Application of Molar Volumes Obtained From a Solvent Extraction Method for Estimation of Sizes of Metal Complexes With Crown Ethers in Phases

Molar volumes (Vj) of ion-pair complexes (MLAz = j; z = 1, 2) and simple complex ions (CdL2+) without a pairing anion A− obtained at 298 K from the plots of the regular solution theory and the geometrical relation Vj = (4π/3)Rj3 were used for an estimation of their apparent radii (Rj) in liquid phases. Here, the cases for Mz+ = Li+-Cs+, Cd2+, and Pb2+ and L = 18-crown-6 ether (18C6) and benzo-18C6 (B18C6) with A− = Cl−-I− and picrate ion (Pic−) were examined. The thus-estimated Rj values were related to h(ML+), λmax, and (r+ + r−) values, where these symbols refer to the number of water molecules attached to the univalent complex ions, ML+, extracted into water-saturated nitrobenzene, absorption maxima of Pic− with ML+ in water-saturated chloroform, and the sum in an effective ionic radius, r+ or r−, between the central Mz+ and the pairing A−, respectively. The data points of j = Li(18C6)Pic, Na(18C6)Pic, M(B18C6)Pic with M = Na-Rb(I), Cd18C62+, Cd(18C6)Cl2, Cd(18C6)Br2, and Cd(18C6)I2 showed larger deviations from the line of Rj = (r+ + r−). While j = M(18C6)Pic with M = K-Cs(I) and M(18C6)Pic2 with Cd(II) and Pb(II) were on the line. It was demonstrated that essentially the former groups are present as water-separated and -shared ion pairs in the phases, while the latter ones as contact pairs.