Identificador: TDX:846
Autors: Maestre Horcajo, Joan Miquel
Resum:
Geometry optimizations were carried out for the cis and trans forms of [Nb2W4O19]4-. Several isomers of the [Nb2W4O19H]3- anion were also studied for the cis form, suggesting that the oxygen bonded to two Nb atoms is the most basic center. The energetic determination of the oxygen basicities in hexametalates was compared with the indirect and less computationally demanding methodology based upon molecular electrostatic potential distributions. In [V2W4O19]4-, the unique OV2 was identified as the most basic center. Although at variance with the niobotungstate anion, the most basic center does not support the highest net negative charge.<br/><br/>Calculations based on density functional theory have been carried out to investigate the electronic and magnetic properties of the -Keggin anions: [XW12O40]n-, (M= W, Mo; X= AlIII, SiIV, PV, FeIII, CoII, CoIII) and [SiM11VO40]m- (M=Mo and W). The atomic populations and the distribution of the electron density computed for the studied clusters support the hypothesis that an oxidized Keggin anion is an XO4n- clathrate inside a neutral M12O36 cage. The energy gap between the band of occupied orbitals, formally delocalized over the oxo ligands, and the unoccupied d-metal orbitals, delocalized over the addenda, has been found to be independent of the central ion. However, substitution of a W or a Mo by V modifies the relative energy of the LUMO and then induces important changes in the redox properties of the cluster. In agreement with the most recent X-ray determination of [CoIIIW12O40]5- and with the simplicity of the spectra observed for this anion the calculations suggest that was a slightly distorted Td geometru. For the parent cluster [CoW12O40]6- the quadruplet corresponding to the anion encapsulating a CoII was found to be 1 eV more stable than the species formed by a CoIII and 1 e delocalized over the sphere of tungstens. The one-electron reduction of [CoIIW12O40]6- and [FeIIIW12O40]5- leads to the formation of the 1 e blue species [CoIIW12O40]7- and [FeIIIW12O40]6-. The blue-iron cluster is considerably antiferromagnetic, and in full agreement with this behavior the low-spin state computed via a Broken Symmetry approch is 196 cm-1 lower than the high-spin solution. In contrast, the cobalt blue anion has a low ferromagnetic coupling with an S-T energy gap of +20 cm-1.<br/>[PMo12O40(VO)2]5- is a highly reduced cluster with eight d metal electrons. The localization-delocalization nature of d metal electrons is one of the points of interest in reduced polyoxoanions. We report DFT calculations wich suggest that the oxidation state of the V atoms is +4 and the other six metal electrons are delocalized between the 12 Mo atoms. The calculations also provide evidence that the anion [PMo12O40(VO)2]5- should be paramagnetic. The crystal environment, wich was included in the calculations by means of two models, increases the tendency to have metal electrons.<br/>Multiplet splittings for several excited configurations of [CoIIW12O40]6- were calculated using DFT methods. In agreement with the experimental interpretation of the spectrum the calculations found that the first strong band corresponds to Co d-d transitions, but it is worth noting that superposed to these transitions there are charge transfer transitions from cobalt to tungsten. The calculations also showed the importance of Jahn-Teller distortions in the excited states. With the exception of the consequences derived from a smaller splitting of d cobalt orbitals the d-d spectrum of [CoCl4]2- is similar to that of the more complex Keggin anion. Finally, the energy of the bielectronic transition 4A24T1(P) was estimated via an approximate procedure based on ligand field theory.
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