The coordination number of [Ln(H 2 O)n]^3 þis normally 9 for the
early lanthanides (La–Eu) and 8 for those later in the series
(Dy–Lu), with the intermediate metals (Sm–Dy) exhibiting a
mixture of species. However, the coordination number can be
dictated by the steric bulk of the coordinating ligands, and
species with coordination numbers as low as 2 and as high as
12 are known(5,17).
With the 4f electrons well shielded from the environment,
the spectroscopic and magnetic properties of the lanthanides
(e.g., electronic spectra and crystal field splittings) are largely
independent of environmental factors (solvent, coordinated
ligands). The number of electronic configurations is a function of
the number of unpaired electrons where 0x14, with the lowest
energy term for each ion consistent with the predictions of Hund's
first and second rules(18,19). Owing in part to spin–orbit coupling,
the lanthanides exhibit a rich energy level pattern, with the lowest
electronic excited states significantly above the ground state ( 20 ).
As f–f transitions are electric dipole forbidden (but magnetic dipole
allowed), lanthanide ion absorptions are very weak
(E0.1 mol–^1 dm^3 cm–^1 )(5,21– 23 ). Electronic transitions must
involve promotion of an electron without a change in its spin
(DS¼0) and with a variation of either total angular momentum
or total angular quantum number of one unit at most (DL¼1.0;
DJ¼1.0). Though absorption of radiation can in theory promote
the lanthanide ion to any energetically accessible state, emission
normally occurs only from the lowest lying energy level of the first
excited term due to rapid internal conversion (IC) ( 19 ). In cases of
low symmetry or vibronic coupling, the f–f transitions can gain
intensity through f- and d-state mixing with higher electronic
states of opposite parity. Broad 4fn!4fn–^1 5d^1 transitions also
can be seen in the infrared region for some lanthanides.
The electronic configurations of Ln^3 þions are split by electronic
repulsion, with term separations on the order of 10^4 cm–^1 (Fig. 2)
( 24 ). These terms are split further by spin–orbit coupling into J
states, with energy differences in the 10^3 cm–^1 range ( 25 ). These
spectroscopic levels can be split once again into what are termed
Stark sublevels due to ligand-field effects from the coordination
sphere around the lanthanide; Stark sublevel splitting is on the
order of 10^2 cm–^1 ( 26 ). The emission peak positions in Ln^3 þ
complexes do not vary substantially, because the f-electrons are
shielded, but an emission profile (defined as the relative intensity
and degree of splitting of an emission band) can vary greatly
(21,27). The number of Stark sublevels depends on the site symme-
try of the lanthanide ion, and these can be thermally populated at
room temperature, yielding more complex emission spectra.
4 MORGAN L. CABLEet al.