Coordination Compounds

Coordination compounds are composed of an array of anions or neutral molecules covalently linked to a central atom. Coordination compounds are also known as coordination complexes. Ligands, also known as complexing agents, are atoms or molecules that interact covalently with the central atom in a coordination compound.

Applications of Coordination Compounds

Metal complexes are Coordination compounds in which the central atom is a metallic element, often having a transitional nature. Notably, the central atom serves as the coordination centre in these complexes.

Coordination Compound Terminology

The following sections cover a wide range of important coordination compound chemistry terms.

Coordination Entity

A coordination entity is a chemical complex consisting of a core ion or atom (or coordination centre) and several additional atoms, molecules, or ions. The coordination entities [CoCl3(NH3)3] and [Fe(CN)6]4- are two examples.

Lewis Acids occupy the Centre

As previously said, the core comprises atoms and ions that are bound to a certain number of other atoms, molecules, or ions. In coordination compounds, the centre atoms or ions are often Lewis Acids, which act as electron-pair acceptors.

Ligands

The ligands are chemically attached atoms, molecules, or ions to the coordination centre or core atom/ion. Ligands may be small ions or molecules (such as Cl– or NH3) or large compounds [such as ethane-1,2-diamine (NH2-CH2-CH2-NH2)].

Coordination no.

The total number of sigma bonds used to bind the ligands to the coordination centre determines the coordination number of the core atom coordination complexes. The coordination complex [Ni(NH3)4]2+, for example, has a coordination number of four.

Coordination Sphere

A non-ionisable complex chemical component consists of a core transition metal ion surrounded by square bracketed atoms or groups. When a chemical complex is depicted as a whole, the coordination sphere comprises the coordination centre, the ligands that link to it, and the net charge of the chemical complex as a whole.

A counter ion is often related to this coordination sphere (the ionisable groups attached to charged coordination complexes).

[Co(NH3)6] is a great example.

Coordination Polyhedron

The coordination polyhedron is the geometric structure generated when ligands are attached to the coordination centre. Tetrahedrons and squares in coordination compounds are examples of such spatial configurations.

After removing all ligand-provided electron pairs, one may calculate the charge associated with the core atom to determine its oxidation number.

The platinum atom in the combination [PtCl6]2-, for example, has an oxidation number of +4. The coordination complex of a homoleptic complex has just one kind of electron pair-donating ligand group, such as [Cu(CN)4]3-

The coordination molecule is a heteroleptic complex wherein the central atom couples to many different ligands, such as [Co(NH3)4Cl2]+.

Properties of Coordination Compounds

This section goes over the general characteristics of coordination compounds.

• During their electronic transitions, unpaired electrons absorb light, lending colour to the coordination compounds generated by transition elements

• For example, iron (II) complexes may be green or light green, while iron (III) coordination compounds can be brown or yellowish-brown

• The existence of unpaired electrons in coordination complexes created when the coordination centre is a metal gives rise to magnetic properties in the ensuing coordination complexes

• Chemically, coordination molecules react in several ways

• They can conduct electron transport activities on both the inner and outer spheres

• Certain ligands in complex compounds may catalyze or help with molecular stoichiometry changes

Complexes of Coordination with Double Salts

Double salts are ionisable in aqueous solutions. Each ion in the solution performs the important confirmatory test. 

Potash alum, for example, is a two-sulfate molecule. The chemical formula is K2SO4.Al2(SO4)3.24H2O. When Al2(SO4)3.24H2O is ionized, the ions K+, SO42-, and Al+3 demonstrate the predicted reactions in the appropriate experiments. 

Coordinate complexes are only partly ionisable in aqueous solutions. All these factors lead to the appearance of non-ionised skin tone.

One such compound is potassium ferrocyanide. K+ and [Fe(CN)6]4- are generated when [K4Fe(CN)6] ionizes. 

Coordination of Compound Naming Guidelines

The International Union of Pure and Applied Chemistry devised a nomenclature for coordination compounds. 

• The nomenclature of advanced coordination complexes often lists the ligands before the core metal ion

• When the coordination centre is linked to many ligands, the names of the ligands appear in alphabetical order

• This convention is regardless of the number of prefixes added to the ligands

• The prefixes di-, tri-, tetra-, and so on represent the number of ligands in a coordination molecule containing several monodentate ligands

• The prefixes bis-, tris-, and so on are used when numerous polydentate ligands are connected to the core metal ion

• Anions in a coordination compound must have names that finish with the letter ‘o,’ which often substitutes the letter ‘e.’ Consequently, the symbol ‘sulfato’ represents the sulfite anion

• At the same time, ‘chlorido’ indicates the chloride ion

• The neutral ligands NH3 (ammine), H2O (aqua), CO (carbonyl), and NO are given separate names in coordination compounds (nitrosyl)

• Following the identification of the ligands, the name of the core metal atom receives its name. The suffix ‘-ate’ is used when a chemical has an anionic charge

• The English name takes priority over the Latin name when writing the name of the central metallic atom in an anionic combination (except for mercury)

• To represent the oxidation state of the centre metal atom/ion, use a pair of parentheses containing Roman digits

• If there is a counterion, the cationic entity must come before the anionic entity

• The examples below demonstrate how to use coordination compound terminology correctly

• K4[Fe(CN)6] is Potassium Hexacyanide ferrate (II)

• [Ni(CN)4]2 is Tetracyano Nickelate(II)

• Ni(CO)4 is Tetracarbonyl Nickel(0) 

Isomerism in Coordination Compounds 

Isomers are compounds with the same chemical formula but different atom configurations. As a result, coordination compounds often exhibit two types of isomerism: stereoisomerism and structural isomerism.

Werner’s Theory of Coordination Compounds

Alfred Werner proposed Werner’s concept in 1898 to explain the structure of coordination chemicals. 

When AgNO3 (silver nitrate) combines with CoCl3.6NH3, all three chloride ions form AgCl (silver chloride). However, when one mixes AgNO3 and CoCl3.5NH3, it produces only two moles of AgCl. Furthermore, the reaction of CoCl3.4NH3 with AgNO3 creates one mole of AgCl. 

Werner proposed the following explanation in light of this discovery.

Werner’s Theory Postulates

The core metal atom of the coordination complex has two types of linkages or valencies: primary and secondary.

• Negative ions complete ionisable main connections

• Secondary links cannot be ionized

• Negative ions meet these requirements

• Furthermore, the secondary valence of any metal is constant and equal to its coordination number

• Various spatial configurations may contain secondary connected ions, corresponding to several coordination numbers

 Limitations of Werner’s Theory

• The colour, magnetic, and optical characteristics of coordination compounds are difficult to explain

• The theory does not explain why all components didn’t connect to create coordination molecules

• It was not possible to explore the directional features of coordination molecule bonds

• This explanation does not account for the complex’s stability

• This strategy disregards the complexities inherent in complexes

Conclusion

As the name suggests, coordination compounds consist of anions or neutral molecules linked together by covalent bonds. 

Coordination compounds and coordination complexes are both terms that describe the same thing. The ligands are the molecules or ions attached to the central atom (also known as complexing agents).

Metal complexes are coordination compounds in which the central atom is a transitional metallic element, known as the coordination centre.