Symmetrization, and Inversion

Isomerization, Symmetrization, and Inversion. During the past year and a half reports of kinetics and mechanisms of isomerizations of platinum(n) complexes have invoked transition states in which the platinum has a co-ordination number of three, four, five, or even six. 

The dependence of isomerization and halide replacement and substitution rates, in methanol, on the shape and size of R in cM -[PtRX(PEt3)2] (R = Me, Et, Ph, o-CeHiY, or P-C6H4Y) strongly suggests that the isomerization is dissociative in character, perhaps with a transient three-co-ordinate intermediate.  

Isomerization of cw-[PtMe(N03)(PMe2Ph)2] is a slow reaction which follows the symmetrization reaction 

The symmetrization produces solely the cw-isomer, at least in dichloromethane solution. The most attractive mechanism for such stereospecific symmetrization involves a cyclic dinuclear transition state (6), in which both platinum atoms are five-co-ordinate. A similar mechanism [5 B2(cyclic)] with an analogous transition state has been suggested for closely related ligand-transfer reactions of the type  

It is almost impossible to tell whether the dinuclear species such as (6) are true transition states or transient intermediates. 

Isomerization, Symmetrization, and Inversion. Rate constants, over the commendably extended temperature range of 293—353 K, and activation energies, enthalpies, and entropies, are reported for the cis to trans isomerization of [PtX2(NH3)j] (X = Br or I) in aqueous solution. An n.m.r. study has given some very qualitative indications of the kinetics and mechanism of isomerization of the complexes [PtCl2(dmso)L] (L= pyridine or 4-methylpyridine).  

Studies on the kinetics and mechanisms of symmetrization reactions of the type shown in equation (5), mentioned in earlier volumes of this Report, continue. The 

Variable-temperature n.m.r. spectra of hexafluoroacetylacetonato-complexes of platinum(ii) and palladium(ii), [M(hfac)(LL)], e.g. (6 R=Bu ) provide evidence for 

spectroscopy has also been used to examine the linkage isomerization process of equation (6). Rate constants and activation parameters were determined, 

In the previous volume we mentioned a preliminary account of variable-temperature n.m.r. studies which showed two coalescence temperatures for solutions containing /rans-[PdX2(SEt2)2] and diethyl sulphide. These coalescences were ascribed to inversion at the sulphur (lower temperature) and ligand exchan (higher temperature). Now a full account of this work has been published, dealing with platinum(ii) as well as palladium(ii) complexes of diethyl sulphide, diethyl selenide, and diethyl 

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