Isomerization dissociative, square-planar complexes

Controversy over the interpretation of the kinetics for cis-trans isomerizations of square-planar complexes continues. The topic is important for the understanding of substitution mechanisms in general. Associative and dissociative mechanisms have been proposed, as well as intramolecular rearrangements via tetrahedral intermediates. ... 

Thus, the five-coordinate intermediates formed by association of a ligand to a square-planar complex can also serve as an intermediate in cis-trans isomerization reactions " of square-planar complexes. Because dissociation of a ligand from square-planar complexes is typically less favorable than association of ligand, isomerization of... 

The cis-trans isomerization of square-planar [Au(Et)Me2(PPh3)] in benzene follows first-order kinetics k a 10 s at 70 °C) and is retarded by added phosphine in the order PMca, PMePhj PPhj. Although associative exchange of phosphine takes place via a five-co-ordinate intermediate it is argued that cis-trans isomerization is a separate process and takes a dissociative pathway involving the rearran ment of a T-shaped [AuEt(Me)2] intermediate. The process occurs in common with, but 100 times faster than reductive elimination (see Chapter 5). Other workers, however, still suggest that isomerization of these Au complexes involves pseudorotation of a five-co-ordinate intermediate. ... 

Several other recent reviews contain material relevant to this section. An article by Blandamer and Burgess on the thermodynamics, kinetics, and mechanisms of solvation, solvolysis, and substitution in nonaqueous solvents contains a contribution on the controversial dissociative mechanism for isomerization of square-planar molecules. This is outlined in Section 5.5. A review of ligand substitution reactions at low-valency transition-metal centers contains sections on five-coordinate metal carbonyl complexes and on ML4 complexes (mainly tetrahedral configurations with L being a tertiary phosphine), as well as on acid- and base-catalyzed reactions. A review by Constable " surveying the reactions of nucleophiles with complexes of chelating heterocyclic imines contains a sizable section on square-planar palladium and platinum derivatives. Most discussion centers on [Pt(bipy)2] and [Pt(phen)2] (bipy = 2,2 -bipyridine phen = 1,10-phenanthroline). The metal center, ligand, or both are susceptible to nucleophilic attack and the mechanisms involved are critically assessed. 

Cis/trans isomerizations of square-planar d complexes normally involve consecutive displacement of coordinated ligands by solvent or other ligand molecules. Stereochemical nonrigidity of a five-coordinate transient adduct or dissociative mechanisms have been less clearly defined. 

Insertion of an alkyne into a Pt-H bond provides a convenient route to alkenylplatinum(ii) complexes. Dimethyl acetylenedicarboxylate reacts with a cationic hydridoplatinum complex with two phosphine donors bonded to a calix-arene 170 to produce the alkenyl complex 171 formed via insertion of the C=C triple bond (Equation (41)). Partial dissociation of a labile amide ligand facilitates m-/r< //i -isomerization of the square-planar intermediate. The intermediate with hydrido and vr-coordinated alkyne ligands at m-positions induces facile insertion, giving the alkenylplatinum complex, followed by its isomerization to the rms-stmcture. 

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