Strong interactions between copper halides and unsaturated systems: new metallocycles? Or the importance of deformation

The complexes formed by CuF with CC double and triple bonds have been studied at the MP2 and CCSD(T) computational levels. The interaction of CuF with acetylene, ethylene and their fluoro derivatives is very strong, with interaction energies close to those of conventional covalent bonds. Hence, these complexes could be actually viewed as a new kind of metallocycles, with significantly strong Cu-C linkages. All electronic indexes analyzed by means of the AIM, ELF and NBO formalisms, indicate that the strength of the interaction should increase with the number of fluorine substituents in both series of compounds. Surprisingly, however, although both series of compounds exhibit the same bonding arrangements, they follow opposite stability trends and the expected increase of the interaction energies with the number of fluorine substituents is only observed in the acetylene series. The reason for this unexpected behavior is once more associated with the effects triggered by the distortion of the interacting subunits. Deformation not only has a direct energetic cost but dramatically affects the intrinsic properties of the interacting systems.

Thermochemistry, bonding, and reactivity of Ni+ and Ni2+ in the gas phase

In this review, we present a general overview on the studies carried out on Ni(+-)- and Ni(2+)-containing systems in the gas phase since 1996. We have focused our attention in the determination of binding energies in parallel with an analysis of the structure and bonding of the complexes formed by the interaction of Ni(+) with one ligand, or in clusters where this metal ion binds several identical or different ligands. Solvation of Ni(2+) by different ligands is also discussed, together with the theoretical information available of doubly charged Ni-containing species. The final section of this review is devoted to an analysis of the gas-phase uni- and bimolecular reactivity of Ni(+) and Ni(2+) complexes.

Effect of Ni(ii), Cu(ii) and Zn(ii) association on the keto-enol tautomerism of thymine in the gas phase

The effect of Ni(II), Cu(II) and Zn(II) association on the diketo/keto-enol tautomerism of thymine has been investigated through the use of B3LYP density functional theory calculations. Final energies were obtained at the B3LYP/6-311+G(2df,2p)//B3LYP/6-311+G(d,p) level of theory. Ni(II) and Cu(II) lead to an oxidation of thymine which for Zn(ii) is only partial and catalyze the tautomerization process, this catalytic effect being much larger for Ni(2+) and Zn(2+) than for Cu(2+). One of the most significant consequences of the oxidation of the base is that the calculated BDE's are primarily dictated by the value of the second ionization potential of the metal, and therefore follow the sequence Cu(2+) > Ni(2+) > Zn(2+). Also importantly, metal dication association leads to a stabilization of the keto-enol tautomer, which becomes the most stable form upon interaction with Ni(2+) and Zn(2+). This stabilization enhancement is the consequence of three concomitant factors, namely, (i) a stronger interaction of the metal cation with the carbonyl oxygen, (ii) the interaction of the metal with the dehydrogenated ring nitrogen, (iii) an aromatization of the six-membered ring.

Unique {H(SiR3)2}, (H2SiR3), H(HSiR3), and (H2)SiR3 Ligand Sets Supported by the {Fe(Cp)(L)} Platform (L=CO, PR3)

This work deals with the type and incidence of nonclassical Si--H and H--H interactions in a family of silylhydride complexes [Fe(Cp)(OC)(SiMe(n)Cl(3-n))H(X)] (X=SiMe(n)Cl(3-n), H, Me, n=0-3) and [Fe(Cp)(Me(3)P)(SiMe(n)Cl(3-n))(2)H] (n=0-3). DFT calculations complemented by atom-in-molecule analysis and calculations of NMR hydrogen-silicon coupling constants revealed a surprising diversity of nonclassical Si--H and H--H interligand interactions. The compounds [Fe(Cp)(L)(SiMe(n)Cl(3-n))(2)H] (L=CO, PMe(3); n=0-3) exhibit an unusual distortion from the ideal piano-stool geometry in that the silyl ligands are strongly shifted toward the hydride and there is a strong trend towards flattening of the {FeSi(2)H} fragment. Such a distortion leads to short Si--H contacts (range 2.030-2.075 A) and large Mayer bond orders. A novel feature of these extended Si--H interactions is that they are rather insensitive towards the substitution at the silicon atom and the orientation of the silyl ligand relatively the Fe--H bond. NMR spectroscopy and bonding features of the related complexes [Fe(Cp)(OC)(SiMe(n)Cl(3-n))H(Me)] (n=0-3) allow for their rationalization as usual eta(2)-Si--H silane sigma-complexes. The series of "dihydride" complexes [Fe(Cp)(OC)(SiMe(n)Cl(3-n))H(2)] (n=0-3) is different from the previous two families in that the type of interligand interactions strongly depends on the substitution on silicon. They can be classified either as usual dihydrogen complexes, for example, [Fe(Cp)(OC)(SiMe(2)Cl)(eta(2)-H(2))], or as compounds with nonclassical H--Si interactions, for example, [Fe(Cp)(OC)(H)(2)(SiMe(3))] (16). These nonclassical interligand interactions are characterized by increased negative J(H,Si) (e.g. -27.5 Hz) and increased J(H,H) (e.g. 67.7 Hz).

Salt Effects on the Conformational Behavior of 5-Carboxy- and 5-Hydroxy-1,3-dioxane1

The varied and essential involvement of metal ions and inorganic salts in biological and chemical processes motivated the present study where 5-carboxy- and 5-hydroxy-1,3-dioxanes are used as model frameworks for the evaluation of the conformational behavior of oxygen-containing receptors in the presence of Li(+), Na(+), K(+), Ag(+), Mg(2+), Ca(2+), Ba(2+), and Zn(2+). Thus, the position of equilibria, established by means of BF(3), between diastereomeric cis- and trans-5-substituted-2-phenyl-1,3-dioxanes, in solvent THF and in the presence of 0, 1, and 5 equiv of salt, has been determined. The observed Delta G(o) degrees values for the conformational equilibria of 5-carboxy-1,3-dioxane show that Ag(+), Li(+), and Ca(2+) complexation leads to increased stability of the axial isomer. In the case of the 5-hydroxy-1,3-dioxane, Mg(2+), Ag(+), and Zn(2+) are the metal ions that stabilize the axial conformer of the heterocycle upon association. Interpretation of the experimental observations was based on DFT molecular modeling studies at the Becke3LYP/6-31G* and Becke3LYP/6-31+G** levels of theory. Although gas-phase calculations give Delta E values that are too large when modeling equilibria involving ionic species in polar solution, the computational results confirm the structural and energetic consequences of metal cation coordination to the oxygen atom in carbonyls or ethers. The results derived from the present study contribute to our understanding of the chemical processes involved in molecular recognition and physiological events.