Ligand substitution mechanisms square-planar, 16-electron

This is most of the time encountered in ligand-substitution reactions of 16-electron complexes, the classic cases being those involving the square-planar d Pd , Pt and Rh complexes. The kinetics (rate = k [MLn] [L ]) and mechanism are analogous to those of organic SN2 reactions. 

In general terms, both steric effects and electronic factors are expected to play a role in determinating the reactivity of square-planar platinum complexes. The presence of planar amine ligands in cis- or /ran.y-Pt(anion )2 complexes and their orientation with respect to the coordination plane, as well as their substituents, can reduce the rates of DNA binding or thio binding compared to aliphatic ammine and amine complexes. Especially, substituents close to the coordination site should be expected to slow down axial substitution reactions at Pt. As there is now little doubt that DNA platina-tion is a key event (or THE key event) in the mechanism of action of platinum anticancer drugs, attention to the process of formation of the major adduct (GG) as an intrastrand cross-link between N(7) atoms of two adjacent guanine (G) residues, will remain important. 

We have already touched on some aspects of inorganic reaction mechanisms kinetically inert metal centres such as Co(III) (Section 21.10) and organometallic reaction types (Section 23.7). Now, we discuss in more detail the mechanisms of ligand substitution and electron-transfer reactions in coordination complexes for the substitution reactions, we confine our attention to square planar and octahedral complexes, for which kinetic data are plentiful. 

Eighteen-electron complexes react more slowly than similar complexes with either more or less electrons. The eighteen-electron rule explains why some reactions are associative and others dissociative. Complexes in which the metal has sixteen or less valence electrons tend to react by associative mechanisms, since the metal has vacant low-energy orbitals which can be used to form a bond with the entering ligand. This orbital can accept an electron pair from an entering ligand and provide a path for associative substitution. Substitution reactions in square planar complexes illustrate this point, reaction (40). 

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