From the experimental polarizability values, alpha, of a large set of solvents containing 426 compounds with very different chemical characteristics, an additive model for the estimation of the polarizability is proposed. The derived average atomic polarizability of 10 elements, C, H, O, N, S, P, F, Cl, Br, and I, allows the calculation of the molecular polarizability of solvents from their chemical composition alone, without any other structural consideration. The average errors are 2.31% and 1.93% for the working set of 340 solvents and the prediction set of 86 solvents, respectively. Semiempirical quantum methods, such as, AM1, PM3, and MNDO, gave errors of about 35%. The density functional theory (DFT) calculations give better results than the semiempirical ones but poorer results than those obtained by the additive approach.

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Quantum calculations with the density functional theory (B3LYP) have been carried out to compare the reactivity of aryl-H and aryl-F bonds toward oxidative addition and to understand the high degree of inertness of the latter. The thermodynamic energy patterns for oxidative addition of 1,4-difluorobenzene toward two very different metal fragments have been examined. In one of them the final product of oxidative addition could be a 16-electron unsaturated complex of the type Os(H)(CO)(C 6 F 2 H 3 )(PH 3 ) 2 and/or Os(F)(CO)(C 6 FH 4 )-(PH 3 ) 2 . In the other system the final product of oxidative addition could be an 18-electron saturated complex CpRh(PH 3 )(H)(C 6 F 2 H 3 ) or CpRh(PH 3 )(F)(C 6 FH 4 ). These two systems are models for experimental complexes which prefer the C-H to the C-F oxidative addition. The calculations reveal that, for both systems, the C-F oxidative addition is thermodynamically preferred, especially in the 16-electron case. The activation energy has been determined in the case of Rh, and it is shown that the activation energy for C-F activation is considerably higher than that for C-H activation. This clearly shows that the inertness of the C-F bond has a kinetic origin.

Computational Neural Network (CNN) based QSPR methodology is applied to a multicomponent system: the prediction of pK a of phenols in different solvents. The system is composed of 94 phenols, 10 solvents, and 276 experimental pK a values. The phenols are characterized by the habitual molecular descriptors, while the solvents are described by a number of physical properties and by several parameters of the most used multiparametric polarity solvent scales. The proposed model, non-linearly derived, contains seven descriptors; five of them belong to the solutes and the other two to the solvents. Good results are obtained with a Root-Mean-Square Error (RMSE) and correlation coefficients (R 2 ) of 0.71 (0.982), 0.83 (0.977), and 0.95 (0.975) for the training, prediction, and cross validation sets, respectively. The robustness of the model is also in accord with the statistical results obtained from particular subsets of phenols with and without orthosubstituents and those obtained from the subsets of values of pK a determined in protic or in aprotic solvents and also in each solvent. The descriptors of the model encode information that reflects characteristics of the molecules of the solutes and the solvents clearly related to the interactions acting in the dissociation process.

A Computational Neural Network (CNN) derived model is proposed for the pK a prediction of benzoic acids in different solvents. The system studied contains 519 pK a values corresponding to 136 benzoic acids determined in water and in 8 organic solvents. The benzoic acids were described by the usual molecular descriptors and the solvents by a number of physical properties and by several parameters of the most widely used polarity solvent scales. The model is composed of seven descriptors -five of them corresponding to the solute and the other two to the solvents -and was validated by an external prediction set. The three sets of values needed in the analysis, training, prediction, and cross-validation, have the same squared correlation coefficient (0.998) and Root-Mean Square Error (RMSE) (0.21) values. The robustness of the model is also given for the statistical results for small subsets, such as ortho-and non-ortho-substituted acids, those obtained in protic or aprotic solvents, those obtained in each solvent, and even those for the set of pK a values of one specific acid in several solvents. The descriptors encode information about the chemical nature of the solutes and the solvents that is clearly related to the interactions present in the dissociation process in solution. The derived model also has the ability to predict pK a values of a larger set of aromatic neutral acids, containing both phenols and benzoic acids.

