Name: stepwise metal-ligand complexation
Uploaded: 2024-04-04T10:51:16.000Z
Duration: 1 min 23 s
Channel: JoVE

Dans les réactions de complexation, les cations métalliques forment des complexes avec des composés appelés ligands avec des paires d’électrons non partagées qui satisfont le nombre de coordination des ions métalliques.
Le cation métallique est un acide de Lewis, un accepteur de paires d’électrons, tandis que le ligand est une base de Lewis, un donneur de paires d’électrons, et la liaison formée entre eux est la liaison covalente dative ou de coordination. 
La constante d’équilibre d’une réaction de complexation est appelée constante de formation Kf ou constante de stabilité Ks.
Plus la valeur de la constante de formation est élevée, plus la stabilité du complexe est grande.
En général, les réactions de complexation se produisent par étapes, et la constante d’équilibre pour chaque étape est appelée constante de formation par étapes.
Cependant, pour l’ensemble de la réaction, la constante de formation cumulative ou globale β est le produit de toutes les constantes de formation par étapes.
En revanche, la réaction de dissociation du complexe est l’opposé de ses équilibres de formation, où la constante d’équilibre est appelée constante de dissociation ou d’instabilité, exprimée comme l’inverse de la constante de formation.

Équilibres de complexation : aperçu

Les réactions de complexation ont lieu lorsque des liaisons covalentes datives ou coordonnées se forment entre les ions métalliques et les ligands. Les composés formés dans ces réactions sont appelés composés de coordination. Le nombre de liaisons formées entre l’ion métallique et les ligands est appelé son numéro de coordination. Généralement, la plupart des ions métalliques présents dans une solution aqueuse sont solvatés par les molécules d’eau et existent donc sous forme de complexes aquatiques.

La constante d'équilibre de la réaction de complexation est représentée par la constante de formation Kf, également connue sous le nom de constante de stabilité Ks. A l'inverse, la dissociation du complexe est l'inverse de la formation du complexe, et la constante d'équilibre de dissociation est appelée constante de dissociation Kd ou constante d'instabilité Ki, qui est égale à l'inverse de la constante de formation Kf.

La complexation se produit généralement par étapes, où un ion métallique se complexe initialement avec un ligand, suivi du deuxième ligand, jusqu'à ce qu'il satisfasse son numéro de coordination. Les constantes d'équilibre pour chaque étape de réaction sont les constantes de formation par étapes représentées par Kf1, Kf2,…., Kfn. La formation globale ou constante cumulative β est le produit des constantes de formation par étapes, ou simplement Kf.

Les valeurs constantes de formation par étapes diminuent régulièrement en raison de facteurs statistiques, coulombiens et stériques. Les facteurs statistiques entrent en jeu une fois que le premier ligand s'est attaché à l'ion métallique, ce qui diminue le nombre de sites disponibles pour le ligand suivant et diminue donc la probabilité de liaison du prochain ligand. Les facteurs coulombiens entrent également en jeu lorsque l'ion métallique se complexe avec le premier ligand, car la charge positive sur l'ion métallique diminue, diminuant ainsi l'attraction coulombienne pour le ligand suivant. Les facteurs stériques ont le plus d'effet lorsqu'un ligand est volumineux, car les répulsions stériques créées par ce ligand affectent la facilité avec laquelle les ligands suivants interagissent avec l'ion métallique.

Dans les réactions de complexation, les cations métalliques forment des complexes avec des composés appelés ligands avec des paires d’électrons non partagées qui satisfont l’ion métallique ...

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Enseignement

JoVE Core

Analytical Chemistry

Chimie analytique

Unit 1

Unité 1

Chemical Equilibria

Équilibres chimiques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

Complexation Equilibria: Overview

stepwise metal-ligand complexation

Stepwise metal-ligand complexation forms coordination compounds when metal ions and ligands share electrons through dative, or coordinate covalent, bonds. In these reactions, the metal ion binds to one or more ligands until its coordination number is reached. This number tells how many bonds form between the metal ion and the ligands.

In aqueous solution, many metal ions are already surrounded by water molecules. These species are called aqua complexes. Complexation reactions are described by the formation constant Kf, which is also called the stability constant Ks. The reverse process is complex dissociation, and its equilibrium constant is the dissociation constant Kd, or instability constant Ki, which equals the reciprocal of Kf.

Complexation usually occurs in stages. A metal ion first binds one ligand, then another, and continues step by step until the coordination number is satisfied. The equilibrium constant for each stage is written as Kf1, Kf2, and so on through Kfn. The overall formation constant, also called the cumulative constant β, is the product of the stepwise formation constants and is often written as Kf.

The stepwise formation constants usually decrease from one step to the next. This drop happens for statistical, coulombic, and steric reasons. Statistical factors matter because each ligand that binds leaves fewer open sites for the next one. Coulombic factors matter because the metal ion becomes less positively charged after binding, so it attracts the next ligand less strongly. Steric factors matter most when a ligand is bulky, since crowding and repulsion make later bindings harder.

