Square-planar substitution reactions bimolecular

Figure 2. Bimolecular displacement mechanism for substitution reactions of square planar complexes. ka is the rate constant for the solvent path and ky is the rate constant for the direct reagent path.

The detection of a reaction intermediate is usually not possible in coordination chemistry because lifetimes of intermediates are commonly extremely short. The simple mechanisms of reaction are commonly designated as an associative mechanism (A, with an intermediate of expanded coordination number formed) or a dissociative mechanism (D, with an intermediate of reduced coordination number formed). Intermediates of expanded coordination number are important in ligand substitution in square-planar complexes and in a few cases can actually be detected. For example, NifCNls " is known from exchange reaction of Ni(CN)4 with CN (288). Even in octahedral complexes, some evidence for associative processes exists indirectly. The [RulNHsle] " ion reacts with NO in acid to form [RuINHslsNO] and NH4 much more rapidly than can be explained by aquation of the hexaamine as the initial step, and a bimolecular mechanism with a 7-coordinate intermediate has been proposed (11, 226). 

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]. 

There is now much kinetic data on substitution reactions of square-planar complexes all of which are explained in terms of a bimolecular (8 2) displacement mechanism. For reactions such as... 

Craig et al. demonstrated that the mechanical activation of a leaving group accelerates the nucleophilic substitution of dimethyl sulfoxide for substituted pyridines at square-planar pincer Pd(ll) metal centers (Figure 14). For the first time, they compared the dynamic SMFS behavior and stress-free kinetic data for bimolecular reactions. The stress-free dissociation rate constants of 1 x (2a)2 and 1 x (2b)2 extrapolated from the corresponding DFS are 0.7 0.4 and 20 3 s , respectively, which are in excellent agreement with the values determined by dynamic NMR (1 and 17 s , respectively). ... 

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