Molar conductances of a large number of copper(I) and cobalt(III) complexes, behaving as strong 1:1 electrolytes, have been measured in acetonitrile (AN) and n-butyronitrile (n-BTN) at 298.15K. The conductance data have been analyzed by the Shedlovsky method to evaluate Λ0 and KA values of these electrolytes. Limiting ion conductances (λi0) for various ions in AN have been calculated by using transference number data. In n-BTN, where no transference number data is available, such values have been calculated by an indirect method using Bu4NBPh4 as a reference electrolyte. The actual ionic radii (ri) for various ions in solution have been calculated using a modified form of Stokes’ law. The ionic radii (ri) for various complex ions have been compared with the ionic radii of two reference ions, Bu4N+ and Ph4B−, which are not solvated in dipolar aprotic solvents, to throw light on the solvation behaviour of these complex ions.

Ultrasonic velocity (u) and density (ρ) of the binary mixtures of acetonitrile (AN) with nitromethane (NM) and dimethylsulfoxide (DMSO) as well as of the solutions of Bu4NBPh4, Bu4NClO4, [Cu(CH3CN)4]ClO4, [Cu(Phen)2]ClO4, [Cu(DMPhen)2]ClO4 and [Cu((NH2)2CS)4]ClO4 have been measured over the entire composition range at 298.15 K. Using these data, isentropic compressibilities (K s) and apparent molal isentropic compressibilities (K s, ϕ ) for various salts have been calculated and used for the evaluation of limiting apparent molal isentropic compressibilities (K s, ϕ 0). The K s, ϕ 0 values of various salts have been split into the contribution of individual ions (K s, ϕ 0)±. The (K s, ϕ 0)± values for all copper (I) cations [Cu(CH3CN)4]+, [Cu(Phen)2]+, [Cu(DMPhen)2]+ and [Cu((NH2)2CS)4]+ are negative and large, for Bu4N+ and Ph4B− are positive and large at all compositions in both AN + NM and AN + DMSO mixtures but for ClO4 − are negative in AN + DMSO mixtures and positive in AN + NM mixtures. The variation of (K s, ϕ 0)± values with solvent composition indicates that both DMSO and NM interact with complex copper (I) cations forming complex ions of the type [Cu(CH3CN)4 − x (S) x ]+ (x = 1–4 and S = co-solvent) and [Cu(ligand)2]+(CH3CN) y (S) z in which AN is systematically replaced by the co-solvents. DMSO interacts more strongly with complex copper (I) cations and ClO4 − ion as compared to NM. Bu4N+ and Ph4B− show some special interaction with AN which decreases with the increase of NM or DMSO composition in the mixture.

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