Complex aqua-complexes form

Solvated ions, which form when dissolving substances in some solvent W, in essence are coordination compounds with a saturated inner coordination sphere. If the solvent is water, the so-called aqua complexes form. In this case, the number of immediately bound monodentate ligands (H2O molecules) is equal to the coordination number, N, of the metal ion. 

Mercury(II) chloride, HgC, corrosive sublimate, m.p. 280 C, b.p. 302"C. Essentially covalent material (Hg plus CL Hg plus aqua regia). Forms complex halide ions, e.g. (HgCU) (HgCL)" in excess HCl and forms complexes. Very poisonous. 

Although the aqua nickel(II) complex A was confirmed to be the active catalyst in the Diels-Alder reaction, no information was available about the structure of complex catalyst in solution because of the paramagnetic character of the nickel(II) ion. Either isolation or characterization of the substrate complex, formed by the further complexation of 3-acryloyl-2-oxazolidinone on to the l ,J -DBFOX/ Ph-Ni(C104)2 complex catalyst, was unsuccessful. One possible solution to this problem could be the NMR study by use of the J ,J -DBFOX/Ph-zinc(II) complex (G and H, Scheme 7.9) [57]. 

The three water ligands located at meridional positions of the J ,J -DBFOX/Ph aqua complexes may be replaced by another molecule of DBFOX/Ph ligand if steric hindrance is negligible. Based on molecular model inspection, the hetero-chiral enantiomer S,S-DBFOX/Ph looks like a candidate to replace the water ligands to form the heterochiral meso-2 l complex J ,J -DBFOX/Ph-S,S-DBFOX/... 

Enantioselectivities were found to change sharply depending upon the reaction conditions including catalyst structure, reaction temperature, solvent, and additives. Some representative examples of such selectivity dependence are listed in Scheme 7.42. The thiol adduct was formed with 79% ee (81% yield) when the reaction was catalyzed by the J ,J -DBFOX/Ph aqua nickel(II) complex at room temperature in dichloromethane. Reactions using either the anhydrous complex or the aqua complex with MS 4 A gave a racemic adduct, however, indicating that the aqua complex should be more favored than the anhydrous complex in thiol conjugate additions. Slow addition of thiophenol to the dichloromethane solution of 3-crotonoyl-2-oxazolidinone was ineffective for enantioselectivity. Enantioselectivity was dramatically lowered and reversed to -17% ee in the reaction at -78 °C. A similar tendency was observed in the reactions in diethyl ether and THF. For example, a satisfactory enantioselectivity (80% ee) was observed in the reaction in THF at room temperature, while the selectivity almost disappeared (7% ee) at 0°C. 

Irradiation of solutions of cw-[Rh(NH3)4Cl2]+ at 366 nm formed mainly tra .y-[Rh(NH3)4Cl(H20)]2+ with a little m-isomer. 7ya .y-[Rh(NH3)4Cl2]+ behaves similarly. The isomeric chloro aqua complexes photochemically interconvert but eventually form a stationary state. 

However, gold does react with aqua regia, a mixture of concentrated nitric and hydrochloric acids, because the complex ion [AuC14] forms ... 

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. 

When Cr3+ is reduced in aqueous solutions, the product is the aqua complex [Cr(H20)6]2+ which has an intense blue color. The reduction of Cr3+ is easily carried out by zinc and hydrochloric acid. When the solution containing Cr21 is added to one containing sodium acetate, a brick-red precipitate of Cr(C2H302)2 forms. Ibis is unusual in the fact that there are few insoluble acetates. Because Al3+ and Cr3+ have similar size-to-charge ratios, there are some similarities between the chemical behavior of the ions. 

The heat of hydration of an ion is related to its size and charge (see Chapter 7). However, in this case the aqua complex that is formed causes the d orbitals to be split in energy, and if the metal ion has electrons in the d orbitals, they will populate the t2g orbitals, which have lower energy. This results in... 

There is another problem that does not have such a simple solution. Consider the following reactions, which represent the first and last step in the reaction of a metal ion, M3+, in the form of an aqua complex, [M(H20)6]3+, with Cl- ... 

Note that in the first case, the Cl ion is approaching an aqua complex that carries a + 3 overall charge. In the second case, the Cl is approaching as aquapentachloro complex that already carries a negative charge (—2), which is electrostatically unfavorable. Therefore, even after statistical correction of the stability constants is made, there is a great deal of difference in the likelihood that [M(H20)5C1]2+ and MCI 3 will form (the values of Kt and K6). 

Another factor that affects trends in the stability constants of complexes formed by a series of metal ions is the crystal field stabilization energy. As was shown in Chapter 17, the aqua complexes for +2 ions of first-row transition metals reflect this effect by giving higher heats of hydration than would be expected on the basis of sizes and charges of the ions. Crystal field stabilization, as discussed in Section 17.4, would also lead to increased stability for complexes containing ligands other than water. It is a pervasive factor in the stability of many types of complexes. Because ligands that form tt bonds... 

What specific properties of these complexes have allowed isolation of five-coordinate Pt(IV), in the form of the trimethyl complex and the dihy-dridosilyl complexes These two types of complexes are significantly different, and their stability is apparently due to different factors. Comparing the trimethyl complex in Scheme 21(A) with the related but six-coordinate complexes of a similarly bulky oc-diimine ligand (98), shown in Scheme 23, is instructive. In Scheme 23A, triflate is clearly coordinated, exhibiting an O-Pt distance of 2.276(3) A (98), which is typical for Pt-coordinated triflate (108). This triflate complex A in Scheme 23 was obtained from dry tetrahydrofuran. The aqua complex cation B, also structurally characterized, was obtained from acetone containing trace water. An equilibrium between coordinated triflate and coordinated water, very likely via a common five-coordinate intermediate, was indicated by NMR spectroscopy (98). 

It has previously been concluded that even in strong acidic solution, the dioxotetracyanoosmate(VI) complex cannot be protonated to form the oxo aqua complex or even the corresponding hydroxo oxo complex. The pA i and pKa2 values have been estimated to be substantially less than -1, which is also supported by the relationship between pKa values and 170 and 13C chemical shifts (Table II). Extreme slow kinetic behavior, as expected in the case of a +6 charged metal center for a dissociative activation exchange process, has been observed, with only an upper limit for the oxygen exchange determined (Table II). 

Substitution of the coordinated aqua ligand trans to the oxo in complexes of the form [Rev0(H20)(SR)4]3+ with sulfur donor ligands (mainly thiourea type) in the equatorial cis plane, denoted by ReO(H2Q)SR. 

Cisplatin diaqua is very reactive, but the deprotonated hydroxo forms are usually considered to be relatively inert, therefore the acidity of the coordinated water molecules in aqua complexes can be directly relevant to their reactivity with target molecules. The pKa values of some Pt-aqua complexes are listed in Table II. 

The dapsox ligand, besides high complex stability, provides appropriate acid-base properties of the complex (Fig. 3), causes an increase in the pAa values of the coordinated water molecules (pAai = 5.8 and pi a2 — 9.5) 45h,46), which are very close to the pi a values of the native Fe -SOD enzyme ( 5 and 9) (3a). Thus at the physiological pH almost 100% of the complex is in the catalytically active aqua-hydroxo form. 

---