A theoretical model, based in density functional theory with the B3LYP functional and the DZVP basis set from Salahub, has been applied for the calculation of the binding affinity and cation basicity between the 20 common amino acids and the monovalent cations Li+, Na+, K+, Cu+ and Ag+. These magnitudes have been calculated for every combination of the five cations with the twenty amino acids, thus totalling 100 reactions. The highest binding affinities correspond to copper(I) (302.2-479.8 kJ mol(-1)), while potassium has the lowest values (115.6-192.4 kJ mol(-1)). The results of the calculations have been compared with both experimental and theoretical values from the literature when they are available. Also, an energy partitioning scheme has been used to evaluate the different factors that have an influence on the value of the amino acid-cation binding energy, mainly the preorganization energy of the ligand and the interaction energy between the cation and the different donor atoms and/or pi system of the amino acid. The procedure developed here can be used with a wide range of metal cations, including those pertaining to the first and second transition series.

Two successive decoalescence events in the hydride region of the 1H NMR spectrum of [ReH5(PPh3)2(py)] (py = pyridine) are now firmly associated with turnstile and pseudorotation fluxionality mechanisms by eliminating an alternative pairwise mechanism. Ab intio (B3LYP) calculations on ReH5(PH3)2L (L = pyridine) have located the transition state for the turnstile mechanism, which proves to be a second dodecahedral tautomer of the starting complex with the pyridine in the normally unfavorable A site. The fluxional process can therefore be considered as an interconversion of two dodecahedral tautomers, and the barrier for the process is identical with the energy difference of the two tautomers. From a comparison in ReH5(PPh3)2L (L = 2-(acetylamino)pyridine and 4-(acetylamino)pyridine), it is clear that having a potentially hydrogen-bonding NH group at the ortho or para positions of the pyridine ring causes an acceleration of the fluxionality, as a result of intramolecular Re−H···H−N hydrogen bonding. The theoretical calculations on ReH5(PH3)2L (L = 2-aminopyridine and 4-aminopyridine) show that the experimental barriers are the result of a compromise between two factors:  hydrogen bonding, which lowers the barrier for the 2-amino compound, and H···H repulsion resulting from an excessively close approach of the two H atoms in the transition state, which raises the barrier. This implies that the particular hydrogen-bonding ligands chosen were too rigid for optimal rate acceleration.

The synthesis of the novel ferrocenyl Schiff base [(η 5 -C 5 H 5 )Fe{(η 5 -C 5 H 4 )CHdN(C 6 H 4 -2-SMe)}] (1c) and the study of its reactivity versus palladium(II) or platinum(II) salts are reported. These studies have allowed us to isolate and characterize [M{[(η 5 -C 5 H 3 )CHdN(C 6 H 4 -2-SMe)]Fe(η 5 -C 5 H 5 )}Cl] {with M ) Pd (2c) or Pt (3c)}, which are the first examples of cyclopallada-and cycloplatinated compounds containing a mer-terdentate [C(sp 2 , ferrocene), N, S] -ligand. Mössbauer spectroscopic studies and the X-ray crystal structures of 1c-3c are reported. The crystal structures of 2c and 3c confirmed the mode of binding of the ligand and revealed that the molecules are associated in pairs, which interact by C-H‚‚‚π interactions forming a chain that stacks along the [101] plane. Electrochemical studies, based on cyclic voltammetry, reveal that the ease with which the oxidation of the ferrocenyl moiety occurs increases according to the sequence 1c < 2c < 3c. Molecular orbital calculations at a DFT level have also been carried out to rationalize the influence of the nature of the Pd-(II) (in 2c) or Pt(II) in (3c) on the electrochemical properties of these compounds, and theoretical studies using time-dependent density functional theory (TD-DFT) calculations have also been carried out in order to assign the bands observed in the electronic spectra of the cyclometalated compounds.