SYSTEMATICS OF LANTHANIDE COORDINATION

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.