Solvolysis reactions planar complex

The planar complex reacts much faster than the other two, with a negative AS suggesting associative activation. The substitution of R-sal by H2-salen for the other two complexes appears to be dominated by rapid planar/octahedral equilibria in solution, and the octahedral form is kinetically inert. The rate differences span four orders of magnitude in acetone, but are reduced by a factor of 30 when MeOH is employed as solvent. This is probably caused by coordination of the more polar methanol, leading to more stable octahedral adducts. Interestingly, the observed rate constant for the reaction of the planar complex is greater in methanol than it is in acetone. The likely explanation is a contribution from an associative solvolysis pathway in the more polar solvent. 

The substitution reactions (23), where the chelate ligands are cyclo-octa-1,5-diene (cod), ethylenediamine (en), or iV,iV -dimethylethylenediamine, offer two further illustrative examples of square-planar substitution reactions, where reversible solvolysis of the substrate complex has to be considered. In these cases also, the observed rate constants depend on the concentration of the leaving ligand (X=C1 ), so the more complete rate law (2) applies. 

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. 

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