Ligand substitution, mechanisms

Although the number of actinides is the same as that of the lanthanides, their availability and chemical characteristics have so far largely restricted the study of their ligand substitution mechanisms to dioxouranium(VI), which is the ionic form of uranium most amenable to such studies in solution. In the solid state, the oxo ligands occupy axial sites above and below the U(VI) center, and four (328), five (329, 330), and six (331, 332) oxygen donor atoms have been reported to occupy equatorial positions. From a mechanistic point of view, this variability of the occupancy of the equatorial plane suggests the possibility of both d- and a-activated ligand substitution pro-... 

Classification of the ligand substitution mechanisms is given in Table 7.2a. 

The catalytic role of die interface was recognized in various liquid/liquid extraction systems. Interfacial adsorption of reactants was the key step in the interfacial catalysis in the extraction of metal ions. The interfacial ligand-substitution mechanism has great importance in the kinetic synergism. Some essential guidelines proposed here are highly useM, not only in solvent extraction but also in interfacial synthesis. 

Although quantitative studies of substitution reactions have only been launched quite recently, a considerable body of experimental data is available, so that theoretical principles of ligand substitution mechanisms in classical inorganic complexes have been developed. Unfortunately the same cannot be said for substitution reactions involving 7r-bonded hydrocarbon ligands, in spite of a continuously expanding number of publications in this field. In the majority of studies substitution reactions are used to obtain novel transition metal 7r-complexes, and a far lesser number of papers deal with the quantitative aspects of the exchange. 

Information on ligand substitution mechanisms should aid us to understand more profoundly homogeneous catalysis by transition metal complexes, where probably consecutive substitution and transfer reactions of ligands from metal to a substrate and back take place continuously. 

Excited-State (ES) lifetimes in ambient Rh(III) haloamine solutions are measured by pulsed laser. From these data the rates of substitution from the ESs are 10 to 5 X 10 s , ca. 14-15 orders of magnitude faster than the analogous thermal rate constants. These ES reaction measurements give insight into the nature of the ES ligand-substitution mechanisms e.g., activation vols for these reactions, measured over 1-2000 atm (0.1 to 200 MPa), indicate the mechanism to be limiting dissociative in character . 

Fig. 1.9. Schematic presentation of the possible ligand substitution mechanisms. In the case of the limiting D and A mechanisms, the transition states indicate the degree of bond breakage or bond formation, respectively.

For a vibronically relaxed bound ES, ligand substitution mechanisms can be discussed in terms of models developed for analogous thermal reactions [36. The limiting mechanisms would be the dissociative (D) and associative (A) pathways, where the rate-limiting steps are, respectively, dissociation of the M-X bond or formation of the M-Y bond to form distinct intermediates (Eqs 6.16 and 6.17). The electronic nature of such intermediates is ambiguous, since these species may also be electronic excited states. For example, the cis to trans isomerization concomitant with the photoaquation of Cl from the Rh(lII) complex cis-Rh(NH3)4Cl2 was successfully explained by a model where Cl dissociation gave a pentacoordinate intermediate in a triplet LF excited state [37, 38]. 

T-bond metathesis and hydrogen transfer/exchange processes can also occur where SISHAs interactions reduce the barriers to such processes much as for H/H2 exchanges above. Chaudret has studied tr-ligand substitution mechan-... 

The Basic Factors that Control Ligand Substitution Mechanisms... 

Y. Onoe, S. Tsukahara, and H. Watarai, Catalytic effect of N,N-dimethyl-4-(2-pyridylazo)aniline on the extraction rate of Ni(II) with l-(2-pyridylazo)-2-naphthol Ligand-substitution mechanism at the liquid-liquid interface. Bull. Chem. Soc. Jpn., 71, 603-608 (1998). 

Figure 4.16 Structure and ligand substitution mechanism for the complexes [Rh(diolefin) (PN3)]BF4.

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