Ligand dissociation, exchange

The diffetentiation of the dissociative and intramolecular mechanisms is usually based on a comparison of the rates of ligand dissociation (exchange or solvolysis) and racemization. If the rates are quite similar, dissociation is assumed, whereas if racemization is much faster, then an intramolecular mechanism is operating. However, this involves the implict assumption that the initial chelate ring opening step is rate limiting for both fnocesses. 

Dissociation of a ligand from a complex may be spontaneous, acid-catalyzed, or consequent on the addition of an appropriate metal ion or ligand. Metal exchange, viz. ML+M ->M L+M, involves either dissociation of a complex (ML M+L) followed by formation of a new complex (M +L M L), or reaction through a... 

H. E. Bryndza, P. J. Domaille, R. A. Paciello, J. E. Bercaw. Kinetics and Mechanism of Phosphine Exchange for Rutheniumill) Complexes in the Series (r 5-CsMes)(PMeo,)2RuX. Ancillary Ligand Effects on Dative Ligand Dissociation. Organometallics 1989, 8, 379-385. 

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. 

The dynamic behavior of tris-chelates was of considerable interest due to the additional constraint of a third bidentate ligand, relative to the extensively studied bis-chelates, with two monodentate ligands (Section III.A.3). Also the question of whether N-Si dissociation takes place during multi-step ligand-site exchange processes was considered. 

The dissociation/recombination mechanism was further confirmed by studying the ligand facial exchange in [La(III)(N03)3 18C6] with the value for k of 35 5 s 1 which is close to k i of 50 s 1. The exchange equilibrium may be written as ligand facial exchange ... 

Many reactions of organic chemistry are catalyzed by complexes of transition-metal ions, most notably those of Group VIII. Here, die catalyst is a system of different complexes that are linked by ligand-exchange and ligand-dissociation equilibria and differ in their catalytic properties, occasionally with rather counterintuitive results such as a decrease in rate with increase in pressure in a reaction in which gas is consumed. 

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