between solvent and acceptor ion for the anion leading to a steady decrease in the effective donor number of the anion as AN,,iv increases, culminating 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 values of the anion donor numbers are related to some experimental results such as spectral, equilibrium, kinetic and thermodynamic data for selected systems. This method of establishing donor numbers for anions is

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