Complexes of lanthanoids with neutral pi donor ligands

The synthesis, structures and reactions of lanthanoid complexes with alkenes, alkynes and arenes have been reviewed. Whilst the discussion is centred on neutral rr donors, including intramolecular a-arene-lanthanoid bonding, some formal [Sm-III(C(5)Me(5))(2)(pi-donor)(-)] complexes, derived from Sm(C(5)Me(5))(2) and neutral pi donors, have been included, especially examples which readily dissociate into the reactants. Metal-carbon bond lengths for lanthanoid(II or III) complexes have been conveniently analysed and compared by subtraction of the appropriate lanthanoid ionic radius for the formal coordination number. Derived values in the approximate range 1.70-2.10 Angstrom are typical, with higher values often associated with intramolecular pi-arene-Ln coordination or dimerization through intermolecular pi-arene-Ln bonding. Lower values, 1.40-1.70 Angstrom, can carry implications of charge transfer, e.g. Ln(II) - L(o) --> Ln(III) - (L)(-1) or may be partly a geometric consequence of other bonding in the molecule. The significantly covalent arene-lanthanoid(0) complexes have stronger bonding than in Ln(II or III)-(pi donor) derivatives and occupy a unique place in organolanthanoid chemistry.