4.1 Substitution Reactions in Square Planar Complexes

Ligand Substitution Mechanisms

Square planar complexes undergo ligand substitution through either associative or dissociative pathways. The dominant mechanism for most square planar $d^8$ complexes (especially $\text{Pt(II)}$) is associative, and understanding why requires looking at the electronic and steric features of the square planar geometry. The open axial faces above and below the plane make these complexes inherently accessible to nucleophilic attack.

Associative Mechanism

The associative (A or $I_a$) pathway is the most common mechanism for square planar substitution. The incoming ligand attacks one of the open axial positions of the complex, forming a five-coordinate intermediate before the leaving group departs.

Steps of the associative mechanism:

1. The incoming nucleophile (Y) approaches along the axis perpendicular to the square plane, where there is minimal steric resistance.
2. A five-coordinate trigonal bipyramidal intermediate forms (or, in many cases, a transition state with pseudo-TBP geometry).
3. The leaving group (X) departs from the intermediate, regenerating a square planar product.

The rate law reflects the involvement of the incoming ligand in the rate-determining step:

$\text{Rate} = k_1[\text{complex}][\text{Y}] + k_s[\text{complex}]$

The $k_s$ term accounts for the solvent pathway, where the solvent acts as the incoming nucleophile before being displaced by Y. This two-term rate law is characteristic of square planar substitution and distinguishes it from purely associative or dissociative kinetics.

Typical complexes that react this way include $\text{Pt(II)}$ species such as $[\text{PtCl}_4]^{2-}$ and $[\text{Pt(NH}_3\text{)}_2\text{Cl}_2]$.

Dissociative Mechanism

Dissociative pathways are far less common for square planar complexes but can become relevant when the ligands around the metal are very bulky.

1. The leaving group (X) dissociates first, generating a three-coordinate, T-shaped (not trigonal planar) intermediate.
2. The incoming ligand (Y) then occupies the vacant coordination site.

In this case, the rate depends only on the concentration of the complex:

$\text{Rate} = k[\text{complex}]$

Because the open axial faces of square planar complexes generally make associative attack easy, a purely dissociative pathway requires enough steric crowding to block nucleophilic approach. Complexes with very bulky phosphine ligands, for example, can be pushed toward dissociative behavior.

Factors Influencing Substitution Rates

Metal Center Properties

The identity of the metal has a dramatic effect on substitution rates. All the classic square planar substrates have a $d^8$ configuration, but their lability varies enormously:

• $\text{Pt(II)}$ complexes are relatively inert, with substitution half-lives ranging from minutes to days. This makes them ideal for mechanistic study.
• $\text{Pd(II)}$ complexes are roughly $10^4$–$10^5$ times more labile than their $\text{Pt(II)}$ analogues, reacting on the millisecond timescale.
• $\text{Ni(II)}$ square planar complexes and $\text{Au(III)}$ (also $d^8$) fall at different points along this lability spectrum.

The slower rates for $\text{Pt(II)}$ arise from stronger metal-ligand bonds (relativistic effects increase the effective nuclear charge felt by the 5d electrons) and higher ligand field stabilization energy in the ground state relative to the five-coordinate transition state.

Ligand Properties: The Trans Effect and Nucleophilicity

Two ligand-related factors dominate square planar substitution kinetics:

The trans effect is the ability of a ligand already coordinated to the metal to labilize the ligand trans to itself. This is a kinetic phenomenon. The approximate trans effect series is:

$\text{H}_2\text{O} < \text{NH}_3 < \text{Cl}^- < \text{Br}^- < \text{I}^- < \text{SCN}^- < \text{NO}_2^- < \text{PR}_3 < \text{CO} \approx \text{CN}^- \approx \text{C}_2\text{H}_4$

Strong trans effect ligands weaken the bond to the ligand sitting across from them, accelerating its departure. Two explanations account for this:

• Ground-state weakening (σ-trans influence): Strong σ-donors compete for the same metal d-orbital as the trans ligand, weakening the trans M–L bond.
• Transition-state stabilization (π-trans effect): Good π-acceptors stabilize the five-coordinate intermediate by delocalizing electron density away from the metal, lowering the activation energy.

