Isomerization of square-planar

Mechanisms for the isomerization of square-planar complexes are either intra- or intermolecular. The simplest intramolecular process is a twist mechanism where the initial square-planar complex rearranges to its isomer via an intermediate or transition state having a T configuration ... 

However, for such well-known reactions 45> as positional isomerization and bimolecular substitution in simple square planar coordination compounds, very recently promissing attempts have been reported to derive selection rules on the basis of the orbital symmetry conservation principle 189> and state correlation diagrams 521> e.g. the cis-trans isomerization of square planar complexes has been predicted as a thermally forbidden and photochemically allowed process, in accordance with experiments 28,31,370) [see aiso section FI]. 

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. ... 

Exchange of free and bound thiocyanate has established that the linkage isomerism of square planar (Et4dien)Pd(SCN) occurs by an intermolecular rearrangement. The reaction probably proceeds through a solvent intermediate that reanates to give Ae isomerized product. ... 

There exists some ambiguity concerning the nature of the cis-trans isomerization of square-planar (LjMXJ (M = Pd or Pt) species in the presence of a catalyst L. ... 

Certain cis-trans isomerizations of square planar [notably Pd(II) or Pt(II)] complexes occur spontaneously but most are catalyzed by donor molecules, which may be free ligands added or generated in solution or coordinating solvents. Two of the mechanisms that have been considered for the catalyzed reaction are consecutive displacements with four-coordinate intermediates or intramolecular rearrangements of five-coordinate transient species. The exchange of methyl groups in 1 is strongly catalyzed... 

A brief report of the cis-trans isomerization of [PtCl2(PhCN)2] in chloroform shows this to be very slow (kc = 3.8 x 10" s kf = 2.9 x IQ" s" at 25°C.) but no mechanism is proposed. Several other papers deal with the subject of cis-trans isomerization of square-planar complexes but these all involve organometallic compounds and are reviewed in a later section of this book. 

Studies of isomerization of square-planar jff-diketonato-complexes are simpler than those of octahedral tris-jff-diketonates. A recent examination of the isomers of [Pd(benzoylacetone)2] paves the way for such a kinetic study in the palladium(ii) series. As mentioned in the platinum(ii) section above, isomerization of the hexa-fluoroacetylacetonato-complexes, for example (7) and the palladium analogue of... 

Platinum.—Continuing studies of the cis-trans isomerization of square-planar m-[Pt(R)Cl(PEt3)2] in alcohols have been quoted as further evidence for a dissociative mechanism in which a three-co-ordinate intermediate [PtR(PEt3)2]+ participates (see also Vol. 5). However, it is argued by Louw that the mass law retardation effect by Cl ions and a similar rate law to that from the dissociative process (5) is obtained for a consecutive displacement mechanism involving initial solvolysis, equations (2)—(4). Kinetic studies by the same author have also shown that the... 

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. 

Geometric isomerization of square-planar platinum(II) and palladium(II) complexes... 

Isomerization of coordination compounds is one important class of thermal transformation. In the case of inert complexes such isomerization may be irreversible and so allow the preparation of specific isomers. Some transformations of this type (e.g. isomerization of square-planar complexes) were introduced in Chapter 1 and additional examples will be discussed in Section 12.10. In this Chapter the use of solid state thermal transformations in synthetic coordination chemistry is discussed. Work on the use of thermal methods as analytical tools in coordination chemistry can be found... 

The cis-trans isomerization of square-planar transition metal complexes, in particular those of platinum(II) and palladium(II), can be understood by comparison of the trans-effects of the individual ligands (Chapter 1). This approach predicts the favored direction of isomerization for a large group of complexes with ligands of different types. In this Section examples of geometric isomerization which are not adequately explained by the trans-effect are considered and it is shown that solid state effects can influence the outcome of such reactions. 

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