Ligand substitutions defined

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

The above examples demonstrate the enormous synthetic potential of the electron-rich complex fragment Re(dppe)2 " which allows well-defined ligand substitutions in the axial positions and a variety of reactions at the coordinated ligands. This has also been observed for other ligands... 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

The most important observation in the pre-steady-state kinetics of the CN system is that after a short lag (100 msec) there is a phase (lasting about 3 sec) where the evolution of H2 is linear and only after these 3 sec does CN reduction occur. This long lag prior to CN reduction would correspond to 18 to 20 electron transfer steps from the Fe protein. More realistically this delay probably involves a CN -induced modification of the enzyme, such as a ligand substitution reaction (this modified state of the enzyme is designated as. E in Figure 21). However, this modification step is too slow to be part of the steady-state cycle for CN reduction. Also, it cannot be a slow activation of the enzyme prior to turnover, since the onset of H2 evolution is the same in both the presence and the absence of CN . Steady-state observations indicate that cyanide binds to a more oxidized form of the MoFe protein than binds N2, but that state cannot be defined because of the long lag phase. 

Catalytic reactions can be analyzed into a cyclic series of stoichiometric steps, for each of which there are many well-understood model systems. The most frequently encountered steps are ligand substitution oxidative addition ligand migration (or migratory insertion) nucleophilic attack reductive elimination and p-and a-elimination. Catalytic cycles are defined by a sequence of several such reactions at the metal centre the organometallic steps are often preceeded or followed by purely organic reactions. 

Over the last 20 years, metathesis catalysts have evolved from poorly defined, heterogeneous mixtures to well-defined, single-component metallocycles and alky-lidenes [9-18]. These complexes react in controlled, consistent ways, and their activities can be attenuated through simple ligand substitution. In contrast to hetero-... 

Let us first consider the situation where initial excitation is followed by relaxation to a bound LEES, which is then responsible for the ligand substitution chemistry. In accord with the above discussion, the quantum yield <1>S for ligand substitution from that state would be fl>lscfcst, where intersystem crossing from the state(s) initially formed, ks is the rate constant for ligand substitution from the LEES, and r = kd1 (kd being the sum of the rate constants for the decay of the LEES). The apparent activation volume for the photoreaction quantum yields is therefore defined as... 

Rate parameters for the reaction of nickel(II) with pyridine, bipyridine, phenanthroline, and terpyridine are available [118] for dimethyl-sulphoxide and acetonitrile solvents along with similar data for terpyridine in methanol and ethylene glycol (Table 12). To test whether or not substitution is normal in type, ligand substitution rate coefficients (fej) are compared with the respective solvent exchange rates (fes) by means of a ratio R defined as... 

Many other reagents and reaction conditions accelerate ligand substitution, but are less defined than the examples described above. A variety of nucleophilic reagents, such as phosphine oxides, accelerate ligand dissociation from stable octahedral carbonyl complexes, but the mechanism that accounts for this acceleration is not well imderstood. ... 

The complexes [Rh(diolefin)(PN3)]BF4 feature P,N-coordinated chelated hg-ands with two pendant amino groups. The corresponding metallacycle has a boat like conformation (racemic chiral), which defines concave and convex sites for the attack of the pendant ligands in an associative ligand substitution process of the coordinated by the pendant amino groups (Figure 4.16) [40]. 

The isomer possibilities in DOTA derivatives can increase dramatically. For example, introducing a chiral center by substitution at Ca, of the acetate arms of DOTA can gives rise to six ligand isomers defined by the absolute configuration at the carbon (SSSS), RSSS (SRRR), and the achiral... 

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