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.