Coordination Compounds are chemical compounds that contain coordination bonds, which involve the sharing of electron pairs between a central atom and one or more surrounding atoms or molecules.

Coordination compounds, also known as coordination complexes, are chemical compounds consisting of a central atom bound to an array of anions or neutral molecules via coordinate covalent bonds. These molecules or ions bound to the central atom are referred to as ligands (or complexing agents).

Table of Contents

Important Terms Involving Coordination Compounds

Properties of Coordination Compounds

Double Salts and Coordination Complex

Types of Coordination Complexes

IUPAC Nomenclature of Coordination Compounds

What are Ligands?

Coordination Isomerism

Werner’s Theory

Magnetic Properties of Complexes

Stability of Complexes

Applications of Coordination Compounds

Many coordination compounds contain a transition element as the central atom, and are therefore referred to as metal complexes. These types of coordination complexes generally consist of a transition element at the centre, which is known as the coordination centre.

Important Terms Involving Coordination Compounds

The definitions of some important terms in the chemistry of coordination compounds can be found below.

Coordination Entity

A coordination entity is a chemical compound in which the central ion or atom (or the coordination centre) is bound to a set number of atoms, molecules, or ions.

Some examples of coordination entities include [CoCl3(NH3)3], and [Fe(CN)6]^4-.

Central Atoms and Central Ions

The central atoms and central ions refer to the atoms and ions to which a specific number of atoms, molecules, or ions are bound, as discussed previously.

The central atoms or ions found in coordination compounds are usually Lewis Acids, and thus, can act as electron-pair acceptors.

Ligands

Ligands are atoms, molecules, or ions that are bound to the coordination centre or the central atom/ion.

These ligands can either be a simple ion or molecule (e.g. Cl⁻ or NH₃) or a relatively large molecule, such as ethane-1,2-diamine (NH₂-CH₂-CH₂-NH₂).

Coordination Number

The Coordination Number of the central atom in a Coordination Compound refers to the total number of σ-bonds through which the Ligands are bound to the Coordination Centre.

For example, the coordination number of nickel in the coordination complex given by [Ni(NH3)4]2+ is 4.

Coordination Sphere

The non-ionizable part of a complex compound consists of a central transition metal ion surrounded by neighbouring atoms or groups, all of which are enclosed in square brackets.

The coordination sphere is composed of the coordination centre, the ligands attached to it, and the net charge of the chemical compound.

The counter ion that attaches to charged coordination complexes is usually accompanied by a coordination sphere.

Answer: [Co(NH_3)_6]^3+/3\textsuperscript{-} - coordination sphere

Coordination Polyhedron

The coordination polyhedron is the geometric shape formed by the attachment of the ligands to the coordination centre.

Examples of spatial arrangements in coordination compounds include tetrahedral and square planar.

Oxidation Number

The oxidation number of the central atom can be calculated by finding the charge associated with it when all electron pairs donated by the ligands are removed.

The oxidation number of the platinum atom in the complex [PtCl6]^2- is +4.

Homoleptic and Heteroleptic Complexes

When the coordination centre is bound to only one type of electron pair donating ligand group, the coordination complex is called a Homoleptic Complex, for example: [Cu(CN)₄]³⁻.

A heteroleptic complex is a coordination compound where the central atom is bound to many different types of ligands. An example of this is [Co(NH3)4Cl2]+.

Properties of Coordination Compounds

The discussion of general properties of coordination compounds is found in this subsection.

The coordination compounds formed by the transition elements are coloured due to the presence of unpaired electrons that absorb light in their electronic transitions. For example, the complexes containing Iron(II) can exhibit green and pale green colours, whereas the coordination compounds containing iron(III) have a brown or yellowish-brown colour.

The presence of unpaired electrons in coordination complexes with a metal as the coordination centre gives them a magnetic nature.

Coordination compounds show a range of chemical reactivity, and can be involved in both inner-sphere and outer-sphere electron transfer reactions.

Complex compounds containing certain ligands can facilitate the transformation of molecules in either a catalytic or a stoichiometric manner.

Double Salts and Coordination Complexes

Double Salts

Double salts are completely ionizable in aqueous solutions, and each ion present in the solution can be identified using a confirmatory test. More info on double salts.

