This article discusses the general trends of lanthanide coordination i
n solution. The bonding in these compounds is best described by an ion
ic model. Minor covalent interactions are inferred from spectral measu
rements; however, these probably involve the 5d or 6s rather than the
4f orbitals. The enthalpy and entropy of these reactions are largely c
ompensatory. Nevertheless, deviations are observed for systems involvi
ng strong metal-donor interactions such as the polyaminocarboxylates.
The kinetics of complexation of these polyaminocarboxylates are interp
reted in terms of a dissociative interchange mechanism. The rate-deter
mining step probably involves the formation of a distorted LnY comple
x (low DELTAH# and negative DELTAS#). The use of NMR and chiroptical m
ethods to study the conformational preference and mode of coordination
in these complexes are also discussed.