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Complex Ions

Lajoy Tucker

Teacher

Lajoy Tucker

Contents

Introduction

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Charges of Complex Ions

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Isomerism in Complex Ions

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Optical Isomerism (Enantiomers)

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Named Complexes to Know

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Introduction

A complex ion consist of a central metal ion bonded to surrounding ligands via coordinate bonds.

  • A ligand is a molecule or ion that donates a lone pair of electrons to the metal ion.

Dot-and-cross diagrams of water (H₂O), ammonia (NH₃), and chloride ion (Cl⁻) showing lone pairs of electrons, which are important for explaining hydrogen bonding and intermolecular forces in A-level chemistry.

  • The coordination number os the number of coordinate bonds formed to the metal ion.

Diagram of a coordination complex showing a central chromium ion bonded to six water ligands by coordinate bonds to form a hydrated metal complex ion.

Shapes of Complex Ions

Octahedral Complexes (Coordination Number = 6)

  • Most common with small, uncharged ligands like and .

  • Six ligands arranged around the central ion.

Example:

  • - iron (III) hexaaqua complex

  • - distorted octahedral

Examples of octahedral transition metal complexes showing six coordinate bonds around a central metal ion, including hexaaquairon(III) and tetraamminediaquacopper(II)

Bond angle:

No answer provided.

Tetrahedral Complexes (Coordination Number = 4)

Often form with larger ligands, such as CI, as fewer ligands can fit around the central metal atom or ions.

Example:

Diagram of the tetrahedral complex ion [CuCl₄]²⁻ showing a central Cu²⁺ ion bonded to four chloride ligands with wedge and dashed bonds indicating the 3D arrangement.

Bond angle:

No answer provided.

Square Planar Complexes (Coordination Number = 4)

  • Typically seen with transition metals like platinum .

Example:

  • Cisplatin, - used in cancer treatment.

Diagram showing tetrahedral bond geometry using wedge and dashed bonds to represent bonds coming out of and going behind the plane in a 3D molecular structure.

Bond angle:

No answer provided.

Linear Complexes (Coordination Number = 2)

  • Rare, typically formed by silver and copper .

Example:

  • - used in Tollens' reagent

Equation showing ligand substitution equilibrium in a complex ion, with arrows indicating reversible exchange of ligands around a central metal ion.

Shape is linear, bond angle =

No answer provided.

Charges of Complex Ions

The charge of a complex ion can be deduced by considering the oxidation state of the central metal ion along with the charges of ligands attached.

The oxidation state of the central metal atom or ion can be deduced if the charge of the complex is known.

The charge is included outside of square brackets.

Example:

Worked examples showing how to calculate the oxidation states of transition metals in complex ions, including Cu in [CuCl₄]²⁻ and Cr in a chromium complex ion.

Ligand Types and Denticity

Denticity refers to how may coordinate bonds a ligand can make to an indicidula metal atom or ion.

Ligand

Denticity

Example

H₂O, NH₃, Cl⁻

Monodentate

Displayed formula of the octahedral complex ion [Cr(H₂O)₆]²⁺ showing a central chromium ion bonded to six water ligands.

 

 

Structures of the bidentate ligands ethane-1,2-diamine (en) and ethanedioate (oxalate) ion, showing the donor atoms that can form coordinate bonds to a metal ion.

Bidentate

(Donate 2 lone pairs)

Displayed formula of a cobalt complex ion showing cobalt bonded to two bidentate ethane-1,2-diamine ligands and two chloride ligands in an octahedral arrangement.

[Co(H2NCH2CH2NH2)3]2+

 

Displayed formula of the complex ion [Cr(C₂O₄)₃]³⁻ showing a chromium ion coordinated to three bidentate oxalate ligands in an octahedral arrangement.

EDTA⁴⁻ (hexadentate)

Haem (tetradentate)

Multidentate*

(Donates more than two lone pairs)

Skeletal formula of a transition metal complex showing multiple organic ligands bonded around a central metal ion in a three-dimensional arrangement.  Displayed structure of a haem complex showing an Fe ion coordinated to four nitrogen atoms in a large porphyrin ring with attached hydrocarbon and carboxylic acid side groups.

You will not be expected to draw complexes with multidentate ligands.

Isomerism in Complex Ions

Transition metal complexes can show stereoisomerism:

Cis-Trans Isomerism (Geometrical)

Isomer

Description

Cis

Identical ligands adjacent (90° apart)

Trans

Identical ligands opposite (180° apart)

Occurs in:

  • Octahedral complexes with 4 of one type and 2 of another type of monodentate ligand (e.g. ), or a pair of bidentate ligands.

Diagrams comparing cis and trans isomers of an octahedral copper complex ion, showing the relative positions of water and ammonia ligands around the central Cu²⁺ ion.

  • Square planar complexes with 2 pairs of ligands attached to the central metal ion, like cisplatin:

Diagrams comparing the square planar platinum complexes cisplatin and transplatin, showing the different spatial arrangements of ammonia and chloride ligands around the Pt ion.

Cisplatin- :

  • Cis form is active as an anti-cancer drug.

  • Transplatin is inactive.

Optical Isomerism (Enantiomers)

These form non-superimposable mirror images (like left and right hands)

Occurs in octahedral complexes with two or more bidentate ligands, such as:

Diagram illustrating ligand exchange equilibrium in the hexaaquachromium(III) complex ion, where water ligands around the central Cr³⁺ ion are replaced by other ligands in solution.

Complexes with two bidentate and two monodentate ligands show cis/trans and optical isomerism. For example:

Displayed formula of the octahedral complex ion [Fe(C₂O₄)₂(H₂O)₂]⁻ showing an Fe ion bonded to two bidentate oxalate ligands and two water ligands.

Named Complexes to Know

Complex

Formula

Shape

Function

Cisplatin

[Pt(NH₃)₂Cl₂]

Square planar

Chemotherapy drug – binds to DNA in cancer cells

Tollens’ reagent

[Ag(NH₃)₂]⁺

Linear

Oxidising agent used to test for aldehydes

Tollens' test:

  • Aldehyde reduces to metallic silver:

This gives a silver mirror on the test tube.

Question 1:

What shape would you expect for the complex ?

Answer:

Octahedral, because six small monodentate ligands coordinate with the ion.

Question 2:

Deduce the oxidation state of in

Answer:

Each ligand has a charge of (the bidantate ethanedioate ligand) contributing .

The overall charge of the complex is

Summary Table

Shape

Coordination No.

Example

Angle

Octahedral

6

[Fe(H₂O)₆]³⁺

90°

Tetrahedral

4

[CuCl₄]²⁻

109.5°

Square planar

4

[Pt(NH₃)₂Cl₂]

90°

Linear

2

[Ag(NH₃)₂]⁺

180°

No answer provided.

Exam Tips

  • For shapes: know the coordination number and typical ligands.

  • Clearly label cis/trans isomers with diagrams is asked.

  • Complexes with the bidentate ligands ethane-1,2-diamine and ethanedioate are commonly assessed. Practice drawing these.

No answer provided.
General Properties of Transition Metals Formation of Coloured Ions