---
title: "Dissociative Mechanism | Inorganic Chemistry II"
description: "Dissociative mechanism in Inorganic Chemistry II is a ligand substitution path where a ligand leaves first, then a new ligand binds to the open site."
canonical: "https://fiveable.me/inorganic-chemistry-ii/key-terms/dissociative-mechanism"
type: "key-term"
subject: "Inorganic Chemistry II"
unit: "Unit 4"
---

# Dissociative Mechanism | Inorganic Chemistry II

## Definition

A dissociative mechanism is a ligand substitution pathway in Inorganic Chemistry II where a ligand leaves first, creating a lower-coordinate intermediate before the incoming ligand attaches.

## What It Is

A dissociative mechanism in Inorganic Chemistry II is a substitution reaction that starts when one ligand comes off a metal complex before the new ligand moves in. That means the metal temporarily loses a coordination partner, so you get an intermediate with one fewer ligand than the starting complex.

The key feature is the order of events. First comes bond breaking at the metal-ligand bond, then bond making with the incoming ligand. Because the leaving step happens first, the rate often depends mostly on how easily the original ligand can dissociate, not on how fast the new ligand arrives.

This is why dissociative pathways are usually discussed as first-order in the complex being substituted. The incoming ligand is not part of the slow step, so changing its concentration may not change the reaction rate much at all. That can feel backward if you are used to simple bimolecular reactions.

In square planar chemistry, this pathway can show up when the metal center is crowded, the leaving ligand is weakly bound, or the complex is set up so the bond can open without a big energy penalty. But for many square planar d8 complexes, especially Pt(II), the associative route is more common, so a dissociative mechanism is often something you compare against rather than assume.

The intermediate is usually short-lived, but it still matters. Once the coordination site opens, the metal can be attacked by a new ligand from solution, and the final product can end up with different reactivity depending on the identity of the ligand that left, the solvent, and the metal's electronic setup. In practice, you look at sterics, ligand strength, and the overall stability of the starting complex to decide whether dissociation is plausible.

## Why It Matters

Dissociative mechanism is one of the main ways you explain how a coordination complex actually changes ligands, instead of just memorizing that substitution happened. In Inorganic Chemistry II, this gives you a way to predict rate behavior, compare metal complexes, and explain why some compounds are stubbornly slow to react while others exchange ligands more easily.

It also helps you read reaction schemes correctly. If the slow step is ligand loss, then the complex's own structure, ligand field, and steric crowding matter more than the concentration of the incoming ligand. That is a different logic from an associative mechanism, where nucleophilic attack shows up first.

This term comes up any time you are asked to justify a mechanism for a square planar complex, especially when you are comparing different metals or ligands. It also connects to ideas like kinetic lability and kinetic inertness, since a complex that dissociates easily is usually more labile. When you see an unexpected substitution rate, dissociation is one of the first explanations to test.

## Connections

### substitution reaction

A dissociative mechanism is one route a substitution reaction can follow. The overall reaction is the same kind of change, one ligand is replaced by another, but the order of bond breaking and bond making tells you which mechanism is operating. If you are writing a mechanism, substitution is the broad category and dissociation is the specific pathway.

### intermediate

The dissociative pathway is defined by formation of a lower-coordinate intermediate after the leaving ligand departs. That intermediate is what makes the mechanism different from a one-step picture. In problems, identifying the intermediate helps you explain why the complex can briefly open up to attack by a new ligand.

### [platinum(ii)](/inorganic-chemistry-ii/key-terms/platinumii)

Pt(II) square planar complexes are a major comparison point in ligand substitution chemistry. They are often used to show why associative pathways are common, which makes a dissociative pathway less typical but still possible under the right steric or electronic conditions. If a problem gives you a Pt(II) complex, mechanism choice matters.

### [kinetic lability](/inorganic-chemistry-ii/key-terms/kinetic-lability)

Kinetic lability describes how quickly a complex undergoes ligand exchange. A dissociative mechanism often appears in more labile systems because a ligand can leave without a huge barrier. If a complex is inert, dissociation is harder, so the substitution rate drops.

## On the AP Exam

A quiz or problem set question might give you a square planar complex, a rate law, and a list of possible mechanisms, then ask you to pick dissociative versus associative. You would look for clues like first-order dependence on the complex, weakly bound ligands, and a plausible lower-coordinate intermediate. If a question includes changing the concentration of the incoming ligand, you should notice that a dissociative rate is usually not very sensitive to that change.

In a mechanism-writeup, you can describe the leaving ligand departing first, show the open coordination site, and then show the incoming ligand binding. If the prompt asks for interpretation, connect the mechanism to sterics, ligand strength, and the stability of the original complex. That is usually the cleanest way to justify your answer.

## dissociative mechanism vs associative mechanism

These two are easy to mix up because both describe ligand substitution. The difference is the order of steps: dissociative means the ligand leaves first and creates a vacancy, while associative means the incoming ligand binds first and raises the coordination number before anything leaves. For square planar complexes, associative is often the more common pathway.

## Key Takeaways

- A dissociative mechanism in coordination chemistry means the leaving ligand departs before the new ligand binds.
- The reaction usually goes through a lower-coordinate intermediate, which is the signature feature of this pathway.
- The rate depends mainly on how easily the original complex loses a ligand, not on how much incoming ligand is present.
- Steric crowding, ligand binding strength, and the metal's electronic setup all affect whether dissociation is likely.
- In square planar chemistry, you often compare dissociative and associative pathways, especially for d8 complexes like Pt(II).

## FAQs

### What is dissociative mechanism in Inorganic Chemistry II?

It is a ligand substitution pathway where a ligand leaves a coordination complex first, creating an open site, and then a new ligand binds. The temporary loss of a ligand gives a lower-coordinate intermediate. In this course, it shows up when you analyze how square planar complexes exchange ligands.

### How is a dissociative mechanism different from an associative mechanism?

Dissociative means bond breaking happens first, while associative means bond making happens first. That changes the coordination number during the slow step and often changes the rate law. If the new ligand concentration barely affects the rate, dissociation is a better fit than association.

### What kind of complexes can follow a dissociative pathway?

You often look for complexes where a ligand can leave without a huge energy cost, especially if the starting complex is crowded or the leaving ligand is weakly bound. Square planar complexes can do this, although many d8 systems, especially Pt(II), are more often discussed with associative substitution. The exact pathway depends on the metal and ligands involved.

### Why does the intermediate matter in a dissociative mechanism?

The intermediate is the moment when the metal has an open coordination site. That open site explains why another ligand can come in after the first one leaves. It also helps you distinguish the mechanism from a concerted or associative pathway, where the coordination number does not drop first.

## Related Study Guides

- [4.1 Substitution Reactions in Square Planar Complexes](/inorganic-chemistry-ii/unit-4/substitution-reactions-square-planar-complexes/study-guide/zBFvnXicbma8JGEL)

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