inorganic chemistry

the fact that the Tb^4 þion has an electronic configuration with an
exactly half-filled 4f-shell( 5 ). As a result, this lanthanide is par-
ticularly vulnerable to perturbation by an electronegative chelat-
ing ligand because the Tb^3 þion has the lowest energy barrier to
losing an electron. Thus, the observed phenomenon of the ligand-
induced gadolinium break is simplya manifestation of the half-
shell effect, where the lanthanides with the lowest ionization
energies are the most significantly affected by electron density
perturbations from a chelating ancillary ligand.
We have discovered that ancillary ligands improve analyte bind-
ing affinity over the lanthanide alone. This enhanced receptor
effect occurs in a variety of systems and appears to be ubiquitous,
that is, independent of analyte or ancillary ligand denticity or
charge, analyte aromaticity, or the lanthanide employed. In every
case, an improvement in the binding affinity of the analyte occurs
on the order of 10-fold. We also have found that the degree of

9.50

9.00

8.50

8.00

log dipicolinate affinity

7.50

7.00

0.90 0.91 0.92

Tb

Eu

Eu(DO2A)+

Sm(DO2A)+

log DPA affinity

lonization energy

Ln

3+

ionization ener

gy

(kJ / mol

)

Ln3+ electronic configuration ([Xe]4fn)

Dy(DO2A)+

Tb(DO2A)+

Sm

Dy

4f^8 → 4f^7

4f^9 → 4f^8

4f^5 → 4f^4

4f^6 → 4f^5

0.93

Ionic radius (Å)

0.94 0.95 0.96 0.97

3700

3900

4100

4300

4500

4700

9 8 6 5

FIG.7 Relationship between [Ln(DO2A)]þdipicolinate binding affin-
ity and Ln^3 þ!Ln^4 þionization energy with lanthanide ionic radius.
The [Tb(DO2A)]þcomplex has the greatest affinity for DPA^2 – because
the low ionization energy of the Tb^3 þion makes it the most susceptible
to perturbation by the DO2A ligand, shifting the electron density of the
lanthanide and thereby generating the most positive binding site for
the DPA^2 – analyte. Ionization energies from Ref. ( 5 ).

20 MORGAN L. CABLEet al.