Lanthanides hard acids

Chemical Properties. Although the chemical properties of the trivalent lanthanides are quite similar, some differences occur as a consequence of the lanthanide contraction (see Table 3). The chemical properties of yttrium are very similar too, on account of its external electronic stmcture and ionic radius. Yttrium and the lanthanides are typical hard acids, and bind preferably with hard bases such as oxygen-based ligands. Nevertheless they also bind with soft bases, typicaUy sulfur and nitrogen-based ligands in the absence of hard base ligands. 

Coordination The lanthanides behave as typical hard acids, bonding preferentially to fluoride and... 

According to the concept of hard and soft acids and bases (HSAB) established by Pearson, lanthanide +3 ions are considered to be hard acids, falling between Mg and Ti in the established scale. Lanthanides therefore ctanplex preferentially to hard bases such as oxygen donor ligands. 

Mainly as a result of the inaccessibility of the f-electrons, directed bonding typically observed in the transition elements is not seen in the lanthanides. In many respects, the lanthanides behave more like the alkaline earth elements than like the transition elements. The lanthanides behave as hard-acid cations in solution, preferring interaction with hard-base donors like oxygen and fluoride to that with sulfur or heavy halide donors. Significant lanthanide interaction with nitrogen donors is observed only when steric factors force the interaction (as in aminopolycarboxylic acid complexes). 

Lanthanides, e.g. La(III), are considered as hard acids in the HSAB classification of Pearson (7), located between Sr (II) and Ti(IV). Divalent lanthanides are Lewis acids, as deduced from their chemical properties.Uranium ions (U(VI), U(IV)) are also hard acids. 

In both families of elements, the eations are typical hard acids. Their outermost electronic configuration resembles that of the closed-shell systems (e.g., inert gases, alkali, and alkaline-earth cations) as the inner f orbitals are largely or completely unavailable for bond formation. There is evidence for covalent bonding in actinide chemistry in the formation of the actinyl ions, AnOj and AnO in which both f and d orbitals participate in the An-O bonds, but the extent and even the existence of covalency in the bonding of simple An" species is a subject of controversy. The presence of a slight amount of covalency in lanthanide bonds has been attributed to involvement of the lanthanide 6s orbitals rather than of the 4f orbitals (Lewis et al. 1962). Differences in interactions with soft donor atoms have been the basis of a... 

The lanthanides are hard acids (Pearson, 1973) and form complexes with considerable ionic character due to the poor overlap exhibited by the contracted f orbitals. This character, as well as other properties, changes little with increasing atomic number. 

As part of a study of the cleavage of carbon-carbon bonds with a hard acid/soft nucleophile system, several lanthanide chlorides were evaluated to see if their Lewis acidity would promote a reaction similar to Luche s aldehyde acetalization (Fuji et al., 1981). Unfortunately, the reaction of lanthanum, ytterbium and cerium chlorides with a-cyano-a, )8-unsaturated esters yielded varying amounts of Michael adduct and unreacted starting material after several days of reaction. 

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