Dissociative ligand

Clearly, a free energy of binding computed with (9), (10) and (13) refers to a highly restricted state of the dissociated ligand. In order to convert such a free energy to a free energy relative to a normal standard state with volume per molecule Vg and no restriction on the molecular orientation, the following term must be added... 

Transmetalation, though, requires enhanced electrophilicity of the Pd. Additionally a free coordination site may be required, which may be freed by dissociation of either a neutral or an anionic ligand. The involvement of five-coordinate species and association-dissociation ligand-exchange mechanisms in the individual steps of Pd-catalyzed reactions also cannot be neglected (Scheme 3).384... 

These and many similar examples resulted in a highly successful general picture of transition-metal ions M coordinated by closed-shell ligands L (anionic or neutral) to form complex cluster ions [ML ]9 in solution. The characteristic coordination shell of each M corresponds to a specific number of sites, with idealized geometry that dictates the possible number of distinct [M(Li) (L2)m. .. ]q structural isomers. Each cluster ion is subject to equilibria with other cluster ions or dissociated ligands in solution,... 

Scheme 17.3 Mechanisms of C02 insertion into a metal-hydrogen bond. L represents a potentially dissociable ligand. Ancillary ligands are not shown.

The contribution of reaction 14 relative to reaction ficant. Rate constants for a variety of substituted quite similar, indicating that a dissociative ligand anism is operative (27). 

The principal photochemical reactions of metal complexes include dissociation, ligand exchange and redox processes. Unlike organic photoreactions (which take place almost exclusively from the S3 or T3 states), the excited state formed on irradiation depends on the wavelength employed. Hence the quantum yield often depends on the wavelength of the irradiating source. The excited-state processes give rise to a reactive intermediate which may find application in the synthesis of new compounds. 

In Cleland nomenclature, the initial velocity and individual rate constants are designated by lower case italicized letters (e.g., v, k, k2, etc.). Dissociation, Michaelis, and equilibrium constants utilize an upper case italicized K with the appropriate unitalicized lower case subscript. For example, the equilibrium constant would be symbolized by whereas the Michaelis constant for substrate B would be designated by K. Dissociation constants for a Michaelis complex contain a subscript i and a letter for the dissociating ligand (e.g., for the EA binary complex, the dissociation constant would be Ki ). Maximum velocities are designated by a capital italicized V, usually with a subscript 1 or 2 depending on whether the forward or reverse reaction is referred to. (If the numerical subscript is not provided, the forward reaction is assumed. In most cases, the unitalicized subscript max is also provided.)... 

A comparison with cross-linker 4a proves the underlying dynamics are controlled by metal-ligand dissociation. Ligand exchange kinetics for 4a are substantially faster than for 4b but the association thermodynamics are very similar, and the effect of those kinetics is dramatic. At 5% cross-linker, the dynamic viscosity of lOOmgmL 4a-PVP is only 6.7 Pa s, a factor of 80 less than that of the isostmctural network 4b PVP. Although the association constants are not identical, the effect of the thermodynamics would be to increase the viscosity of 4a PVP relative to 4b PVP, the opposite direction of that observed. The kinetics dominate even the extent of cross-linking 5% 4a PVP is less viscous by a factor of 5 than is 2% 4b PVP. 

Molybdenum complexes in this class are formed on dissolution of the Mo—-Mo quadruply-bonded Mo2Cl4L2L 2 species, which dissociate ligands and undergo [2 + 2] cycloaddition ... 

Photochemical Reactions of Metal Complexes. The major photoinduced reactions of metal complexes are dissociation, ligand exchange and reduc-tion/oxidation processes. The quantum yields of these reactions often depend on the wavelength of the irradiating light, since different excited states are populated. This is seldom the case with organic molecules in which reactions take place almost exclusively from the lowest states of each multiplicity Sj and Tj. 

In the kinetic trans effect, the departure of the trans ligand is probably aided by a stabilization of the transition state via the same mechanisms operative for the trans influence.114 Both associative and dissociative ligand substitution processes seem to be facilitated in this way.117... 

Although this was at first thought to indicate that an associative mechanism was indeed operative in these reactions, over the years a body of further data accumulated to suggest that this was not the case. It is now clear that the deprotonation of a co-ordinated amine is the key step in this mechanism, which is based upon dissociative ligand loss from an amido intermediate. This process is known as the SN1 cZ mechanism, and was mentioned briefly in Chapter 2, and illustrated in Fig. 2-13. The first step involves the deprotonation of the co-ordinated amine (Fig. 5-40). 

Chemical or electrochemical reduction of 18-electron precursors is, in principle, the most straightforward route to 19-electron radicals. The electrochemical reduction of stable organometallics has been studied for many years.89 By reference to Scheme 1, one can see that ligand dissociation may accompany reduction of M—L to M—L , and to generate the 19-electron complex as the predominant species it is necessary that Ksq for the 19e <-> 17e interconversion be small. This is more likely to occur as E° becomes less negative, i.e., an easily reduced (electron-poor) M—L is less prone to dissociate ligand L after reduction. Even if these conditions are met, the formation of 18-electron M2- may still be thermodynamically favored, and radical species may not be seen. Equations (11) and (15) are... 

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