Solvation metal complexes

The reactivity of a number of alkane complexes has been examined and this field has been reviewed through 1996 by Hall and Perutz. Flash photolysis of Cr(CO)6 in cyclohexane showed that solvation occurs within the first picosecond after photolysis, a fact that appears to rule out spin crossing as an important component in the dissociation of CO from Cr(CO)6. The stability of CpRe(CO)2(alkane) is particularly striking. Comparison of the rate constants for heptane solvated metal complexes with CO, Table 1, reveals that the rate constant for CpRe(CO)2(heptane) is five orders of magnitude slower than that of CpV(CO)3 (heptane). In fact, the stability of the CpRe(CO)2(alkane) complexes is so high that it has been possible to carry out low-temperature NMR on the cyclopentane complex generated by continuous photolysis of... 

In summary, ZnCl2 and the lanthanide catalysts give products derived from a classical [4 + 2] cycloaddition reaction. The side chain on the aldehyde occupies an endo position in the transition state, leading to cis (endo) products. This is probably due to the large steric bulk of the solvated metal complex, which binds anti to the aldehyde side chain. In addition, vinylogous ortho ester products, which are the expected products for a cycloaddition process, can be isolated when using these catalysts. MgBr2 also... 

Figure 3. Thermochemical cycles for electron attachment to free metal ions, gas-phase metal complexes, and solvated metal complexes. refers to heterolytic bond disruption (M-L- M + L).

Much work has been done on solvated metal complexes by Merbach and co-workers. These pressure studies have been extended to organometallic CO and SO complexes plus, interestingly, the first dihydrogen aqua-complex, Ru(H2)(H20)s, (20), produmi as shown in Equation (9) ... 

Borrowing from the tools of crystal engineering [25], here we use the crystal structures of lanthanide complexes to study interactions whose effects on solution chemistry, including extraction, may be important but are not easily measured. The examples discussed here will include crystal structores of a lanthanide cation complexed to an extractant-functionahzed EL anion, a co-crystal of an ionic lanthanide complex with neutral ligands, and a salt composed of solvated metal complexes as both cations and anions. Each of these crystalline compounds is ultimately a pure salt, and in examining their crystal structures, we will pay particular attention to how high ionic strength affects both the inner- and outer-sphere interactions observed in these stmctures. 

Magnetic circular dicliroism (MCD) is independent of, and thus complementary to, the natural CD associated with chirality of nuclear stmcture or solvation. Closely related to the Zeeman effect, MCD is most often associated with orbital and spin degeneracies in cliromophores. Chemical applications are thus typically found in systems where a chromophore of high symmetry is present metal complexes, poriihyrins and other aromatics, and haem proteins are... 

The water of hydration of these complexes can be replaced with other coordinating solvents. For example, the ethanol and methanol solvates were made by dissolving the hydrates in triethyl and trimethyl orthoformate, respectively (81,82). The acetic acid solvates are made by treating the hydrates with acetic anhydride (83). Conductivity and visible spectra, where appHcable, of the Co, Ni, Zn, and Cu fluoroborates in A/A/-dimethylacetamide (L) showed that all metal ions were present as the MLg cations (84). Solvated fluoroborate complexes of, Fe +, Co +, , Cu +, and in diethyl... 

Phase-transfer catalysis succeeds for two reasons. First, it provides a mechanism for introducing an anion into the medium that contains the reactive substrate. More important, the anion is introduced in a weakly solvated, highly reactive state. You ve already seen phase-transfer catalysis in another fonn in Section 16.4, where the metal-complexing properties of crown ethers were described. Crown ethers pennit metal salts to dissolve in nonpolai solvents by sunounding the cation with a lipophilic cloak, leaving the anion free to react without the encumbrance of strong solvation forces. 

The solvated sulfenamides [Li2( BuNSC6H4Me-4)2(THF)n] (n = 2,4) have dimeric structures with a central Li2N2 ring. The coordination mode is determined by the extent of solvation of the Li" ions monosolvation allows for rj -N,S coordination whereas disolvation restricts the coordination mode to // -M Variable temperature NMR studies indicated that a dynamic exchange between these two structural types occurs in THF solution (Scheme 10.10). The dihapto coordination mode is observed exclusively in transition-metal complexes and the... 

Solubility depends on the nature of the IL and on solvation or complex formation. Most metal ions display preferential partitioning into water in IL aqueous systems and are hence less soluble in the IL than in water. 

In the case of electrochemical reactions the partial anodic reaction results in the formation of a solvated metal cation, a charged or uncharged metal complex MX or a solid compound MX, where AT is a halogen ion, organic acid aninn, etc. 

Considering their generally similar dimensions, there are not expected to be major differences between the solvation of the open-chain and macrocyclic metal complexes arising from size differences. Nevertheless. 

Where solvent exchange controls the formation kinetics, substitution of a ligand for a solvent molecule in a solvated metal ion has commonly been considered to reflect the two-step process illustrated by [7.1]. A mechanism of this type has been termed a dissociative interchange or 7d process. Initially, complexation involves rapid formation of an outer-sphere complex (of ion-ion or ion-dipole nature) which is characterized by the equilibrium constant Kos. In some cases, the value of Kos may be determined experimentally alternatively, it may be estimated from first principles (Margerum, Cayley, Weatherburn Pagenkopf, 1978). The second step is then the conversion of the outer-sphere complex to an inner-sphere one, the formation of which is controlled by the natural rate of solvent exchange on the metal. Solvent exchange may be defined in terms of its characteristic first-order rate constant, kex, whose value varies widely from one metal to the next. 

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