Lanthanide coupling scheme

One of the main issues with the use of magnesium and zinc as inexpensive co-reductants is their reactivity with alkyl halides. Also, additives such as TMSOTf are relatively expensive. In an attempt to address these limitations, Namy utilised mischmetal (La 33%, Ce 50%, Nd 12%, Pr 4%, Sm and other lanthanides 1%) as the stoichiometric reductant for the regeneration of the Sm(II).31,32 This reagent system provides an important alternative since it does not require the use of additives and mischmetal is relatively inexpensive. This system has been utilised successfully in Barbier and Reformatsky reactions, halide reductions and pinacol couplings (Scheme 7.7).31,32... 

The spectra of lanthanide ions are not as sensitive to the environment as the transition metal ions. The small changes in absorption spectra of Eu(NC>3)3 and EuCb were attributed to higher symmetry of the environment in the nitrate compound [128,129]. Later systematic investigations on the band intensities of the rare earth ions were made [130— 135]. The procedure involving an intermediate coupling scheme was used in the calculation of energy levels [123,125]. 

Most published work on the energy levels in the trivalent lanthanides and actinides has been carried out in crystalline media, where the identity of a level in terms of a given coupling scheme can be experimentally established 8, 19). In attempting similar correlations in aqueous solution, one must rely heavily on the level identifications established in crystals. Where crystal data is not available, extrapolation of parameters... 

The electronic configurations of the lanthanides are described by using the Russell-Saunders coupling scheme. Values of the quantum numbers S and L corresponding to the lowest energy are derived in the conventional manner. These are then expressed for each ion in the form of a ground term with the symbolism that S, P, D, F,. .. correspond to L = 0, 1,... 

The two movements of the electrons are in fact not independent, they couple and the strength of the interaction usually increases with the atomic number. To simplify the treatment of this interaction, Russel and Saunders have proposed to consider this coupling at the level of the overall angular momenta and not for each individual electron. This model is valid for lighter elements and not quite adequate for lanthanides for which an intermediate coupling scheme should be applied however, it is in large use in view of its simplicity and we shall keep with it in this chapter. The Hamiltonian becomes ... 

When the intermediate coupling scheme is used for identification of the energy levels of the lanthanide ion, the matrix element of unit tensor operator U > in the expression for the transition amplitude (10.17) (or the line strength) exists only if the triangular condition is satisfied, namely... 

The affinity of cyanide groups for lanthanide ions has motivated the use of [M(CN)6]3 tectons (with M = Cr3+, Mn3+, Fe2+, Fe3+, Co3+) that can give rise to a wide variety of one-dimensional cyanide-bridged structures ((E) topologic mode in Scheme 4.2) [90]. Some noticeable compounds are [Ln(DMF)4(H20)2Mn(CN)6]-H20 K chains (DMF, dimethylformamide), where antiferromagnetic coupling was observed between Mn3+ tecton and Sm3+, Tb3+,... 

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