Coordination compounds and complexes

A complex is a compound which usually consists of a central transition metal ion or atom which is surrounded by a number of atoms, molecules or ions that are bonded to the central transition metal atom/ion by coordinate or dative covalent bonds. The image below shows a complex formed between a transition metal ion and six water molecules. In this complex the water molecules use one of their lone pairs of electrons to form a dative or coordinate covalent bond to the transition metal ion. That is the metal ion is acting as a lone pair acceptor or a Lewis acid while the water molecules are acting as a Lewis base or an electron pair donor.

Ligands

The molecules and ions that bond to the central transition metal ion in these complexes are called ligands (from the Latin word ligare; which means to bind). These ligands are species that are able to donate a pair of electrons into empty orbitals in the central transition metal ion, that is the ligands are Lewis bases or electron pair donors while the central transition metal ion is a Lewis acid or an electron pair acceptor. The coordinate bonds formed by the ligands to the central metal atom/ion are often shown using arrows, as outlined in the diagram opposite. Common ligands that you are likely to meet include:

• Water (H₂0), which is named as "aqua" when it acts as a ligand in a complex.


• Ammonia (NH₃), which is named as "ammine" when it acts as a ligand in a complex.


• Cyanide ion (CN^-), which is named as "cyano" when it acts as a ligand in a complex.


• Hydroxide ion (OH^-), which is named as "hydroxo" when it acts as a ligand in a complex.


• Chloride ion (Cl^-), which is named as "chloro" when it acts as a ligand in a complex.

Complexes

As a simple example consider the reaction of ammonia (NH₃) with silver ions (Ag⁺), here the complex formed has a linear shape. Two ammonia (NH₃) molecules are able to act as Lewis bases and form coordinate bonds with vacant orbitals on the silver ion, equations for this Lewis acid-Lewis base reaction are shown below:

In this simple example the complex is shown in square brackets [ ] while the ammonia ligands are shown using in parenthesis ( ), the overall charge on the complex is show as a superscript outside the square brackets.

As a second example consider the square planar complex formed between copper ions (Cu²⁺) and four ammonia ligands (shown in the image opposite). In this example the formula of the complex is written as [Cu(NH₃)₄]²⁺, the ammonia ligands (NH₃) are neutral molecules and so have no charge; this means that the 2⁺ charge on the complex is due only to the charge on the copper ion (Cu²⁺).

Example 3- The [Cr(H₂O)₄Cl₂]⁺ complex is shown in the image below:

In the [Cr(H₂O)₄Cl₂]⁺ complex there are two types of ligands, four neutral water molecules and two negatively charged chloride ions (Cl^-); overall this complex has a charge of +1; this means that the chromium ion must have a charge of +3 since there will be a total charge of -2 from the two chloride ion (Cl^-) and the presence of a Cr³⁺ ion will give the complex an overall charge of +1.

Many of these complexes have an overall charge and are therefore likely to exit as ionic compounds where a counter ion or ions are present such as a sodium ions (Na⁺) or potassium ions (K⁺), these ionic salts are often referred to as coordination compounds though you will often see the words coordination compound and complex used interchangeably, for example

• Potassium hexacyanoferrate(II)- K₄[Fe(CN)₆]: Is an ionic salt since it contains potassium cations (K⁺) and a complex anion [Fe(CN)₆]^4-. The four potassium ions are present in this ionic salt to balance out the 4^- charge on the complex containing the iron ion (Fe²⁺) and the six cyanide ions (CN^-).


• Hexaamminecobalt(III) chloride [Co(NH₃)₆]Cl₃: This is also an ionic salt since it contains a complex cation [Co(NH₃)₆]³⁺ and three chloride anions (Cl^-).

Ionic and covalent

You should be aware of the fact that within these salts the bonding between the complex and the counter ions present may be ionic but the bonding within the complex itself is coordinate ; that is dative covalent bonds exist between the metal ion and the ligands. Therefore, these coordination compounds exhibit a combination of both ionic and covalent bonding, contributing to their unique properties and structures.

However not all complexes are ionic; many are not; for example the tetracarbonylnickel Ni(CO)₄ complex (shown in the image opposite) is neutral with no ionic components. The nickel atom is covalently bonded to four carbon monoxide or carbonyl ligands (CO).

The shapes of complexes

The number of ligands that surround the central metal ion or atom in a coordination compound or a complex is called the coordination number. The coordination number of a metal ion in a complex depends on a number of factors; which include:

• The size of the metal ion; larger metal ions can accommodate more ligands around them, leading to higher coordination numbers.


• The size of the ligands themselves plays a role in the number that can fit around the central metal ion or atom, though as long as the ligands are not excessively large or bulky the size of the ligand is less important than the size of the metal ion. Though it would seem obvious that it would be possible to fit more smaller ligands around any given transition metal ion than to try and coordinate the same number of larger bulkier ligands around the same metal ion, for example Fe³⁺ ions are able to fit six fluoride ions (F^-) to form the complex [FeF₆]^3- but is only able to coordinate with four chloride ions (Cl^-) to for the complex [FeCl₄]^-, this is outlined in the image below:


• Ligand charge: Ligands with high charges can limit the coordination number due to the electrostatic repulsions which will be present between neighbouring ligands.

Shapes

The most common shapes for the complexes formed by the first row of transition metals in the periodic table are shown in the image below.

The octahedral geometry is the most common amongst complexes which are six coordinate, while four coordinate complexes are either tetrahedral or square planar; the tetrahedral arrangement is much more common than the square planar geometry tough the square planar geometry is often found in transition metal ions with 8d electrons in the valence shell; for example in platinum (II) and copper(II) ions. Silver (I), Copper (I), Gold (I) and other ions with a d¹⁰ electronic configuration ions frequently form linear or two coordinate complexes; for example the following coordination compounds all have a linear geometry:

• Linear complexes containing silver(I) ions include: [Ag(CN)₂]^- and [Ag(NH₃)₂]⁺


• Linear complexes containing copper(I) ions include:[Ag(Cl₂)]^-


• Linear complexes containing gold(I) ions include:[Au(CN)₂]^-

Key Points

• A transition metal complex is a compound which usually consists of a central transition metal ion or atom which is surrounded by a number of atoms, molecules or ions that are bonded to the central transition metal atom/ion by coordinate or dative covalent bonds.


• Ligands are species that are able to donate a pair of electrons into empty or vacant orbitals in the transition metal ion or atom, that is the ligands are Lewis bases or electron pair donors while the central transition metal ion or atom is a Lewis acid or an electron pair acceptor.


• All the first row transition metals are able to form octahedral complexes with ligands such as water and ammonia.


• The size and charge on the ligands and the size of the charge and the transition metal ion/atom are common factors that will affect the coordination number and the shape of a transition metal complex.

Check your understanding - Questions on transition metal complexes.

Check your understanding - Additional questions on transition metal complexes.

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