Stepwise Metal-Ligand Complexation

ionic strength: overview

Ionic Strength in Salt and Acid Solutions

Ionic strength describes the total electrolyte concentration in a solution. It is a quantitative measure of how many ions are present and how strongly they contribute to the solution. The concept was introduced in 1921 by Gilbert N. Lewis and Merle Randall while they were describing the activity coefficient of strong electrolytes. To calculate ionic strength, written as I or μ, both cations and anions are included. The concentration of each ion matters, but the charge number z matters even...

ionic strength: effects on chemical equilibria

ionic-strength-effects-on-chemical-equilibria

Salt Effect on Salt Solubility

Salt effect can change the solubility of a sparingly soluble salt in water. When an inert ionic compound is added, the dissolved salt may become more soluble. A common example is potassium nitrate added to a saturated calcium sulfate solution, which increases the amount of calcium sulfate that dissolves. Le Châtelier’s principle does not explain this shift well. Instead, the change is described by the effective concentration of ions in solution. Each ion is surrounded by an ionic atmosphere,...

thermodynamics: chemical potential and activity

thermodynamics-chemical-potential-and-activity

Chemical Potential in Solution Activity

Chemical potential and activity describe how a species behaves in a solution. Activity is the effective concentration of a species. It accounts for the electrolytes near that species and depends on the ionic strength of the solution. Activity is calculated as molar concentration multiplied by the activity coefficient. This makes it a more precise way to describe a species in solution than concentration alone. The thermodynamic equilibrium constant is also defined more accurately using activity...

thermodynamics: activity coefficient

Activity Coefficient in Ionic Solutions

Activity coefficient in ionic solutions describes how much a species behaves like its measured concentration. Activity is the effective concentration of a solute in solution. It equals the molar concentration multiplied by the activity coefficient, which is a unitless number. The activity coefficient depends on the total ionic strength of the solution. It shows how far a solution is from ideal behavior. When ionic strength is very low, the activity coefficient of an ionic species is close to...

chemical equilibria: redefining equilibrium constant

chemical-equilibria-redefining-equilibrium-constant

Salt Effect in Equilibrium Constant Calculations

Salt effect changes how a sparingly soluble salt dissolves when an inert salt is added. The size of this effect depends on the ionic strength of the solution. Ionic strength reflects how strongly the ions in the solution interact. In solution, the activity of a species is the product of its concentration and its activity coefficient. A redefined equilibrium constant must account for these activity effects when the ionic strength is moderately high. This form is also called the thermodynamic...

factors affecting activity coefficient

How Ionic Strength Changes Activity Coefficient

The activity coefficient of an ion in water depends on ionic strength, ion charge, and ion size. The extended Debye-Hückel equation describes these effects for aqueous solutions at 25°C. These three factors are partly linked, so changes in one can influence the others. When a solution has almost zero ionic strength, the activity coefficient is close to one. That means the solution behaves almost ideally. As ionic strength rises from 0 to 0.1 mol/L, the activity coefficient usually drops.

chemical equilibria: systematic approach to equilibrium calculations

chemical-equilibria-systematic-approach-to-equilibrium-calculations

Equilibrium Calculations with Mass and Charge

Chemical equilibrium calculations become easier when they are organized around mass balance and charge balance. This approach is useful for systems with multiple equilibria, where several reactions must be considered together. It is especially helpful when finding the solubility of a sparingly soluble salt in water with a common ion present. The first step is to identify every chemical reaction in the system. Next, write the equilibrium constant expression for each reaction. These expressions...

acid&#8211;base equilibria: activity-based definition of ph

acidbase-equilibria-activity-based-definition-of-ph

How Ionic Strength Changes pH

pH depends on hydrogen ion activity, not just hydrogen ion concentration. In an ideal solution, pH is the negative logarithm of the hydrogen ion concentration. In a non-ideal solution, a more accurate pH uses hydrogen ion activity, which combines concentration with an activity coefficient. This difference matters most in very low ionic strength solutions, such as pure water. At first, the hydrogen ion activity coefficient is close to one. When an electrolyte that does not donate or accept a...

ladder diagrams: acid&#8211;base equilibria

Using Ladder Diagrams to Predict pH

Ladder diagrams help show how acid-base equilibria change with pH. They are graphical tools that make the chemistry of a system easier to read during analysis. In a ladder diagram, the vertical axis shows pH. Horizontal bars, or steps, place the pKa values in the system.

ladder diagrams: redox equilibria

Redox Potential Shifts in Ladder Diagrams

Ladder diagrams show redox equilibrium using electrochemical potential, E, as the vertical scale. They help students see how changes in concentration affect which species is most stable. The areas of predominance are set with the Nernst equation. A useful example is the Fe3+/Fe2+ half-reaction, which has a standard-state potential of +0.771 V. At potentials more positive than +0.771 V, Fe3+ predominates. At potentials more negative than +0.771 V, Fe2+ predominates. Ladder diagrams also show...

