General Quantum-Mechanical Study on the Hydrolysis Equilibria for a Tetravalent Aquaion: The Extreme Case of the Po(IV) in Water

Abstract

A systematic study of the different hydrolyzed species derived from the hydrated Po(IV) in water, [Po(H(2)O)(n)(OH)(m)]((4-m)) for 1 m 4, and 4 m + n 9, has been carried out by means of quantum mechanical computations. The effects of outer solvation shells have been included using a polarizable continuum dielectric model. For a fixed number of hydroxyl groups, the preferred hydration number for the Po(IV) can be determined in terms of Gibbs energy. It is shown that the hydration number (n) systematically decreases with the increase in the number of hydroxyl groups (m) in such a way the total coordination number (n + m) becomes smaller, being 9 in the aquocomplex and 4 in the neutral hydroxo-complex. Free energies for the hydrolysis processes involving Po(IV) complexes and a different number of hydroxyl groups have been computed, revealing the strong tendency toward hydrolysis exhibited by these complexes. The predominant species of Po(IV) in aqueous solutions are ruled by a dynamical equilibrium involving aggregates containing in the first coordination shell OH(-) groups and water molecules. Although there is not experimental information to check the theoretical predictions, theoretical computations in solution seem to suggest that the most likely clusters are [Po(H(2)O)(5)(OH)(2)](2+) and [Po(H(2)O)(4)(OH)(2)](2+). The geometry of the different clusters is ruled by the trend of hydroxyl groups to be mutually orthogonal and to promote a strong perturbation of the water molecule in trans-position by lengthening the Po-H(2)O distances and tilting the corresponding bond angle. A general thermodynamic cycle is defined to compute the Gibbs free energy associated to the formation of the different hydrolyzed forms in solution. From it, the estimates of pK(a) values associated to the different protolytic equilibria are provided and discussed. Comparison of the relative values of pK(a) along a hydrolysis series with the experimental values for other tetravalent cations supports its consistency.

Effect of Basicity on the Hydrolysis of the Bi(III) Aqua Ion in Solution: An Ab Initio Molecular Dynamics Study

“…The total coordination number also decreases when going from simulation 1 (7) to simulation 2B, but it does not change in simulations 2B and 2C (5.5–6). As previously found, the hydrolysis processes are associated with dehydration events. ,,− Starting from the Bi­(III) aqua ion, the increase in the basicity leads to an increase in the degree of hydrolysis of the Bi­(III) aqua ion complexes, as well as to a decrease in its global coordination number.…”

“…In our previous work, the possibility of four or more OH – ligands around the Bi­(III) was not considered. However, the study of other highly charged cations like Po­(IV) and Te­(IV) ,, indicates that a first coordination shell with four OH – ligands can give rise to two OH – groups opposite to each other. In this case, metal–O OH distances are larger than those found in less hydrolyzed clusters.…”

UV–Vis Absorption Spectroscopy of Polonium(IV) Chloride Complexes: An Electronic Structure Theory Study

“…The following radii have been considered: 2.354 Å for Po 34 (as in previous studies by Ayala et al 33,[35][36][37] ), 1.980 Å for Cl 38 and 1.520 Å for O. 34 No radius has been applied for the H atom (united atom fashion 38 ).…”

Characterizing Pt‐Derived Anticancer Drugs from First Principles: The Case of Oxaliplatin in Aqueous Solution

“…[37] The structural response to the solvent observed for oxaliplatin is completely parallel if one considers the amino ligand to play the role of water ligands in aqua ions and the oxalate ligand to emulate the behavior of hydroxyl ligands in hydroxo-aqua ions.…”