Answer: Potash Alum is a double sulphate of potassium and aluminium, with the chemical formula K2SO4.Al2(SO4)3.24H2O. On ionization, it gives:

K⁺, SO₂^-4 and Al⁺³ ions which respond to the corresponding tests.

Coordination Complex

Co-ordinated complexes are incompletely ionisable in aqueous solutions, resulting in a complex which does not demonstrate complete ionisation.

Potassium Ferrocyanide [K4Fe(CN)6] ionizes to give K+ and [Fe(CN)6]−4 [ferro cyanide ions].

Types of Coordination Complexes

Cationic Complexes: In this co-ordination sphere, there is a cation, such as [Co(NH3)6]Cl3.

Anionic complexes: In this co-ordination sphere, the Anion is present. For example: K4[Fe(CH)6]

Neutral Complexes: In this co-ordination sphere, there is neither a cation nor an anion. For example, [Ni(CO)4].

Homoleptic Complex: The complex consists of ligands of the same type. For example: K₄[Fe(CN)₆]

Heteroleptic Complexes: These consist of different types of ligands, such as [Co(NH3)5Cl]SO4.

Mononuclear complexes: In this co-ordination sphere, there is a single transition metal ion, such as K4[Fe(CN)6].

Polynuclear complexes: More than one transition metal ion is present. Example: Octahedral complex

Polynuclear complexes

IUPAC Nomenclature of Coordination Compounds

Rules for Naming Coordination Compounds

The standard rules for nomenclature of coordination compounds are outlined below.

1. In the naming of complex coordination complexes, the ligands are always written before the central metal ion.

2. When the coordination centre is bound to more than one ligand, the names of the ligands must be written in alphabetical order, disregarding any numerical prefixes.

When there are multiple monodentate ligands present in the coordination compound, the prefixes used to indicate the number of ligands are:

• di- (2 ligands)
• tri- (3 ligands)
• tetra- (4 ligands)
• etc.

4. When there are many polydentate ligands attached to the central metal ion, the prefixes used are “bis-”, “tris-”, and so on.

The names of the anions present in a coordination compound must end with the letter ‘o’, which generally replaces the letter ‘e’. Thus, the sulfate anion is written as sulfato and the chloride anion is written as chlorido.

6. Coordination compounds assign specific names to the following neutral ligands:

• NH3 (ammine)
• H2O (aqua or aquo)
• CO (carbonyl)
• NO (nitrosyl)

7. After the ligands are named, the name of the central metal atom is written. If the complex has an anionic charge associated with it, the suffix -ate is added.

When writing the name of the central metallic atom in an anionic complex, priority is given to the Latin name of the metal (with the exception of mercury) if it exists.

9. The oxidation state of the central metal atom/ion must be specified with the help of roman numerals enclosed in parentheses (e.g. (IV)).

10. The cationic entity must be written before the anionic entity if the coordination compound is accompanied by a counter ion.

⇒ Further Reading: Nomenclature of Organic Compounds

Naming Coordination Compounds: Examples

Examples of coordination compounds and their nomenclature can be found below.

K4[Fe(CN)6]: Potassium Hexacyanoferrate (II)

Tetracyanonickelate(II) Ion: [Ni(CN)_4]^2-

[Zn(OH)_4]^2- : Tetrahydroxide Zincate (II) Ion

[Tetra carbonyl Nickel (O)]: Ni(CO)4

Ligands in coordination compounds are molecules or ions that are bound to the central metal atom or ion. They are typically electron-pair donors, such as water, ammonia, or halide ions.

The surrounding atoms, ions, and molecules around the central transition metal ion are known as Ligands. They act as Lewis bases, donating electron pairs to the transition metal ion, thus forming a dative bond between the Ligands and the transition metal ion. Therefore, these compounds are known as coordination complexes.

Also Check Out: Ligands

Types of Ligands

Based on the nature of the bond between the ligand and the central atom, ligands are classified as follows:

Anionic Ligands: CN⁻, Br⁻, Cl⁻

Cationic Ligands: NO+

Neutral Ligands: CO, H2O, NH3

Ligands can be further classified as:

Unidentate Ligands

Unidentate ligands are those ligands that only have one atom that can bind to the coordination centre. Ammonia (NH3) is a prime example of this type of ligand. Other common unidentate ligands include Cl– and H2O.

