Lanthanides: Comparison to 3d Metals

Abstract

Abstract Lanthanides superficially resemble the 3 d transition metals, for example, in forming coloured compounds, but there are significant differences. Their reactivity is greater, resembling the Group II elements such as magnesium, reflecting the large negative reduction potentials of the ions. Unlike the 3 d metals, their chemistry is largely of one oxidation state; this is due to large reduction potentials for the oxidation states related by one‐electron reductions common in 3 d chemistry. The lack in 4f chemistry of the very high oxidation states familiar in the 3 d transition series can be linked with the unwillingness of the 4f metals to form strong Ln O double bonds, while the lack of significant variation in the third ionization energy I 3 is an important factor in maintaining the +3 oxidation state at the end of the lanthanide series. Lanthanide complexes are relatively labile, in the absence of polydentate ligands; one result of this is that certain oxidation states cannot be stabilized by suitable choice of ligand, as with 3 d metals. The lanthanides are developing a rich and complex organometallic chemistry. Unlike the transition metals, little or no use is made of π‐acceptor ligands. One consequence of this is that the organometallics isolated are largely determined by steric factors, rather than the electronic factors embodied in the 18‐electron rule applicable in transition metal chemistry. Very unstable carbonyls are only found at low temperatures, but there are sandwich compounds stabilized by very bulky π‐arenes. In contrast with the 3 d metals, where spin‐orbit coupling is weak and ligand fields strong, the converse applies to the 4f metals. This is again a consequence of the shielding of the 4f orbitals, so that 4f electrons are relatively unperturbed by the ligands. Optical spectra are characterized by weak but very sharp transitions, while the magnetic moments are usually well described by predictions using the Russell‐Saunders coupling scheme for the ground state.