Square planar substitution

In substitution reactions of both square planar platinum(ll) complexes and octahedral cobalt(in) complexes, the influence of solvent is quite marked. Note that in all solution processes the solvent plays an important role. Thus the behavior observed in H2O may differ markedly from that found in other solvents. 

It is widely accepted that square planar platinum(ll) complexes react by an 7, mechanism. BCinetic work on substitution reactions of other square complexes suggests that 7, processes predominate in these systems. When the entering ligand plays a role in determining the rate of a reaction, it is important to leam which ligands promote the most rapid reactions. 

Rate studies have indicated that ligands that exhibit a large trans effect (Section 4.8) also add rapidly to platinum(ii) complexes. Groups such as phosphines, SCN , and 1 react rapidly with platinum(II) complexes amines, Br, and Cl react at an intermediate rate and H2O and OH react slowly. This behavior reflects in part the nucleophilicity (the attraction toward a positive center) of these groups and indicates that OH is a poor nucleophile, toward platinum(li). However, the order of reactivity is clearly not an indication of only the attraction of ligands toward a positive center. If it were. Cl should react more rapidly than the larger anions Br and I . The 

Relatively few rate and mechanistic studies on tetrahedral complexes have been reported. These complexes are less common than octahedral complexes and their substitution reactions are frequently so fast that their study requires techniques such as stopped flow, temperature jump, or nuclear magnetic resonance. That the reactions are fast indicates that associative processes are occurring, since the activation energy for a reaction will be reduced when an incoming group can assist in the cleavage of a metal—ligand bond. 

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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The ki term in the rate law for square planar substitution is very clearly connected with an associative mechanism. The k term may be also. Consider the pathway (S = solvent)... 

Fig. 13. Change in the metal Pi orbital structure in square- planar substitution.

Fe—2S] ferredoxins, 38 238-239 xylene monooxygenase, 38 239 Pseudorotation, square-planar substitution reactions, 34 239-247 5-coordinate intermediate, 34 245... 

Squai e-planar rhodium I) complexes, phosphorus-nitrogen donor ligands, 44 295 Square-planar substitution reactions, 34 219-221... 

Fig. 13.3 Reaction coordinate/energy profile for a square planar substitution reaction having (a) a trigonal bipyramidal activated complex and (b) a trigonal bipyramidal intermediate. [From Burdetl, 1. K. Inorg. Chem. 1977, 16, 3013-3025. Used with permission.)...

For the remainder of this section on square planar substitution reactions, we will confine our attention to those proceeding by a nucleophilic path. We turn now to consideration of the mechanistic details of these reactions. 

An Appraisal of Square-Planar Substitution Reactions R. J. Cross... 

Square-planar substitution reactions can be presented generically as ... 

Square-planar substitution reactions frequently show two term rate laws, of the form... 

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