Ligand substitution organometallic systems

A recurrent problem in this area and for other substitution reactions is the separation of the electronic and steric effects of the nonreacting ligands. In organometallic systems, the steric effect is usually measured by the Tolman cone angle, 0, but the electronic effect is a mixture of the a-donor and n-acceptor abilities of the nonreacting ligands. These features have been discussed previously in Section 3.5.6. 

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. 

Ligand substitution studies continue apace in organometallic and bioinorganic chemistry, where the ligand systems are much more complicated than those considered in this chapter. It is in geochemical contexts, however, that the kinetics and mechanisms of simple substitution reactions at metal centers have recently found new... 

Reaction 10.5 initiates the chain reaction via H-atom transfer to generate Re(CO)5. Rapid associative ligand substitution occurs in reaction 10.6. H-atom transfer between the substituted metal radical and the starting hydride completes the chain, as shown in Equation 10.7. All of these reactions occur much faster in radical systems than their saturated counterparts. The result is that ligand substitution in the net reaction 10.8 (reactions 10.6 and 10.7) occurs most rapidly by a pathway involving largely unseen radical intermediates. The paper by Byers and Brown also contains two statements of relevance to those new to the area of organometallic radical chemistry ... 

In the application of these parameters to organometallic systems, the measured quantity that varies as the ligand is changed, Ay, is substituted into Equation 4 along with Eg and Cg leading to a series of simultaneous equations. The least square fit of this data produces E, and W. ... 

The application of classical and non-classical (photoacoustic) reaction-solution calorimetry to probe the energetics of metal-ligand bonds in organometallic systems is briefly analysed and illustrated by thermochemical results involving two families of compounds. The classical reaction-solution studies enabled the discussion of the systematics of metal-carbon bond enthalpies in several complexes M(t) -C H ) L. The photoacoustic studies addressed the effect of phenyl goups on the energetics of silicon-hydrogen bonds in phenyl-substituted silanes. 

Nineteen-electron organometallic complexes are rare and generally quite reactive. Recently, the ligand substitution kinetics have been studied for the following system ... 

Such a reductive elimination parallels the elimination of a pair of ligands upon the oxidation of typical dialkyl-substituted organometallics such as (bipy)2FeEt2 [223], Me4Au [224], and related LnMR2 complexes. Further exploitation of the potential photo-instability of these systems (with metals other than boron) is clearly desirable. 

Rate constants for the reactions of [Ru(NH3)5(OH2)] with ethyl glycinate, methyl sarcosinate, and methylamine have been determined at 298.2 K in aqueous solution. These have been compared with the rate constant for reaction of [Ru(NH3)s(OH2)] with ammonia, to show that alkyl substitution in ammonia reduces the affinity of ruthenium(II) for such ligands.A link between classical formation reactions of this type and organometallic systems is provided by a study of reactions of [Ru(NH3)5(OH2)] with a selection of alkenes and alkynes. ... 

It is also possible to design mixed inorganic-organometallic systems by appropriate decoration of such clusters, as in the case of MogBri4 " below. The 6 apical Br ligands can be substituted by triflates, then by functional pyridines to synthesize dicationic cluster-cored ferrocenyl dendrimers ... 

Serron S, Nolan SP, Moloy KG (1996) Solution thermochemical study of tertiary phosphine ligand substitution reactions in the RhCl(CO)(PR3)2 system. Organometallics 15 4301-4306... 

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