The Structure of Water in the Vicinity of Complex Ions of Transition Elements in Their Aqueous Solutions as Revealed by the Shift of the Temperature of Maximum Density of Water

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Abstract
The temperature of the maximum density (TMD) of aqueous solutions was measured for twenty complex ions of transition metals. From the degrees of depression of TMD, Despretz’s constant kionD and the constant of excess depression of mixing Δkion were determined. These constants along with Bion coefficient of viscosity give us information on the structure of water in the vicinity of ions. [Co(H2O)6]2+, [Ni(H2O)6]2+, [Cu(H2O)6]2+, and [Zn(H2O)6]2+ are electrostrictive structure-breakers in the natural order of Irving-Williams, Co(II)<Ni(II)Zn(II). Of the complexes [Co(NH3)6]3+, [Co(en)3]3+, and [Co(pn)3]3+, the pn-complex is the strongest hydrophobic structure-maker due to the hydrophobic nature of its aliphatic constituent. Both [Fe(CN)6]3− and [Co(CN)6]3− are fairly strong electrostrictive structure-breakers, while [Fe(phen)3]2+, [Co(phen)3]2+, and [Co (phen)3]3+ are clathrate-like structure-makers. The EDTA and CyDTA complex ions of Co(II), Ni(II), Cu(II), and Zn(II) show no significant difference in kionD, the situation being complicated by the hydrophobicity of the ligands and the radius effect of the complex ion. The actions of ions as structure-makers should be classified into three categories; electrostrictive, hydrophobic (iceberg formation), and hydrophobic (clathrate-like structure formation). The characteristic influence of the structure-making nature on kionD, Δkion, and Bion is discussed.