Coordination Compounds

A coordination compound is a chemical complex in which a central atom is linked to a collection of anions or neutral molecules via covalent coordinate bonds. Coordination compounds and coordination complexes are both terms that describe the same thing. The ligands are the molecules or ions that are attached to the central atom (also known as complexing agents).

Compounds with metals as their core atoms are called metal complexes. The core atom of these coordination complexes is usually a transition metal. This complex’s coordination centre atom can be identified by its name.

To understand coordination compounds, first we need to understand some important terms 

Important Factors

• Coordination Entity

An ion, molecule, or ion that is bound to an established number of other atoms, molecules, or other ions is known as a coordination entity.

Example : [CoCl3(NH3)3], and [Fe(CN)4]4-

• Coordination Number

It is the number of sigma bonds through which the ligands are attached to the coordination centre that is counted as the coordination number.

Example :  [Ni(NH3)4]2+ , the coordination number of nickel is 4

• Ligands 

“Ligand” refers to a group of small molecules or atoms that are linked to the central atom/ion by a strong interaction with the coordination centre.

Some of these ligands are very small molecules, like Cl– or NH3, whereas others are much larger.

Example : 1,2-diaminoethane (NH2-CH2-CH2-NH2).

• Coordination spher

The non-ionizable component of a complex molecule has a core transition metal ion surrounded by atoms or groups in square brackets.

Writing down the coordination sphere’s components, the ligands, and the net charge of the chemical molecule all together forms the coordination sphere.

• Oxidation number

To determine the oxidation number of the central atom, all of the electron pairs donated by the ligands must be removed from the atom.

Example : [PtCl6]2- here, the oxidation number is +4.

• Coordination Polydron

The coordination polyhedron is the geometric shape generated by binding the ligands to the coordination centre.

Coordination compounds with tetrahedral and square planar spatial layouts are examples of this type of organisation.

Homo-leptic and Heteroleptic Compounds

Homoleptic: Homoleptic are those with the same type of ligands bound to the core of the metal atom. 

Example:  [Co (NH3)6] Cl3 is homoleptic as the same ligands are present. 

Heteroleptic: Heteroleptic in cases when the ligands connected differ in kind.

Example :  [CoCl3(NH3)3]3+ is heterolytic because different ligands are present.

Properties of Coordination Compounds

The properties of coordination compounds are following: 

• Due to unpaired electrons that absorb light in their electronic transitions, the coordination compounds generated by the transition elements are coloured. Green and pale green colours can be seen in iron(II) complexes, while brown or yellowish-brown colours are seen in iron(III) coordination compounds.

• Because of unpaired electrons in metal coordination centres, the related coordination complexes are magnetic.

• Numerous chemical reactions can be found in coordination molecules. They can participate in both inner-sphere and outer-sphere electron transfer reactions.

• Complex compounds can aid molecules’ catalytic or stoichiometric transition with specific ligands.

Double Salt

Double salts are salts that have more than one cation or anion. Two different salts crystallised in the same ionic lattice are combined to produce them.

Example: The salt of Rochelle is a potassium sodium tartrate (KNaC4H4O6.4H2O).

Coordination Complex

In aqueous solutions, coordinate complexes are only partially ionizable. Despite the lack of complete ionisation, they have a bluish tint.

Potassium Ferrocyanide is a good example. [K4Fe(CN)6] K,+  and [Fe(CN)6]4 are produced as it ionises. [Ions of ferrocyanide]

There are 7 types of coordination complexes, namely

• Cationic complexes 

A cation is present in this coordination sphere.

• Anionic complexes 

An anion is present in this coordination sphere .

• Neutral complexes

Neither anion or cation is present in the coordination sphere .

• Homoleptic complexes

Ligands of the same type form the complex. 

• Heteroleptic complexes

These are made up of various ligands. 

• Mononuclear complexes 

A transition metal ion is the only ion in this coordination sphere. 

• Polynuclear complexes

Several transition metal ions can be found in this sample.

Rules of Naming Coordination Compounds

The rules of naming coordination compounds are as follows : 

• In the nomenclature of complicated coordination complexes, the ligands are always written before the central metal ion.

• No numerical prefixes are needed when the coordination centre is linked to multiple molecules. The ligand names are written alphabetically without regard to the number of ligand prefixes.

• Prefixes of the type di-, tri-, tetra-, and so on indicate the number of monodentate ligands present in the coordination molecule.

• Polydentate ligands are connected to a central metal ion in multiples of two, three, four, and so on.

• In coordination compounds, the anions’ names must end with the letter “o,” which commonly replaces the letter “e,” in order to avoid confusion. Because of this, sulphate anion must be expressed as sulfato,’ and chloride anion as ‘chlorido,’ in the formula.

• Ammine, H2O, CO, and NH3 are neutral ligands with particular names in coordination compounds (nitrosyl).

• Names for the ligands come first, followed by the atom’s central metal name. It is necessary to use the suffix ‘-ate’ when the complex possesses an anionic charge.

• If a Latin name for the metal is available, it takes precedence for naming the anionic complex’s centre metallic atom (except mercury).

• Using roman numerals contained in parenthesis, indicate the oxidation state of the central metal atom or ion.

• A counter ion is required if a cationic substance accompanies a coordination chemical.

Coordination Compounds Example 

The example of  coordination compounds are as follows : 

K4[Fe(CN)6]: Potassium hexa cyanide ferrate (II)

[Ni(CN)4]−2: Tetra cyanie Nickelate (II) ion.

[Zn(OH)4]−2: Tetra hydroxide zincate (II) ion.

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

Bonding in Coordination Compounds

The bonding of coordination compounds can be described by three bonding ideas. Valence bond theory is the first. When Linus Pauling proposed the valence bond concept in 1931, he explained the covalent bonding in molecules of main group elements.

Secondly, the crystal field theory, and thirdly, the ligand field theory.

Conclusion

Living organisms rely on naturally occurring coordination molecules. In addition, metal complexes have several vital roles in biological systems. Metal complexes are found naturally as catalysts in several enzymes that govern biological processes (otherwise called metalloenzymes). For example, carboxypeptidase contains a zinc ion, which coordinates too many amino acid residues of the protein in the hydrolytic enzyme. 

A hydrogen peroxide decomposer enzyme, catalase, contains iron-porphyrin complexes, highly effective catalysts. The catalytic activity sites in both situations are likely to be coordinated metal ions. Additionally, iron-porphyrin complexes in haemoglobin function in the iron atom’s capacity to reversibly coordinate oxygen molecules in its job as an oxygen transporter. Vitamin B12, a cobalt complex with a macrocyclic ligand called corrin, and chlorophyll are examples of other coordination molecules necessary to life (a magnesium-porphyrin complex).

There are numerous and diverse uses for coordination compounds in chemistry and technology. Intense and dazzling colours, such as Prussian blue, make coordination compounds of tremendous utility as dyes and pigments. Fabric dyes comprising phthalocyanine complexes (such as copper phthalocyanine) that contain large-ring ligands (such as porphyrins) are an essential part of the dyes for the fabrics family.