Nucleophilicity of the incoming ligand directly affects the $k_1$ term in the rate law. A nucleophilicity parameter $n_{\text{Pt}}$ (measured relative to methanol as the reference nucleophile reacting with $\text{trans-}[\text{Pt(py)}_2\text{Cl}_2]$) ranks incoming ligands:

$\text{CH}_3\text{OH} \approx 0 < \text{Cl}^- \approx 3.0 < \text{NH}_3 \approx 3.1 < \text{Br}^- \approx 4.2 < \text{I}^- \approx 5.5 < \text{PR}_3 \approx 8.9$

Soft, polarizable nucleophiles react fastest with the soft $\text{Pt(II)}$ center, consistent with the HSAB principle.

Steric and Solvent Effects

• Steric effects of spectator ligands: Bulky cis ligands slow associative substitution by blocking the incoming nucleophile's approach to the axial face. This is why cis-positioned bulky phosphines can retard substitution dramatically.
• Polar, coordinating solvents (water, DMSO) participate in the $k_s$ solvent pathway and can stabilize the charged transition state. Non-coordinating solvents suppress the solvent pathway, making the reaction rate more dependent on the incoming nucleophile.
• Temperature increases rates as expected from the Eyring equation. Activation parameters ($\Delta H^\ddagger$ and $\Delta S^\ddagger$) help distinguish mechanisms: associative pathways typically show negative $\Delta S^\ddagger$ (more ordered transition state), while dissociative pathways show positive $\Delta S^\ddagger$.

Stereochemistry of Square Planar Substitution

Retention of Configuration

A defining feature of associative substitution in square planar complexes is that it proceeds with retention of stereochemistry. Here's why:

1. The incoming ligand enters along the axial direction, forming a trigonal bipyramidal intermediate.
2. In this TBP intermediate, the entering and leaving groups both occupy positions in the trigonal plane.
3. The leaving group departs from the trigonal plane, and the remaining ligands relax back to a square planar arrangement that preserves the original cis/trans relationships of the spectator ligands.

This means that if you start with a cis isomer, you get a cis product, and likewise for trans. Isomerization during substitution is uncommon through the standard associative pathway.

The Trans Effect and Synthetic Stereochemistry

The trans effect is the primary tool for controlling which product isomer you obtain during stepwise synthesis. The classic example is the synthesis of cis- vs. trans-$[\text{Pt(NH}_3\text{)}_2\text{Cl}_2]$:

To make cis-platin ($\textit{cis}$-$[\text{Pt(NH}_3\text{)}_2\text{Cl}_2]$):

1. Start with $[\text{PtCl}_4]^{2-}$.
2. Add $\text{NH}_3$. It replaces one $\text{Cl}^-$. Now the ligand trans to the remaining $\text{Cl}^-$ ligands is either $\text{Cl}^-$ or $\text{NH}_3$.
3. Add a second $\text{NH}_3$. Because $\text{Cl}^-$ has a stronger trans effect than $\text{NH}_3$, the $\text{Cl}^-$ trans to another $\text{Cl}^-$ is labilized preferentially. The second $\text{NH}_3$ enters trans to $\text{Cl}^-$, giving the cis product.

To make trans-platin ($\textit{trans}$-$[\text{Pt(NH}_3\text{)}_2\text{Cl}_2]$):

1. Start with $[\text{Pt(NH}_3\text{)}_4]^{2+}$.
2. Add $\text{Cl}^-$. It replaces one $\text{NH}_3$.
3. Add a second $\text{Cl}^-$. Now $\text{Cl}^-$ (stronger trans effect) labilizes the $\text{NH}_3$ trans to it, so the second $\text{Cl}^-$ enters trans to the first, giving the trans product.

Trans Influence vs. Trans Effect

Don't confuse these two related but distinct concepts:

• The trans effect is kinetic: it describes how fast the trans ligand is replaced (rate of substitution).
• The trans influence is thermodynamic: it describes how much a ligand weakens the ground-state bond to the trans ligand (observable by bond lengths in crystal structures or stretching frequencies in IR).

Both follow roughly the same ordering of ligands, but they operate through different mechanisms and are measured differently.