Bidentate Ligands

Ligands which possess two donor atoms allowing them to bind to the central atom, such as ethane-1,2-diamine, are known as bidentate ligands.

Oxalate ion is a bidentate as it can bond through two atoms to the central atom in a coordination compound and Ethane-1,2-diamine.

Polydentate Ligands

Polydentate ligands are those ligands which have multiple donor atoms that can bind to the coordination centre.

The EDTA4- ion (ethylene diamine tetraacetate ion) is an excellent example of a polydentate ligand, as it can bind to the coordination centre through its four oxygen atoms and two nitrogen atoms.

Chelate Ligand

When a polydentate ligand attaches itself to the same central metal atom through two or more donor atoms, it is known as a chelate ligand. The atoms that ligate to the metal ion are termed as the denticity of such ligands.

Ambidentate Ligand

Some ligands have the ability to bind to the central atom through atoms of two different elements.

The SCN- ion can bind to a ligand through either the nitrogen atom or the sulfur atom, thus making them known as ambidentate ligands.

Isomerism in Coordination Compounds

Isomers are compounds with the same chemical formula but different arrangements of atoms. Coordination compounds mainly display two types of isomerism: Stereo-isomerism and Structural Isomerism.

Stereoisomerism

Coordination compounds which have the same chemical bonds but have different spatial arrangement are known as stereoisomers. These stereoisomers are further divided into two categories: optical isomerism and geometrical isomerism.

Optical Isomerism in Coordination Compounds

Optical isomers or enantiomers are isomers that form non-superimposable mirror images. These isomers come in two types.

The isomer that rotates plane-polarized light towards clockwise direction is referred to as the dextro or ’d’ or ‘+’ isomer.

The isomer that rotates plane-polarized light to the counterclockwise direction is the levo isomer or ‘L’, ‘-’, isomer.

The racemic mixture is known as the equimolar mixture of the ’d’ and ’l’ isomers.

Optical Isomerism: Optical isomerism is a form of stereoisomerism that occurs when two molecules have the same molecular formula but differ in the arrangement of their atoms in space, resulting in molecules that are mirror images of each other.

Geometrical Isomerism

Geometrical isomerism is observed in heteroleptic complexes (complexes with more than one type of ligands) due to the existence of various possible geometric arrangements of the ligands.

Geometrical isomerism of complexes with coordination number 4 is mainly observed in coordination compounds having coordination numbers equal to 4 and 6.

ML4 Tetrahedral Complexes do not exhibit Cis-Trans Isomerism since the ligands are oriented in different directions.

MABCD has 3 geometrical isomers: 2-cis and 1-trans.

The MA2B2 complex displays both cis and trans isomers.

Example:

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Answer:

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The ML6 octahedral complex does not show geometrical isomerism, whereas the MA2B4 complex does show cis-trans isomerism.

Co(NH3)4Cl2]+

The MA3B3 complex displays facial-meridional isomerism.

![Structural Isomerism]()

Structural Isomerism

Structural Isomerism is exhibited by coordination compounds having the same chemical formula, but a different arrangement of atoms. This is further divided into four types:

Linkage Isomerism

Coordination compounds with Ambidentate ligands exhibit Linkage isomerism.

For example: [Co(NH₃)₅NO]SO₄ and [Co(NH₃)₅ONO]SO₄

Coordination Isomerism

Incoordination Isomerism, the exchange of Ligands between Cationic and Anionic Entities of different Metal Ions present in Coordination Compounds, occurs.

For example: [Co(NH3)_6][Cr(CN)_6] and [Cr(NH3)_6][Co(CN)_6].

Ionization Isomerism

Ionisation Isomerism arises when the counter ion in a complex salt, which is a potential ligand, replaces the ligand.

For example: [Co(NH3)₅Br]SO₄ and [Co(NH3)₅(SO₄)]Br

Solvate Isomerism

Solvate isomers are a special case of ionisation isomerism, wherein compounds differ depending on the number of solvent molecules directly bonded to the metal ion.

