Square Planar Substitution Reactions

Notice that the most positive LFAEs occur for the d, LS d, and d electron configurations, which helps explain why Cr , Co , and Pt are substitutionally inert 

Example 17-1. Some octahedral substitution reactions are believed to occur by an I, mechanism, where the transition state is seven-coordinate. Assuming a pentagonal bipyramidal transition state, calculate the LFAEs for d -d . Which electron configurations will lead to substitutionally inert metal ions, assuming an I, mechanism  

In general, the LFAEs are more positive for the associative mechanism than for the dissociative mechanism, adding further support to the theory that most octahedral substitution reactions are dissociative. The largest LFAEs for the associative mechanism largely mirror those for the dissociative mechanism d, LS d, and d , with the main exception being that LS d is also now included as Inert. 

The compound [Pt(NH3)2Cl2] was first prepared In 1845 by two different synthetic routes. The a form of the compound (known as Peyrone s salt) was made by the reaction of [PtC ] with NH3, while the / form (also called Resiet s second chloride) resulted when [Pt(NH3)4]Cl2 was heated to 250 °C. Many years later, Alfred Werner showed that the two compounds were isomers of each other and suggested that they had square planar molecular geometries. Currently, we know the a form as the anticancer drug cisplatin, c/s-[Pt(NH3)2Cl2], and the form as its geometrical isomer trans-[Pt(NH3)2Cl2]. 

From his further studies of Pt(ll) chemistry, Werner postulated that the addition and elimination steps in square planar substitution reactions were both stereospecific. Consider, for example, Peyrone s reaction, which is shown in Equation (17.37). Addition of only one equivalent of NH3 or py to the tetrachloroplatinate(ll) ion leads to the monosubstituted [PtACy, where A=a N-donor ligand. The addition 

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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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