4.1 Substitution Reactions in Square Planar Complexes

Square planar complexes undergo ligand substitution reactions through associative or dissociative mechanisms. These reactions are influenced by the metal center, ligands, and reaction conditions, determining the rate and pathway of the substitution process.

Understanding the factors that affect substitution reactions in square planar complexes is crucial for predicting and controlling the stereochemistry of the products. This knowledge is essential for designing and synthesizing coordination compounds with specific properties and applications.

Ligand Substitution Mechanisms

Associative Mechanism

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• Square planar complexes undergo ligand substitution reactions through an associative mechanism where the incoming ligand approaches the complex and forms a five-coordinate trigonal bipyramidal intermediate before the leaving ligand departs
• The associative mechanism is a two-step process with the rate-determining step being the formation of the five-coordinate intermediate
• Examples of square planar complexes that undergo associative substitution include $\text{Pt(NH}_3\text{)}_4^{2+}$ and $\text{Pd(PPh}_3\text{)}_4$

Dissociative Mechanism

• In the dissociative mechanism, the leaving ligand departs first, forming a three-coordinate trigonal planar intermediate, followed by the attachment of the incoming ligand
• The dissociative mechanism is also a two-step process with the rate-determining step being the dissociation of the leaving ligand
• The choice between the associative and dissociative mechanisms depends on factors such as the nature of the metal center ($\text{Pt(II)}$ vs. $\text{Pd(II)}$), the ligands involved (bulky vs. small), and the reaction conditions (polar vs. non-polar solvents)

Factors Influencing Substitution Rates

Metal Center Properties

• The metal center's electronic configuration and oxidation state play a crucial role in determining the rate and mechanism of substitution reactions
• Complexes with a $d^8$ electronic configuration ($\text{Pt(II)}$ and $\text{Pd(II)}$) tend to undergo substitution reactions more readily than those with other electronic configurations
• Higher oxidation states generally lead to slower substitution rates due to the increased charge on the metal center and stronger metal-ligand bonds

Ligand Properties

• The nature of the ligands, including their size, charge, and electron-donating or electron-withdrawing properties, affects the rate and mechanism of substitution reactions
• Bulky ligands ($\text{PPh}_3$) can hinder the approach of incoming ligands, favoring a dissociative mechanism, while smaller ligands ($\text{NH}_3$) may facilitate an associative mechanism
• Ligands with strong electron-donating properties ($\text{NH}_3$) can increase the electron density on the metal center, making it more susceptible to nucleophilic attack and favoring an associative mechanism
• Ligands with strong electron-withdrawing properties ($\text{CO}$) can decrease the electron density on the metal center, making it less susceptible to nucleophilic attack and favoring a dissociative mechanism

Reaction Conditions

• The reaction conditions, such as temperature, solvent, and the presence of catalysts or inhibitors, can influence the rate and mechanism of substitution reactions
• Higher temperatures generally increase the rate of substitution reactions by providing more energy for the system to overcome activation barriers
• Polar solvents (water, DMSO) can stabilize charged intermediates and transition states, favoring an associative mechanism, while non-polar solvents (benzene, toluene) may favor a dissociative mechanism
• Catalysts (acids, bases) can lower the activation energy of the reaction, increasing the rate of substitution, while inhibitors (chelating agents) can slow down the reaction by blocking active sites or forming stable complexes with the metal center

Stereochemistry of Square Planar Products

Associative Mechanism Stereochemistry

• In the associative mechanism, the stereochemistry of the product is determined by the approach of the incoming ligand relative to the leaving ligand
• If the incoming ligand approaches from the same side as the leaving ligand (cis-attack), the product will have a cis-configuration
• If the incoming ligand approaches from the opposite side of the leaving ligand (trans-attack), the product will have a trans-configuration

Dissociative Mechanism Stereochemistry

• In the dissociative mechanism, the stereochemistry of the product is determined by the geometry of the three-coordinate trigonal planar intermediate and the approach of the incoming ligand
• If the incoming ligand approaches the vacant site from the same side as the remaining ligands, the product will have a cis-configuration
• If the incoming ligand approaches the vacant site from the opposite side of the remaining ligands, the product will have a trans-configuration

Influence of Chirality and Trans-Effect

• The chirality of the starting complex can influence the stereochemistry of the product in substitution reactions
• If the starting complex is chiral, the substitution reaction can lead to the formation of stereoisomers, such as enantiomers or diastereomers, depending on the mechanism and the approach of the incoming ligand
• In some cases, the chirality of the starting complex can be retained in the product, while in others, the chirality may be inverted or racemized
• The trans-effect, where certain ligands ($\text{Cl}^-$, $\text{CN}^-$) preferentially direct incoming ligands to the trans-position, can also influence the stereochemistry of the products in substitution reactions of square planar complexes