Lanthanide/group 3 amido ligands

A recent exhaustive review [1] presents the known and well characterised lanthanide hydrides. Recently, a few additional original complexes have been synthesised. These include phospholyl [7] and indenyl derivatives [8], a bimetallic metallocenic structure (Scheme 1) [4], a samarium hydride supported by a calix-tetrapyrrolyl ring, with a triply bonded hydride ligand (Scheme 2) [9], metallocenes with chiral groups which has been used for the asymmetric hydrogenation [10], a tris(pyrazolyl)borate ytterbium(II) hydride [11], Cp amido-[12] and bis(amido)yttrium hydrides [13]. The latter was not isolated but obtained in situ from the methyl derivative (Scheme 2). 

Early transition metal, lanthanide and actinide alkoxy and amido complexes are common, and they often are stable because of the interaction between the filled p orbital of the O or N atom of the ligand and an empty d metal orbital. The alkoxy and aryloxy ligands play a crucial role in the catalytic properties of group 5-7 metal-alkylidene and metal-alkylidyne complexes for the metathesis of simple, double and triple bonds. - On the other hand, the behavior of late transition-metal alkoxy and amido complexes is less known. Many of them are stable, however, in spite of the possible repulsion between the filled d orbital and the p orbital of the heteroatom. The metal-heteroatom bonds are robust, and the main characteristic of these is that they are strongly polar and possess a significant ionic character. They exhibit nucleophilic reactivity and sometimes form strong bonds to proton donors (they even deprotonate relatively weak acids). 

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