For example:

For instance:

[Co(H2O)₆]Cl₃

[Co(H₂O)₅Cl]Cl₂·H₂O

[Co(H₂O)₄Cl₂]Cl₂·2H₂O

[Co(H₂O)₃Cl₃].3H₂O

Ligand Isomerism

In this type, ligands exhibit isomerism.

For Example:

For Example:

Werner’s Theory

Alfred Werner proposed Werner’s theory in 1898, which explained the structure of coordination compounds.

Werner’s Experiment: When AgNO3 (silver nitrate) was mixed with CoCl3·6NH3, all three chloride ions were converted to AgCl (silver chloride). However, when AgNO3 was mixed with CoCl3·5NH3, two moles of AgCl were formed.

Based on the observation that one mole of AgCl was formed when CoCl3·4NH3 was mixed with AgNO3, Werner’s theory was postulated.

Postulates of Werner’s Theory

The central metal atom in the coordination compound exhibits two types of valency, namely, primary and secondary linkages or valencies.

Primary linkages are ionizable and are fulfilled by the positive ions.

Secondary linkages are non-ionizable and are satisfied by negative ions. Additionally, the secondary valence of any metal is fixed and is equal to its coordination number.

The ions bound to the metal by the secondary linkages exhibit characteristic spatial arrangements which correspond to different coordination numbers.

What is the Difference Between Primary and Secondary Valency in Coordination Compounds?

| Werner’s Theory |

| Primary Valency | Secondary Valency |

These are Ionizable	These are Non-ionizable
Acids	Sugars
Bases	Fats

| Satisfied by charged ions | Satisfied by ligands |

Primary valency	Secondary valency
Does not help	Helps in structure

| It can not function as a primary valency | It can also function as a secondary valence |

The Werner complex is represented by the formula CoCl$_3$.6NH$_3$.

The terms inside the square brackets [coordination complexes] and the ions outside the square brackets [counter-ions] form a spatial arrangement known as a coordination polyhedra.

Limitations of Werner’s Theory

1. Coordination compounds exhibit magnetic, colour, and optical properties that are not explained by this statement.

2. The reason why not all elements form coordination compounds was not explained.

3. It failed to explain the directional nature of bonds in coordination compounds.

4. This theory does not account for the durability of the complex.

5. This theory failed to provide an explanation for the nature of complexes

Effective Atomic Number Rule

The Effective Atomic Number Rule was proposed by Sidgwick. It states that the total number of electrons passed by a central transition metal ion after the donation of electrons by the ligand is the effective atomic number.

A complex is stable if the effective atomic number is equal to the atomic number of the nearest inert gas.

What is the effective atomic number of the following complexes?

K4[Fe(CN)6]

[Co(NH3)$_3$]Cl$_3$

1. [K4Fe(CN)6]

Number of electrons in Fe²⁺ = 24

Number of electrons for Six CN = 2 x 6 = 12

Total number of electrons possessed by Fe²⁺ = 36

Therefore, the effective atomic number is 36.

2. [Co(NH3)$_3$]Cl$_3$

Number of electrons in Co⁺³ = 24

Number of electrons for Six NH3 = 2 x 6 = 12

Total number of electrons possessed by Co³⁺ = 24 + 12

Therefore, the effective atomic number is 36.

The Magnetic Properties of Complexes

1. The complex in which a central transition metal ion has unpaired electrons is Paramagnetic.

2. The complex in which the central transition metal ion has no unpaired electrons is diamagnetic.

3. The spin only formula is used to calculate the magnetic moment of a complex.

M = $\sqrt{n(n+2)}BM$

BM = Bohr Magneton

The magnetic moment of complex compounds depends upon:

Hybridization

The Oxidation State of a Central Transition Metal Ion

The number of unpaired electrons

Spectrochemical Series

I– < Br– < SCN– < Cl– < S^2- < F– < OH– < C₂O_24- < H₂O < NCS^- < EDTA^4- < NH₃ < en < CN < CO

Stability of Complexes

The formation of complex ML4 is a multi-step process, wherein each process step is reversible. The equilibrium constant for each step is known as the stepwise formation constant.

