Lanthanide Contraction: What is Normal?

Abstract

Renewed interest in lanthanide contraction results from its possible effect on the properties and applications of Ln­(III) compounds and the theory related to these issues. To understand this effect, it is important to know what is a normal dependence of the contraction on the number of 4f electrons, n. The normal trend is based on recent values of ionic radii that have a linear dependence on n for coordination numbers (CNs) of 6, 8, and 9. If the normal trend is not followed, then some other interactions in the system are affecting the extent of contraction. However, the suggestion that the variation is curved and fitted by a quadratic function has become popular in recent years. This report examines the Ln­(III)-to-ligand atom distances for coordination compounds with CNs of 6–9 and the nitrides and phosphides. Least-squares fits to the linear and quadratic models are applied to all of the bond distances to determine when a quadratic model is justified. The result is that complex systems show a mixture of linear and quadratic dependencies when individual bond distances are considered and that the linear model is most common and representative of the true lanthanide contraction.

Aqueous Structure of Lanthanide–EDTA Coordination Complexes Determined by a Combined DFT/EXAFS Approach

“…15,75 The general trends are the same and show that distances between the coordinating ligands and the lanthanide ions decrease as we move to the right across the periodic table. This trend is consistent with decreasing ionic radius across the lanthanide series (lanthanide contraction 76 ) and is observed in other studies of lanthanides bound by ligands. 15,28,44,74 ■ DISCUSSION Studies examining the complexation of lanthanides by EDTA have primarily used solid-phase XRD to determine coordination structure.…”

“…The difference in distances between EXAFS and DFT structures are of a similar magnitude to differences observed in other studies of lanthanide and actinide complexes with polyaminocarboxylates. , The general trends are the same and show that distances between the coordinating ligands and the lanthanide ions decrease as we move to the right across the periodic table. This trend is consistent with decreasing ionic radius across the lanthanide series (lanthanide contraction) and is observed in other studies of lanthanides bound by ligands. ,,, …”

Aqueous Interfaces in Chemical Separations

“…Notably, this transition occurs in the middle of the lanthanide series, between Gd and Tb, which is also where the water coordination number of lanthanides drops from 9 to 8, and various other trends observed in the lanthanide series, such as molar volume, heat capacity, molar entropy, and viscosity, display a so-called “gadolinium break” . However, this terminology has been reevaluated recently as studies show that hydration number does not change abruptly around Gd but gradually decreases from 9 to 8 throughout the series. − …”

Donor Radii in Rare-Earth Complexes

“…Recent work suggests that a linear variation of Ln-donor distances across the series is more common than a curved dependence. 24,25 The calculation of Shannon's effective ionic radii relies on the assumption that interatomic distances can be approximated as the sum of cation and anion radii, which change with the coordination number. Some authors, who argued that it is doubtful that an unambiguous separation of a bond into a cation and an anion part can be reached, have heavily criticized this assumption.…”

“…Nevertheless, the Ln­(III) ions certainly comprise the most coherent family of metal ions across the periodic table in relation to their coordination properties. Recent work suggests that a linear variation of Ln-donor distances across the series is more common than a curved dependence. , …”