Lanthanides: Chemistry and Use in Organic Synthesis
The use of lanthanides in organic chemistry and in organic synthesis has attracted broad interest recently because of the unique reactivities and selectivities exhibited by compounds of these rare earth elements. In particular, several major advances have
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Reiner Anwander
they are for gaseous ions. These transitions are “LaPorte-forbidden” and result in weak intensities which are responsible for the pale color of the trivalent species. General principles of d-transiton metal ligand bonding such as m-donor//acceptor interaction, the “18-electron rule”, and the formation of classic carbene, carbyne, or carbon monoxide complexes are not observed in lanthanide chemistry, neither do they form Ln=O or Ln>N multiple bonds. However, the lack of orbital restrictions, e.g. the necessity to maximize orbital overlap as in d-transition metal chemistry, allows “orbitally forbidden” reactions. Because of very small crystal-field splitting and very large spin-orbit coupling (high Z) the energy states of the 4fn electronic configurations are usually approximated by the Russel–Saunders coupling scheme [18]. The peculiar electronic properties of the f-elements have proved attractive for numerous intriguing opto- and magneto-chemical applications (“probes in life”) [15]. The inert gas-core electronic configuration also implies a conform chemical behavior of all of the Ln(III) derivatives including Sc(III), Y(III) and La(III). The contracted nature of the 4f-orbitals and concomitant poor overlap with the ligand orbitals contribute to the predominantly ionic character of organolanthanide complexes. The existing electrostatic metal ligand interactions are reflected in molecular structures of irregular geometry and varying coordination numbers. According to the HSAB terminology of Pearson [19], lanthanide cations are considered as hard acids being located between Sr(II) and Ti(IV). As a consequence, “hard ligands” such as alkoxides and amides, and also cyclopentadienyl ligands show almost constant effective ligand anion radii (alkoxide: 2.21±0.03 Å; amide: 1.46±0.02; cyclopentadienyl: 1.61±0.03) [20] and therefore fit the evaluation criteria of ionic compounds according to Eigenbroth and Raymond [21]. The ionic bonding contributions in combination with the high Lewis acidity cause the strong oxophilicity of the lanthanide cations which can be expressed in terms of the dissociation energy of LnO [12]. The interaction of the oxophilic metal center with substrate molecules is often an important factor in governing chemo-, regio- and stereoselectivities in organolanthanide-catalyzed transformations [22]. Complexation of the “softer” phosphorus and sulfur counterions is applied to detect extended covalency in these molecular systems [23,24]. Scheme 1 further indicates the tendency of the Ln(III) cations to form the more unusual oxidation states in solution [25]. Hitherto, organometallic compounds of Ce(IV), Eu(II), Yb(II) and Sm(II) have been described in detail [4]. More sophisticated synthetic approaches involving metal vapor co-condensation give access to lower oxidation states of other lanthanide elements [26]. Charge dependent properties such as cation radii and Lewis acidity significantly differ from those of the trivalent species. Ln(II) and Ce(IV) ions show very intense and ligand-dependent c
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