Coordinated water

The marked increase in the acidity of water when it becomes metal-coordinated, as shown in Table 6.5, has very important ramifications. Many metal complexes will be involved in an aqua-hydroxo equilibrium in the common pH region of 3 to 11. Since the hydroxo form often 

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In an aquo-complex, loss of protons from the coordinated water molecules can occur, as with hydrated non-transition metal ions (p. 45). To prevent proton loss by aquo complexes, therefore, acid must usually be added. It is for these conditions that redox potentials in Chapter 4 are usually quoted. Thus, in acid solutions, we have... 

Recently Desimoni et used the same bis(oxazoline) ligand in the magnesium(II) catalysed Diels-Alder reaction of the N-acyloxazolidinone depicted in Scheme 3.4. In dichloromethane a modest preference was observed for the formation of the S-enantiomer. Interestingly, upon addition of two equivalents of water, the R-enantiomer was obtained in excess. This remarkable observation was interpreted in terms of a change from tetrahedral to octahedral coordination upon the introduction of the strongly coordinating water molecules. 

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other paHadium-cataly2ed oxidations of olefins ia various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

Hydroxyl Compounds. The aqueous chemistry of zirconium is complex, and in the past its understanding was compHcated by differing interpretations. In a study of zirconium oxide chloride and zirconium oxide bromide, the polymeric cation [Zr4(OH)g (H20)jg was identified (189) the earlier postulated moiety [Zr=0] was discarded. In the tetramer, the zirconium atoms are coimected by double hydroxyl bridges (shown without the coordinating water molecules) ... 

H20)4].3H20 is 10-coordinate and its stmcture is probably best regarded as being based on the same structure but with an extra water squeezed between the three coordinated water molecules. 

The coordination number of Ni rarely exceeds 6 and its principal stereochemistries are octahedral and square planar (4-coordinalc) with rather fewer examples of trigonal bipyramidal (5), square pyramidal (5), and tetrahedral (4). Octahedral complexes of Ni arc obtained (often from aqueous solution by replacement of coordinated water) especially with neutral N-donor ligands such as NH3, en, bipy and phen, but also with NCS, N02 and the 0-donor dimethylsulfoxide. dmso (Me2SO). 

The thermodynamic stability of a species is a measure of the extent to which this species will be formed from other species under certain conditions, provided that the system is allowed to reach equilibrium. Consider a metal ion M in solution together with a monodentate ligand L, then the system may be described by the following stepwise equilibria, in which, for convenience, coordinated water molecules are not shown ... 

One important difference between the present and the previous case should be noted. For the hydroquinone clathrates, where the wall of a cavity consists of 12 OH groups, 6 adjacent carbon atoms, and 6 CH groups in ortho position to the OH groups, it seemed best to consider the product z qjk) as one unknown. For hydrates one may not do this the walls of both types of cavities consist exclusively of tetrahedrally-coordinated water molecules. Hence, one should use the same value of (,eg/k) —characteristic for a water molecule in a hydrate lattice—for both types of cavities and multi-... 

The investigation of coordinated water in paramagnetic metalloproteins through N.M.R. spectroscopy. I. Bertini, Comments Inorg. Chem., 1981,1, 227-243 (68). 

Some species have a large number of exchangeable hydrogens. Thus, metal ions with many coordinated water molecules will become completely deuterated in D20. For example, Co(H20) + becomes Co(D20) +. If there were a 2 percent secondary kie per bond, it would be amplified to (1.02)12 or 1.27. It would not be a simple matter to demonstrate the operation of a primary kie in such a system because of this, given the p/f difference between the two metal ions and the general effects of the solvent change. 

This is indeed observed and, particularly in higher oxidation states, coordinated water molecules are relatively acidic (Table 9-5). Water coordinated to an iron(iii) center is a stronger acid than acetic acid ... 

These structural data are in agreement and support EXAFS data for MOP (214) as well as for xanthine oxidase (in both oxidized and reduced forms) (198, 215), but the coordinated water ligand was iden-... 

The first step consists in the attack of a proton on the W-H bond to yield a labile dihydrogen intermediate (Eq. (3)) that rapidly releases H2 to form a coordi-natively unsaturated complex (Eq. (4)). This complex adds water in the next step to form an aqua complex (Eq. (5)) that completes the reaction by substituting the coordinated water by the X anion (Eq. (6)). Steps (3)-(6) are repeated for each W-H bond and the factor of 3 in the rate constants appears as a consequence of the statistical kinetics at the three metal centers. The rate constants for both the initial attack by the acid (ki) and water attack to the coordinatively unsaturated intermediate (k2) are faster in the sulfur complex, whereas the substitution of coordinated water (k3) is faster for the selenium compound. 

The H2O is considered to come from coordinated water in Rh(III)Cl3.3 H2O. Assuming a steady state for Rh(III) Cl i yields... 

Fackler, J.J., Grant, T.A., Hanson, B.E.and Staples, R.J. (1999) Characterisation of luminescent, homoleptic, three coordinate, water soluble Au(I) complex of trisulfonated triphenylphosphine (TPPTS) as the caesium salt, Cs8[Au(TPPTS)j]. 5.25H2O. Cold Bulletin, 31, 20-23. 

ESR spectra (Table 1). The JV-cyclohexylthiosemicarbazone, 13, complex formed the expected [Fe(13-H)2] with FeCl as the counterion [141]. However, [Fe(13) (13-H)H20]C104 was isolated from ethanol. Bulkiness of the cyclohexyl group, and the perchlorate ion s greater ability to hydrogen bond are probably both important to the stability of this cation. The iron(III) center is considered six-coordinate with a tridentate 13-H, bidentate 13, and a coordinated water molecule. 

The Variation of the Lability of Coordinated Water in Nickel(II) Complexes... 

Rate constant for the exchange of a particular coordinated water molecule. b RRSS form of macrocycle. c Five-coordinate RSRS form of macrocycle. d The two inequivalent H20 exchange at different rates. 

Where kf is the rate constant for the exchange of an unspecified one of the four coordinated water molecules recalculated to second-order units [ is the first-order rate constant (s 1) for the exchange of a particular coordinated solvent molecule]. 

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