Ligand Exchange and Substitution

As a general statement, the reaction rate in each direction follows second-order kinetics for all the rhenium compounds studied. Moreover, the rate constants depend on the identities of L and Ly. Both findings argue for an associative (displacement) mechanism, which is also supported by the large and negative values of AS, that often reach —120 J K-1 mol-1 (39). 

It would be difficult to envisage entry of Ly into the coordination sphere of rhenium other than at the vacant position trans to the oxo group. It 

It is useful to note that the same issue does not arise in complexes that have a horizontal symmetry plane. There, a wedge geometry allows L and hj to attain equivalent positions above and below that plane. These conceptual issues have been addressed in the case of oxorhenium(V) complexes by two experimental studies, each of which supports intervention of intermediate(s) that undergo turnstile or trigonal twist mechanisms. In so doing, L and L7 attain equivalent or at least interchangeable positions. These studies are the subjects of the next two sections. 

On the basis of the preceding discussion, these complexes, where Py = RC5H4N, should undergo racemization, a process that will accompany pyridine exchange. 

When MeReO(mtp)Py complexes were examined, yet another phenomenon was revealed. With this ligand, in which the two sulfur donor atoms are inequivalent, two geometric isomers of the compound 

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E. The Chemical Mechanism at the PR3 Stage V. Digression to Ligand Exchange and Substitution... 

Heteroaromatic Sulfoxides and Sulfones Ligand Exchange and Coupling in Sulfuranes and Ipso-Substitutions... 

The formation of Mo02+ complexes and their substitution, ligand exchange and rearrangement (fluxionality) reactions have been reviewed.195... 

Heteroaromatic sulfoxides and sulfones ligand exchange and coupling in sulfuranes and //Avo-substitutions, 49, 1 Heteroaromatic systems, Claisen rearrangements in, 42, 203 Heteroaromatics, quantitative analysis of steric effects in, 43, 173 Heterocycles aromaticity of, 17, 255 chiral induction using, 45, 1 containing the sulfamide moiety,... 

Lactim ethers, chemistry of, 12, 185 Ligand exchange and coupling in sulfuranes and ipxo-substitutions, 48, 1... 

The chemistry of the coordinated nitrido group is relatively sparse. The strong trans-labilizing influence of the nitrido group, and the effect this has on the facile exchange and substitution of the trans ligand and the structure of nitrido complexes has been discussed (132). 

The electron spin echo modulation technique detects directly the coordination environment around a paramagnetic ion by observing the dipolar coupling to nuclei of weakly coordinated ligands. This technique has been used extensively by Kevan, for example, to examine transition metal ion exchanged and substituted zeolite materials [19]. 

It has been recognized for many decades that there is an intimate relationship between coordinated ligand type and physical properties. In particular, the rate at which a donor ligand can be displaced by another in the coordination sphere of an inert metal ion is markedly dependent on the type of ligand involved. If a variety of donors are bound to a metal ion, one particular site may be far more likely to undergo ligand exchange or substitution than others this selectivity is important in the efficient operation of many metalloenzymes and in the operation of certain catalysts. Even with simple octahedral corn-... 

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