ladder diagrams: complexation equilibria

Reading Metal-Ligand Ladder Diagrams

Ladder diagrams help show equilibria in metal-ligand complexes. In these diagrams, the vertical scale shows pL, or the concentration of free ligand that has not reacted. The horizontal lines mark the log of formation constants and show which species is dominant in each region. For the Cd and ammonia system, the formation constant K1 for Cd(NH3)2+ is 3.55 × 10^2. Its log value is 2.55, so pNH3 = 2.55 is the dividing line between Cd2+ and Cd(NH3)2+ dominance. Above 2.55, Cd2+ is the predominant...

solubility equilibria: overview

How Solubility Reaches Equilibrium

Solubility equilibrium describes what happens when a sparingly soluble salt dissolves in water and then reaches a balance with the solid left over. In a saturated solution, dissolved ions and the undissolved solid are in dynamic equilibrium. The process is reversible, so this topic is also called precipitation equilibrium. This balance matters in real life. Sugar-rich foods can produce organic acids that dissolve hydroxyapatite, the mineral in tooth enamel, which can lead to tooth decay. A...

solubility equilibria: ionic product of water

solubility-equilibria-ionic-product-of-water

Water Ionization and Kw in Equilibrium

Water ionization and the ionic product of water describe how pure water reaches equilibrium. Water is a weak electrolyte, so only a small amount breaks into hydrogen ions and hydroxide ions. At a fixed temperature, the amount of undissociated water stays almost constant. Because of this, the ionic product of water is the product of the hydrogen ion concentration and the hydroxide ion concentration. It is written as K w . The square root of K w gives the concentration of each ion in pure water.

complexation equilibria: overview

Stepwise metal-ligand complexation forms coordination compounds when metal ions and ligands share electrons through dative, or coordinate covalent, bonds. In these reactions, the metal ion binds to one or more ligands until its coordination number is reached. This number tells how many bonds form between the metal ion and the ligands. In aqueous solution, many metal ions are already surrounded by water molecules. These species are called aqua complexes. Complexation reactions are described by...

complexation equilibria: the chelate effect

complexation-equilibria-the-chelate-effect

Chelating Ligands and Metal Complex Stability

Metal complexes form when metal atoms or cations interact with ligands in a complexation reaction. These donor-acceptor adducts are central to understanding how metal ions bind in chemistry. Ligands can bind in different ways. A monodentate ligand uses one donor site. A bidentate ligand uses two donor sites, and a polydentate ligand uses more than two. Ethylene diamine is a bidentate ligand that binds through two nitrogen donor atoms and forms a five-membered ring. Polydentate ligands can...

complexation equilibria: factors influencing stability of complexes

complexation-equilibria-factors-influencing-stability-of-complexes

Complex Ions: Metal and Ligand Stability

Complex ions form when a metal cation accepts electron pairs from ligands, which are the electron pair donors. The stability of these metal complexes depends mainly on the metal ion and on the ligands attached to it. The metal ion’s size and charge play a major role in stability. As the size of the metal ion increases, complex stability usually decreases when the valency of the metal ion and the ligands stays the same. As the charge on the metal ion increases, complex stability increases. <br...

redox equilibria: overview

Redox Electron Transfer and Half-Reactions

Redox reactions involve the transfer of electrons from one species to another. In this type of reaction, electrons are not shared between atoms. They move from the species that loses them to the species that gains them. The species that loses electrons is the reducing agent, also called the reductant. It is oxidized during the reaction. The species that gains electrons is the oxidizing agent, or oxidant, and it is reduced. Redox reactions are often written as two half-reactions. One...

titrimetric methods: types and commonly used strategies

titrimetric-methods-types-and-commonly-used-strategies

Direct, Back, and Displacement Titrations

Titrimetric methods use a standard solution to measure the amount of an analyte in chemistry. The main categories are volumetric, gravimetric, and coulometric methods. Each one reaches the endpoint in a different way. In volumetric titrations, the key measurement is the volume of a titrant with a known concentration. The titrant is added until it reacts completely with the analyte. In gravimetric titrations, the standard solution forms an insoluble precipitate with the analyte. That solid is...

classification of titrimetric analysis based on reaction types

classification-of-titrimetric-analysis-based-on-reaction-types

Four Reaction Types in Titrimetric Analysis

Titrimetric analysis measures solution volume and is also called volumetric analysis. A standard solution with a known concentration is placed in the burette and is called the titrant. The solution with an unknown concentration in the flask is the analyte, or titrand. Titrimetric analysis is grouped into four types based on the reaction between the titrant and the analyte. Acid-base titrations involve a neutralization reaction between an acid and a base. This reaction forms water molecules.

key terms and concepts

Explore how metal ions bind ligands step by step, forming coordination compounds and equilibrium constants such as Kf, Kd, and β.

Stepwise Metal-Ligand Complexation | JoVE K12

analytical chemistry

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Stepwise Metal-Ligand Complexation

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