us decipher much of the history of processes in our solar system,
including the early development of the feldspar-rich lunar crust ( 7 ).
In aqueous solution, lanthanides are most stable in the
tripositive oxidation state, making them difficult to separate
and purify. The preference for this oxidation state is due in part
to the energy of the 4f electrons being below those of the 5d and
6s electrons (except in the cases of La and Ce). When forming
ions, electrons from the 6s and 5d orbitals are lost first so that
all Ln^3 þ ions have [Xe] 4fn electronic configurations. Under
reducing conditions, certain lanthanides (europium, samarium,
and ytterbium) can be stable as dipositive ions, and cerium can
adopt aþ4 oxidation state( 5 ).
Lanthanide ions are hard Lewis acids: the 4f orbitals are well
shielded by the 5s and 6p orbitals and do not participate directly
in bonding,p-bonding is disfavored, and there are no complexes
with LnO or Ln N multiple bonds. Coordination to Ln^3 þis
ionic in character, leading to a strong preference for negatively
charged or neutral donor groups with large ground state dipole
moments (Fig. 1). Combinations of amines and carboxylic acid
groups are therefore often used for lanthanide complexation
(8,9). Coordination geometries in lanthanides are determined by
ligand steric factors as opposed to orbital overlap or crystal field
effects (10,11). In aqueous solution, donor groups containing
neutral oxygen or nitrogen atoms generally bind when present in
multidentate ligands, such as podands, crown ethers, and cryp-
tates ( 12 – 15 ). Relatively few complexes of monodentate nitrogen
donors are known, suggesting an oxophilic tendency of lanthanide
binding. This preference for oxygen donors also makes lanthanides
quite lithophilic and explains their occurrence in silicates as
opposed to metallic or sulfidic minerals ( 16 ).
FIG. 1. Binding of an analyte to a lanthanide-based receptor pro-
duces a luminescent ternary complex. The ancillary (receptor or helper)
ligand is shown in dark gray.
LUMINESCENT LANTHANIDE SENSORS 3