Labile Capping Bonds in Lanthanide(III) Complexes: Shorter and Weaker

For
a wide range of trivalent lanthanide ion coordination complexes
of tricapped trigonal prism or monocapped square antiprism configurations,
the bonds between the central lanthanide ions and the capping ligands
are found to violate Badger’s rule: they can get weaker as
they get shorter. We demonstrate that this observation originates
from the screening and repulsion effect of the prism ligands. Both
effects enhance as the electric field of the central ion or the softness
of the prism ligands increases. Thus, for heavier lanthanides, despite
the fact that the capping bond could be shorter, it is more efficient
to be weakened by the prism ligands, being inherently labile. This
concept of “labile capping bonds phenomenon” is then
successfully used to interpret many problems in lanthanide­(III) hydration,
e.g., why the water exchange rate of a lanthanide­(III) complex is
much higher in a twisted square antiprism than in square antiprism
configuration. Thus, the theory proposed in this paper offers new
insights in understanding chemical problems.