Nucleophilic Attack Other Than at the Metal Atom

Nucleophilic Attack Other Than at the Metal Atom 

At square-planar complexes conjugate base (CB) formation has received relatively little attention compared with octahedral molecules. The reasons are probably that it does not produce spectacular rate enhancements and that proving the operation of this route is more difficult. Nevertheless, more and more examples are being recognized, and they fit a general scheme of reaction types at these molecules. 

Moving to palladium complexes, related early studies on [PdX(Et4-dien)]+ also indicated the possible operation of a CB mechanism. Hydroxide ions reacted differently to other nucleophiles at these steri-cally hindered molecules in showing a distinct bimolecular (A2) dependence as well as a solvolytic pathway (85). This was unexpected because OH is known to be a poor nucleophile in these systems, and it led to the proposal of the conjugate base contribution. The ions [PdX-(Et4Me-dien)]+, which have no acidic protons, did not show this behavior. The CBs reacted some 30 times faster than their precursors (a reactivity enhancement somewhat less than their gold(III) counterparts with the same ligands), again probably by an associative route. 

Normal associative ligand substitutions at these sterically hindered compounds are dominated by the solvento (Ai) route, and scope for forming the CB of aquo intermediates at high pH [(Eq. (16)] was recognized as a potential mechanistic complication (86a). 

Iodide anation studies showed that substitution at this hydroxide CB did not in fact occur, water being a better leaving group than OH . Complications of the kinetics at high pH were more likely to be due to proton extraction from the Et4-dien (86). 

---

III. Nucleophilic Attack Other Than at the Metal Atom... 

---