Metals water exchange

Water exchange kinetics in labile aquo and substituted aquo transition metal ions by means of 170 n.m.r. studies. J. P. Hunt, Coord. Chem. Rev., 1971, 7,1-10 (29). 

Quantum chemical calculations have recently been extended to In.891 Adsorption has been found to be nondissociative and the metal-water interaction has been proposed to be in the sequence Hg < Ag(100) < In < Cu(100). Compared with the data in Tables 27 and 28, it appears that the positions of In and Ag(100) are exchanged. 

Fig. 1. Mean lifetimes of a single water molecule in the first coordination sphere of a given metal ion, th2o> and the corresponding water exchange rate constants, h2o- The tall bars indicate directly determined values, and the short bars indicate values deduced from ligand substitution studies. References to the plotted values appear in the text.

Fig. 7. The variation of AG-i- for water exchange on high-spin [M(H20)6]2+ and low-spin [M(H20)6]2+,3+ at 298.2 K with dn, where the closed squares represent directly determined values, and the open squares represent estimated values. The LFAE calculated for D and A mechanisms are indicated by open and closed circles, respectively. The AV are indicated by circles enclosing the rM of the metal ions in picometers.

Di or trivalent cations are able to induce the dissociation of coordinated water molecules to produce acidic species such as MOH+ (or MOH2+ for trivalent metal cations) and H+. Several infrared studies concerning rare-earth or alkali-earth metal cation exchanged Y zeolites have demonstrated the existence of such species (MOH+ or MOH2+) [3, 4, 5, 6]. However, the literature is relatively poor concerning the IR characterization of these acidic sites for LTA zeolites. The aim of the present work is to characterize 5A zeolite acidity by different techniques and adsorption tests carried on 5A zeolite samples with different ion exchange. 

Charge density is known to influence water exchange rates at a metal center (160). Since OH- is a stronger donor than OH2, this... 

The first and very fundamental question we had to address was how many solvent molecules coordinate to a metal ion. In the case of the Be2"1" cation, the coordination of four water molecules to form [Be(H20)4]2+ (at pH<3) is corroborated based on NMR (62-68), X-ray (69-74), or even neutron diffraction data (75). In parallel, these observations are also made by different types of computer-based simulations (76-79). In the case of Li+ one can find different values in the literature. While most X-ray structures demonstrate the existence of [Li(H20)4] + (80-82), [Li(H20)5]+ (83), and [Li(H20)6]+ (84) are also found. Even if one is doubtful and sceptical from a modern crystallographic point of view, e.g., [Li(H20)5]+ and [Li(H20)6]+ were studied at room temperature, we need to clarify the coordination number before the water exchange mechanism can be investigated. 

The simplest reactions to study, those of coordination complexes with solvent, are used to classify metal ions as labile or inert. Factors affecting metal ion lability include size, charge, electron configuration, and coordination number. Solvents can by classified as to their size, polarity, and the nature of the donor atom. Using the water exchange reaction for the aqua ion [M(H20) ]m+, metal ions are divided by Cotton, Wilkinson, and Gaus7 into four classes ... 

Table 1.9 Rate Constants for Water Exchange in Metal Aqua Ions...

Solvent exchange reactions have been reviewed several times in the last 10 years. A comprehensive review by Lincoln and Merbach was published in this series in 1995 (6). More recent reviews focused more on high pressure techniques for the assignment of reaction mechanisms (7-9) or on water exchange (10). This review is a follow up of the exhaustive Lincoln and Merbach review (6). The main features of solvent exchange on metal ions will be pointed out, taking into account developments and new results from the last 10 years. 

---