Complex ions: formation, shape and isomerism

In a nutshell

A complex ion comprises of a central metal ion surrounded by co-ordinately bonded ligands. Cis/trans isomerism occurs when a complex ion contains two ligands of the same type. Haemoglobin is a complex ion which helps transport oxygen in the body.

Equations

oxidation \space number \space of \space metal \space ion = complex \space ion \space oxidation \space number \space - \space sum \space of \space charges \space of \space ligands

​$$oxidation \space number \space of \space metal \space ion = overall \space charge \space of \space complex \space ion \space - \space charges \space of \space ligands$$​​

Key words

Complex ion structure 

A complex ion comprises of a central metal ion surrounded by ligands that are co-ordinately bonded to it. 

In a complex ion, the ligands donate electron pairs to the central metal ion to form co-ordinate bonds. A ligand must have one or more lone pairs of electrons in order to form coordinate bonds. 

Types of ligands

Monodentate ligands 

Monodentate ligands can donate one lone electron pair to a central metal ion.

Examples

​$$H_2O,\space OH^-, \space Cl^-, \space NH_3$$​​

Bidentate ligands 

Bidentate ligands have two lone electron pairs. Each bidentate ligand can form two co-ordinate bonds with the central metal ion.

Examples

Ethylenediamine and oxalate ion.

​​ETHYLENEDIAMINE​	​​oxalate ion​

Multidentate ligands 

Multidentate ligands have more than two lone electron pairs.

Examples

EDTA and diethylenetriamine

​DIETHYLENETRIAMINE​	​EDTA​

Oxidation number of metal ions

The oxidation number of a complex ion is the same as its overall charge. The overall charge of a complex ion is placed outside square brackets.

To work out the oxidation number of a metal ion the following equation is used:

$$oxidation \space number \space of \space metal \space ion = overall \space charge \space of \space complex \space ion \space - \space charges \space of \space ligands$$oxidation number of metal ion=complex ion oxidation number − sum of charges of ligandsoxidation \space number \space of \space metal \space ion = complex \space ion \space oxidation \space number \space - \space sum \space of \space charges \space of \space ligands

procedure 

​​

Example

Find the oxidation number of chromium in the the complex ion $$[CrCl_2(H_2O)_4]^+$$.

First, find the overall charge of the complex ion $$[CrCl_2(H_2O)_4]^+$$:

The overall charge is $$+1$$​. 

Next, find the oxidation number of type of ligand:

The water ligands are neutral. Each $$Cl^-$$ ligand has a $$-1$$ oxidation number. 

Then, find the total oxidation number for the chlorine ligands:

​There are two $$Cl^-$$ligands.  In total the ligands have a $$-2$$​ oxidation number.

​

To get the oxidation number of the chromium ion, the total oxidation number of the ligands is subtracted from the charge of the complex ion: 

​$$oxidation \space number \space of \space chromium \space ion= 1 \space - 4(0) \space -2(-1) = +3$$​​

The oxidation number of chromium is  $$\underline{+3}$$​​

Number of ligands

The number of coordinate bonds the ligands form with the central metal ion is the coordination number. Coordination numbers are typically four or six.

A coordination number of six is possible if the ligands are small. If the ligands are large there will only be space for four ligands around the central metal ion.

The bonding electrons in coordinate bonds repel each together so the ligands will position themselves as far as possible from each other; this gives rise to complex ions with distinctive shapes.

Six-fold coordination

Complex ions with a coordination number of six have an octahedral shape. All the bond angles in octahedral complexes are $$90 \degree$$​​

Examples

​

Four-fold coordination

Complex ions with a coordination number of four usually have a tetrahedral shape. All the bond angles in tetrahedral complexes are $$109.5 \degree$$​ 

Sometimes complexes with four-fold coordination have a square planar shape. The bond angles in a square planar complex ion are $$90\degree$$​.

Examples

​Tetrahedral​	​​square planar​

Two-fold coordination

Some silver complexes contain two coordinate bonds and have a linear shape. The bond angles in linear shapes are $$180 \degree$$​

Example

The complex ion $$[Ag(NH_3)_2]^+$$ found in Tollens' reagent has a linear shape.​

Cis-trans isomers 

Cis-trans isomerism is a type of stereoisomerism that falls under E/Z isomerism. Cis-trans isomerism arises when a complex ion contains two ligands of the same type. 

In a cis isomer the two ligands are next to each other and have a bond angle of $$90 \degree$$​. In a trans isomer the two ligands are opposite each other and have a bond angle of $$180 \degree$$​

Cis-trans isomerism can exist in square planar and octahedral complex ions.

Examples

Cis-trans isomerism in a octahedral complex ion.

Cis-trans isomerism in a square planar complex ion. Cisplatin is an anti-cancer drug.

​​cis-platin           ​	​Trans-platin​

Haem 

Blood contains haemoglobin, a protein which helps transport oxygen. Haemoglobin has an octahedral structure and contains a hexa-coordinated $$Fe^{2+}$$ ion, six lone pairs are donated to the $$Fe^{2+}$$​ ion.

The haem part of haemoglobin comprises of a single multidentate ligand which circles the $$Fe^{2+}$$ ion. Four nitrogen atoms in the multidentate ligand form four coordinate bonds with the $$Fe^{2+}$$ ion.Fe^{2+}

Globin, a protein, also forms a coordinate bond with the $$Fe^{2+}$$​ ion. The last coordinate bond forms with either an oxygen or a water molecule. 

When haemoglobin reaches the lungs, an oxygen molecule substitutes the water ligand; this happens because oxygen concentration in the lungs is high. The oxyhaemoglobin formed is transported around the body.

Fe^{2+}

When oxyhaemoglobin reaches an oxygen deficient area, the oxygen molecules exchange with water molecules. Haemoglobin returns to the lung and the process is repeated.

When carbon monoxide is inhaled, it can take the place of water ligands in haemoglobin to form carboxyhaemoglobin. Carbon monoxide binds strongly to haemoglobin and does not readily exchange with water or oxygen; this reduces the amount of oxygen that can be carried by haemoglobin and is referred to as carbon monoxide poisoning. 

Fe^{2+}Fe^{2+

Fe^{2