Difference Between Hapticity and Denticity

The key difference between hapticity and denticity is that hapticity refers to the coordination of a ligand to a metal centre via a series of contiguous atoms whereas denticity refers to the binding of a ligand to a metal centre via covalent chemical bond formation.

The terms hapticity and denticity come under the subtopic of coordination chemistry where we talk about the formation of a coordination complex through the association of a metal centre and ligands. These ligands may bind with the metal in different ways. If it is via a series of contiguous atoms, then it refers to hapticity, but if the ligands form covalent bonds with the metal, then it refers to denticity.

CONTENTS

1. Overview and Key Difference
2. What is Hapticity
3. What is Denticity
4. Side by Side Comparison – Hapticity vs Denticity in Tabular Form
5. Summary

What is Hapticity?

Hapticity is the coordination of a ligand to a metal centre via uninterrupted and contiguous series of atoms. We can denote it by η. i.e. if a ligand coordinates through two contiguous atoms, then we say the hapticity of the ligand is η2. Usually, we use this notation only if there are multiple atoms involved in the coordination process. Let us consider ferrocene as an example;

The metal centre of ferrocene is iron (Fe), and there are two ligands of cyclopentadienyl. The hapticity of each ligand is five because the electron cloud coordinates with the metal centre and this electron cloud forms from the contribution of all five carbon atoms of the ligand. The notation is then η5 cyclopentadienyl.

However, during a reaction, the hapticity of a coordination complex can change. Let us consider the example below. During this reaction,  η6-benzene rings changes to a η4-benzene.

What is Denticity?

Denticity is the number of donor groups of the same ligand that binds with a metal centre. Most of the times, only one atom of the ligand binds with the metal. In this case, we name the ligand as a monodentate ligand. If there are more than one donor groups that bind with the metal enter, then we name the ligand as a polydentate ligand. The denotation of these ligands is via k-notation method. i.e. if we say a ligand attached to a metal via six donor groups, then the notation is k6.

Usually, polydentate ligands are chelating agents. Therefore we classify them based on the denticity. The names of these ligands originate from the number of donor groups, i.e. if there are two donor groups, then the ligand is bidentate. Sometimes, the ligand has many donor groups, but some of them are used in the coordination process, and others are not used. And, these donor groups are available to react with another chemical species.

What is the Difference Between Hapticity and Denticity?

We need to understand the difference between hapticity and denticity clearly because we often use these terms incorrectly, thinking they are similar. The key difference between hapticity and denticity is that hapticity refers to the coordination of a ligand to a metal centre via a series of contiguous atoms, whereas denticity refers to the binding of a ligand to a metal centre via covalent chemical bond formation. Therefore, in theory, hapticity gives the number of contiguous atoms involved in the coordination process while denticity gives the number of donor groups of the ligand that attaches with a metal centre. Moreover, we use η-notation for hapticity and k-notation for denticity.

Below infographic summarizes the difference between hapticity and denticity.

Summary – Hapticity vs Denticity

In summary, hapticity and denticity are two different terms in coordination chemistry. The key difference between hapticity and denticity is that hapticity refers to the coordination of a ligand to a metal centre via a series of contiguous atoms, whereas denticity refers to the binding of a ligand to a metal centre via covalent chemical bond formation.

Reference:

1.  “A. Denticity.” Chemistry LibreTexts, Libretexts, 23 Feb. 2019, Available here.

Image Courtesy:

1. “Ferrocene” By Roland Mattern – Roland1952 (Public Domain) via Commons Wikimedia
2. “EofRu(bz)2” By Smokefoot – Own work (Public Domain) via Commons Wikimedia
3. “Oxaliplatin” By Calvero. – Selfmade with ChemDraw (Public Domain) via Commons Wikimedia