The solvation behavior of Pt(II) versus Pd(II) has been studied in ambient water using ab initio molecular dynamics. Beyond the well-defined square-planar first solvation shell encompassing four tightly bonded water molecules as predicted by ligand field theory, a second coordination shell containing about 10 H2O is found in the equatorial region. Additional solvation in the axial regions is observed for both metals which is demonstrated to be induced by the condensed phase. For the Pt(II) aqua complex, however, this water molecule is bonded with one of its hydrogen atoms toward the cation, thus establishing a typical anionic solvation pattern, which is traced back to the electronic structure of Pt(2+) versus Pd(2+) cations, in particular to the anisotropic polarizability of their tetrahydrates. Systematic model calculations based on suitable aqua complex fragments embedded in a polarizable continuum solvent support the idea that anionic hydration is facilitated by the liquid. Furthermore, transient protolysis of water molecules in the first shell is observed for both divalent transition metal cations, being more pronounced for Pt(II) versus Pd(II). The relevance of these solvation features is discussed with respect to the different acidity of Pt(2+) versus Pd(2+) aqua ions in water, their different water ligand exchange rates, and force field modeling approaches.

The thermodynamical stability of free, pristine gold clusters at finite temperature, and of cluster+ligands complexes at finite temperature and in the presence of an atmosphere composed of O2 and CO, is studied employing parallel tempering and ab initio atomistic thermodynamics. We focus on Au13, which displays a significant fluxional behavior: Even at low temperature (100 K) this cluster exhibits a multitude of structures that dynamically transform into each other. At finite temperature, the preference of this cluster for three-dimensional versus planar structures is found to result from entropic effects. For gold clusters containing one to four gold atoms in an O2 + CO atmosphere, we apply ab initio atomistic thermodynamics. On the basis of these considerations, we single out a likely reaction path for CO oxidation catalyzed by gold clusters.

The aqua ions of Pd II and Pt II form well-defined square-planar structures in aqueous solutions.[1] Their hydration and related physicochemical properties are relevant in a twofold sense. The first one is for understanding the solvent structure in the non-equatorial regions. In general terms, a highly charged atomic cation interacts with the solvent, thus generating a roughly spherical shell of strongly perturbed solvent molecules surrounding the cation, named first solvation shell. At a distance far enough from the cation, the solvent recovers the bulk behavior. Then, an intermediate region, with a spherical shell shape, can be defined in which the solvent molecules slowly lose their perturbed character as they are further away from the cation and closer to the bulk. This intermediate region is called second solvation shell. This widely used description of the interaction between the cation and the solvent is known as the concentric shell model of Frank and Evans.[2] However, when the electronic properties of the aqua ion impose a planar coordination in the first hydration shell, this Copernican view of Frank and Evans is no longer applicable. Thus, the characterization of the axial regions in Pd II and Pt II aqua ions is attracting much attention from both experimental [3] and theoretical [4] points of view. The second aspect of interest is associated to the highly relevant antitumoral applications of Pt II square-planar complexes, such as cisplatin cis-PtA C H T U N G T R E N N U N G (NH 3 ) 2 (Cl) 2 , which become medically active when some of the first-shell ligands exchange for water molecules. The homologous compounds of Pd II are inert in this respect, which is supposedly due to their extremely different exchanging rate constants relative to those of the Pt II compounds.[5] This dissimilar behavior may have its origin either in the structural or in the electronic properties of the compounds. A first step towards unraveling this issue consists in studying the tetrahydrates in water solution.This Communication reports Car-Parrinello molecular dynamics (CP-MD) simulations [6] of the Pd II and Pt II aqua ions in water, thereby providing unusual insight into both dynamical and electronic structure phenomena occurring in the first and second solvation shells. The focus is on two remarkable features: 1) the "anionic solvation" of axial H 2 O molecules and 2) the transient proton transfer between equatorial first-and second-shell H 2 O molecules. General StructureMÀO and MÀH radial distribution functions (RDFs) for aqueous solutions of Pd II and Pt II are plotted in Figure 1.The first solvation shells of both Pd II and Pt II are planar, on average, and composed of four H 2 O molecules, which remain in the first-shell region during the simulation. The RDFs show an MÀO maximum at 2.04 in both cases, in agreement with previous experimental [3] and theoretical [4a-d] works (where the maximum was observed between 2.00 and 2.07 ). On the other hand, the MÀH maximum (observed at 2.61 ) can only be compared to prev...

Solution chemistry of Pd(II) and Pt(II) complexes is relevant to many fields of chemistry given the widespread applications of their compounds in homogeneous and heterogeneous catalysis, intermediate reaction synthesis, and antitumoral drugs. The well-defined square-planar arrangement of their complexes contrasts with the rather diffuse axial environment in solution. A theoretical proposal for a characteristic hydration shell in this axial region, called the meso-shell, stimulated further experimental and theoretical studies which have led to different pictures. The present work characterizes the structure of the axial region of the Pd(II) aqua ion in solution using a combination of neutron and X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy, with empirical potential structure refinement (EPSR). The results confirm the existence of the axial region and structurally characterize the water molecules within it. An important finding not previously reported is that the counterion, in this case the perchlorate anion, competes with water molecules for the meso-shell occupancy. The important role played by the axial region in many ligand substitution reactions is therefore intimately connected with the presence of the counterion and not just hydration water. This must call the attention of the experimental community to the important role that the counterion of the precursor salt must play in the synthesis.