Lanthanides water exchange

In aqueous solution metal ions are surrounded by water molecules.36 In some cases, such as the alkali ions, they are weakly bound, whereas in others, such as [Cr(H20)6]3+ or [Rh(H20)6]3+, they may be firmly bound and exchange with solvent water molecules only very slowly for the lanthanides water exchange decreases with decreasing ionic radii.37 Coordination numbers vary extensively, depending on the size of the metal ion. For example, coordination number four is common for lithium, six is most frequently found for transition metal ions 38 higher coordination numbers are not unusual for larger ions, e.g., Bi3+ can form Bi(H20)93+.39... 

Water exchange on [Ln(H20)8]3+ for the heavier lanthanides Gd3+-Yb3+ is characterized by a systematic decrease in /feHa0 and an increase in AH as the ionic radius decreases from Tb3+ to Yb3+, and both A Si and AV-t are negative (311-313). The AV are significantly less than either the value of -12.9 cm3 mol-1 calculated for water... 

Rate Constants and Activation Parameters for Water Exchange on Lanthanide Aqua Ions... 

Fig. 10. Possible mechanistic paths for water exchange on eight- and nine-coordinate lanthanides.

EXAFS study on Eu2+ and Sr2+ in both solid state and aqueous solution gave coordination numbers of 8.0 for strontium(II) and 7.2 for europium(II) (228). The water exchange rate measured on the divalent europium aqua ion is the fastest ever measured by 170 NMR (Table XVI) (2). The activation volume is much more negative (—11.7 cm3 mol-1) than those determined on trivalent lanthanide aqua ions clearly indicating an a-activation mechanism which is most probably a limiting... 

A. Water exchange on trivalent aqua-ions of lanthanides... 

A. Water Exchange on Trivalent Aqua-Ions of Lanthanides... 

Fig. 12. Mechanistic pathways for water exchange on ennea and octa aqua ions across the lanthanide(III) series equilibrium between ennea and octa coordinated ions for Sm and Eu . ...

Water exchange on cationic lanthanide chelates can also be influenced by the nature of the counter-anions (170,171). Anions like halides, sulfate, nitrate, acetate, and fluoroacetate impose different order on the second coordination shell around the chelate by influencing the hydrogen bond network. Anions with a high charge density like CU and S04 can break up the hydrogen bond network between water molecules around the metal center and by that, slow down the water exchange rate of the inner shell water molecule (171). 

In recent years, X-ray diffraction studies of aqueous solutions have established primary hydration numbers for several fast-exchange cations 45,187-190 the timescale of X-ray diffraction is very much shorter than that of NMR spectroscopy. Octahedral hydration shells have been indicated for Tl3+,191 Cd2+, Ca2+, Na and K+, for example. For the lanthanides, [Ln(OH2)9]3+ is indicated for La, Pr and Nd, but [Ln(OH2)8]3 for the smaller Tb to Lu.192,193 Sometimes there are difficulties and uncertainties in extracting primary hydration numbers from X-ray data. Thus hydration numbers of eight and of six have been suggested for Na+ and for K+,194 and for Ca2+,195 and 8 and 9 for La3+, 196 In some cases rates of water exchange between primary and secondary hydration shells are so fast as to raise philosophical questions in relation to specific definitions of hydration numbers.197... 

The change in structure along the lanthanide series, going from La3+ to Lu3+, can be ascribed to the decrease in ionic radius of the metal center which causes an increase of the rigidity and in the steric constraint on the water binding site [35,36]. For this reason the 9-coordinate Gd complex shows a remarkably high water exchange rate. 

Neutral N-derivatized octadentate ligands based on cyclen (1,4,7,10-tetraazacy-clododecane) form tripositive cationic complexes with the trivalent lanthanides [116-124]. The N-substituted tetraamide derivatives have proven useful in understanding the relationship between the solution structure of the Ln3+ complex and its water exchange rate, a critical issue in attaining optimal relaxation efficiency of CA s [125-131]. 

Trivalent lanthanide ions are known as being kinetically very labile, with water exchange rates in the range 107—109 s 1. Complexation reactions with non-cyclic, simple, ligands... 

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