Coordination Bonds and Complex Ions

What Are Coordination Bonds?

Definition

Coordination bond

A coordination bond (also known as a dative covalent bond) forms when both electrons in the shared pair come from the same atom.

In the context of complex ions, this occurs when a ligand donates a lone pair of electrons to a transition metal cation.

Key Definitions

Ligand: A molecule or ion that donates a lone pair of electrons to a central metal ion to form a coordination bond.

Example

Common ligands include water ( $H_{2} O$ ), ammonia ( $N H_{3}$ ), and chloride ions ( $C l^{-}$ ).

Transition Metal Cation: A positively charged ion of a transition metal, which has vacant orbitals capable of accepting electron pairs from ligands.
Complex Ion: A charged species consisting of a central metal ion surrounded by ligands bonded through coordination bonds.

Tip

Ligands must have at least one lone pair of electrons to form a coordination bond with the metal ion.

How Are Coordination Bonds Formed?

Coordination bonds are formed when ligands donate their lone pairs of electrons to the empty orbitals of a transition metal cation.
This process can be broken down into the following steps:
1. Electron Pair Donation: The ligand, which has a lone pair of electrons, approaches the transition metal cation.
2. Orbital Overlap: The lone pair of electrons from the ligand overlaps with an empty orbital on the metal ion.
3. Bond Formation: A coordination bond is established, resulting in the formation of a complex ion.

Why Transition Metals?

Transition metals are particularly suited to forming complex ions because:

They have vacant d-orbitals that can accept electron pairs.
Their high charge density attracts electron-rich ligands.
They can exhibit multiple oxidation states, allowing for a variety of coordination environments.

Analogy

Think of the transition metal cation as a host at a party with empty seats (orbitals) at their table. The ligands are guests who bring their own chairs (electron pairs) to fill the empty spots and form bonds.

Example

The [Cu(NH₃)₄]²⁺ Complex Ion

Let’s examine the formation of the tetraamminecopper(II) ion, $[C u (N H_{3})_{4}]^{2 +}$ , as an example of a complex ion.

The Metal Ion: Copper(II) ion ( $C u^{2 +}$ ) acts as the central metal ion. It has a high charge density and vacant orbitals to accept electron pairs.
The Ligand: Ammonia ( $N H_{3}$ ) molecules act as ligands. Each $N H_{3}$ molecule has a lone pair of electrons on the nitrogen atom.
Bond Formation: Four $N H_{3}$ molecules donate their lone pairs to the $C u^{2 +}$ ion, forming four coordination bonds. This results in the complex ion $[C u (N H_{3})_{4}]^{2 +}$ .

Representation of the Complex Ion

The structure of $[C u (N H_{3})_{4}]^{2 +}$ can be represented as:

$[C u (N H_{3})_{4}]^{2 +}$

Here:

The central $C u^{2 +}$ ion is surrounded by four $N H_{3}$ ligands.
The overall charge of the complex ion is $+ 2$ , which is the sum of the charge on the metal ion and the neutral ligands.

Note

In the $[C u (N H_{3})_{4}]^{2 +}$ complex, the copper ion accepts four lone pairs of electrons from four ammonia molecules, forming a stable coordination complex.

Determining the Charge on a Complex Ion

To deduce the charge on a complex ion, you can use the oxidation state of the central metal ion and the charges of the ligands present.
Ligands can be either neutral (e.g., $H_{2} O$ , $N H_{3}$ ) or negatively charged (e.g., $C l^{-}$ , $C N^{-}$ ).
The overall charge of the complex ion is the sum of the oxidation state of the metal and the charges contributed by all ligands.

Example

In the complex ion $[{Fe(CN)}_{6}]^{4 -}$ :

Cyanide ( $C N^{-}$ ) carries a charge of -1 each, contributing a total charge of $- 6$ ( $6 \times - 1$ ).
The overall charge of the complex ion is -4, so the oxidation state of iron must be +2 since $+ 2 + (- 6) = - 4$ .

Tip

By carefully summing the metal oxidation state and ligand charges, you can systematically determine the charge of any complex ion.

Common ions formed by the transition elements:

Common oxidation states of transition elements.

Properties of Complex Ions

Complex ions exhibit unique properties due to the interaction between the metal ion and the ligands. These include:

Color: Many complex ions are colored due to electronic transitions within the d-orbitals of the metal ion.
Magnetism: The magnetic properties of a complex ion depend on the number of unpaired electrons in the metal ion.
Stability: The stability of a complex ion depends on factors such as the charge density of the metal ion and the nature of the ligands.

Tip

The color of a complex ion can often be used to identify the metal ion and its oxidation state.

Common Mistakes to Avoid

Common Mistake

Do not confuse coordination bonds with ionic or covalent bonds. Coordination bonds involve the donation of a lone pair from a ligand to a metal ion, whereas covalent bonds involve the sharing of electrons between two atoms.

Common Mistake

Remember that not all molecules can act as ligands. Only species with lone pairs of electrons can form coordination bonds.

Reflection

Self review

What is the role of a ligand in the formation of a coordination bond?
Why are transition metals particularly suited to forming complex ions?

Theory of Knowledge

How does the ability of transition metals to form complex ions illustrate the interplay between structure and function in chemistry?
Can this concept be extended to other areas of knowledge, such as biology or materials science?

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R3.4.8 Ligands and coordination bond (Higher Level Only)

Coordination Bonds and Complex Ions

What Are Coordination Bonds?

Key Definitions

How Are Coordination Bonds Formed?

Why Transition Metals?

The [Cu(NH₃)₄]²⁺ Complex Ion

Representation of the Complex Ion

Determining the Charge on a Complex Ion

Common ions formed by the transition elements:

Properties of Complex Ions

Common Mistakes to Avoid

Reflection

You've read 2/2 free chapters this week.

Coordination Bonds and Complex Ions