ISSN:
1572-8951
Keywords:
Mixed-valence magnetochemistry
;
spin density distribution
;
excess diamagnetism
;
spin-orbit coupling
;
magnetic coupling
;
ring currents
;
electron delocalization
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract A special case of the influence of electron delocalization in the magnetic properties of mixed-valence inorganic compounds is presented. All the ions studied are Keggin or Wells-Dawson heteropolyanions. The data shown in this paper reflects the need of a theory that may explain the electronic properties of a type of compound intermediate between classical coordination complexes and solid state compounds. Measurements of the magnetic properties (susceptibility and NMR studies), of a series of mixed-valence oxide clusters as compared to their oxidized counterparts, show the existence of several important features. First, an excess diamagnetism is observed for diamagnetic two-electron species. This excess diamagnetism corresponds exactly to that expected for a pair of electrons delocalizable over a sphere of the radius of the molecule studied, and, therefore, may be understood as a ring-current effect similar to that found in organic aromatic systems. The correspondence of theoretical and observed diamagnetism in spherical molecules may be seen as an evidence that the ground state in two-electron species can be described as an orbitally symmetric A state. Second, the thermal study of the magnetic properties of delocalized one-electron species may be explained only by assuming a large covalency factor in the limit of what classical magnetochemistry may allow. Species containing paramagnetic transition ions see their magnetic properties changed upon reduction by a diamagnetic delocalized two electron pair. Such changes, observed as a decrease of second order effects, such as spin-orbit couplings, may only be interpreted, again, as the result of a considerable increase in covalency upon reduction. Third, if two antiferromagnetically coupled ions are present in the complex, the two electron reduction decreases this coupling, as if a shielding effect created by the ring current was present. On the other hand,183W NMR studies of paramagnetic tungstates establish that the two electron delocalization increases the electronic interaction throughout the molecule, allowing an increase of magnetic spin delocalization through covalent bonding.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00999629
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