![Stability of Complex]()

1/β = Instability constant and β = K1 × K2 × K3 × K4

Factors Contributing to the Stability of Complexes

1. Small size and high nuclear charge of central transition metal ion.

2. The Crystal Field Stabilizing Energy (CFSE) should be increased.

Complexes containing chelating ligands are more thermodynamically stable.

Octahedral complexes are generally more stable than tetrahedral complexes.

Check out this link for more info on Crystal Field Theory: Crystal Field Theory

“The Colour of Complexes”

Complexes containing a central transition metal ion with unpaired electrons exhibit color, which is a result of ’d – d’ transition. The color of these complexes depends upon:

Number of Unpaired Electrons in a Transition Metal Ion

Nature of Ligands

The Oxidation State of a Central Transition Metal Ion

The wavelength of light absorbed and emitted

The Proportion of Ligands in the Coordination Sphere

Answer: [Ni(H2O)6]²⁺ + en^(aq) → [Ni(H2O)4en]²⁺ – Green Pale blue

Bonding in Metal Complexes: Metal Carbonyls

Metal carbonyls are complexes in which carbon monoxide acts as ligands.

In these complexes, a σ bond is formed by the overlapping of the vacant d orbital of the metal ion and the filled orbital of the C-atom (carbon), such as [Ni(CO)4] Tetracarbonyl Nickel (0) and [Fe(CO)5] Penta Carbonyl Iron (0).

A π bond is formed when filled inner orbitals of a metal ion and vacant orbitals of a carbon atom overlap laterally. This leads to the formation of a synergic bond in metal carbonyls.

Applications of Coordination Compounds

Coordination compounds possess many useful properties, making them invaluable in many processes and industries. Some examples of these applications include:

Transition metals found in coordination compounds produce a wide range of colors, making them highly useful in industries for the coloring of materials. These compounds have applications in the dye and pigment industries.

Complex compounds containing cyanide as a ligand are utilized in the process of electroplating and are also beneficial in photography.

Coordination complexes are highly beneficial in the extraction of numerous metals from their ores. For instance, nickel and cobalt can be extracted from their ores through hydro-metallurgical processes that involve ions of coordination compounds as seen here.

Applications of Biology

Haemoglobin consists of a Haeme complex-ion which has a tetrapyrrole Porphyrin ring structure with a central Fe²⁺ ion.

Vitamin B12 consists of a tetrapyrrole porphyrin ring complex with a central Co+3 ion and a coordination number of 6.

Applications of Laboratory

The concentration of Ni²⁺ is estimated using a complexing agent Dimethylglyoxime (DMG), while the hardness of water is estimated using complexes of Ca²⁺ and Mg²⁺ with EDTA.

In Medicine: Cisplatin is used as an anti-cancer drug.

In Photography: The process of developing film is complex.

In Metallurgy: The MacArthur Forest Process for the extraction of gold and silver involves a complex of cyanide ions.

Coordination Compounds - Rapid Revision

Frequently Asked Questions

1. What is the formula for the coordination compounds?

Iron (III) Hexacyanidoferrate (II)

Pentaamminechloridocobalt(III) Chloride

C[Amminebromidochloridonitrito-N-Platinat(II)]

The cat is black.

Answer: The cat’s fur is black.

a. Fe³⁺[Fe(CN)₆]^3-

b. [Co(NH₃)₆]Cl₃

c. [Pt\ (NH_3)\ Br\ Cl\ (NO_2)]\u2013

2. Write the IUPAC names for the following compounds:

[CO(NH₃)₄(H₂O)₂]Cl₃

b. K3[Fe(CN)6]

C. K2[PdCl4]

This sentence is written in bold.

Answer: This sentence is written in bold.

A. Tetraamminediaquacobalt (III) Chloride

b. Potassium Hexacyanoferrate (III)

c. Potassium Tetrachloropalladate (II)

The [Fe(H2O)6]3+ ion has an unpaired electron in its d-orbital, which makes it a strong paramagnetic compound. On the other hand, the [Fe(CN)6]3- ion does not have an unpaired electron in its d-orbital and thus is a weak paramagnetic complex.

I love reading books!

Answer: I enjoy reading books!

[Fe(H₂O)₆]³⁺

[Fe(CN)₆]³⁻