Coordination Compounds Chapter - Chemistry JEE Main

Coordination compounds, often referred to as complex compounds, are a fascinating and diverse class of compounds that play a vital role in chemistry and various applications. Understanding the fundamentals of coordination compounds is essential for students preparing for competitive exams like JEE (Joint Entrance Examination). This comprehensive study covers the introduction to coordination compounds, Werner's theory, ligands, coordination number, denticity, chelation, and their significance.

Coordination Compound: Definition

In inorganic chemistry, coordination compound is any of the grades of substances that are composed of chemical structures in which a central metal atom is surrounded by non-metal atoms or groups of atoms, called ligands. The examples of coordination compounds are hemoglobin, vitamin B12, dyes and pigments, chlorophyll, and the catalysts used in preparing organic substances.

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Important Terms Linked to Coordination Compounds in Chemistry 

1. Coordination Entity: When the central metal atom is surrounded by ligands or ions and forms a complex, it is called the coordination entity. For example, [PtCl₂(NH₃)₂].

2. Coordination Number: The total number of ligands bonded together to the central metal atom is the coordination number. For example, in [NiCl₂(H₂O)₄], the coordination number for this complex will be 6.

3. Central Atom: It is a metal atom to which all the ions or clusters are strapped in the complex compound. For example, in IUPAC name of coordination compound [PtCl₂(NH₃)₂], Pt is the central atom.

4. Ligands: The ions or groups that are held together to the central metal are what is termed as ligands. These ligands can either be ions or neutral molecules. For example in the IUPAC name of coordination compound [PtCl₂(NH₃)₂], Cl and NH₃ are ligands.

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Werner's Theory of Coordination Compounds

Looking at the overview of Werner's theory of coordination compounds, it can be understood that it studies a large number of coordination compounds, their physical properties, attributes, and structures. Based on the studies conducted by Alfred Werner, a theory was proposed called the Werner's theory. The chief principles of his theory are as follows.

1. In complex compounds, central metal reveals 2 forms of valencies, namely primary and secondary valency.

2. The primary valency is ionizable and is satiated by the negative ions.

3. The secondary valency is satiated by negative ions or neutral molecules.

4. Due to this secondary valency, ligands or ions bind to the central metal atom in a particular order and therefore these molecules obtain a certain shape.

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Isomerism in Coordination Compounds

Isomerism is basically a theory that reveals two or more compounds having the same chemical formula but they vary in their chemical and physical properties. In chemistry of coordination compounds, this isomerism is of the following two types.

1. Stereo-isomerism

2. Structural isomerism

Stereo-isomerism is further classified into 2 divisions.

(i) Geometrical Isomerism: This isomerism is the case when the ligands are held together in different geometric order.

(ii) Optical Isomerism: This isomerism takes place when two isomers are just identical to one another and these mirror images are not superimposable to each other. In optical isomerism isomers called enantiomers.

Structural isomerism is further classified into 4 divisions.

(i)  Coordination Isomerism: This type of isomerism is the case when ligands between the anionic and cationic species interchange. For example, interchangeability arising between [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].

(ii) Ionization Isomerism: This type of isomerism arises when the counter ion itself is a prospective ligand and has the ability to replace a ligand from the entity. The compounds [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄ are examples of ionization isomerism. 

(iii) Linkage Isomerism: This isomerism takes place in those coordination compounds in which the ambidentate ligands exist. For example, in thiocyanate ligand NCS-, this ligand can be bound to the central metal atom either through the sulphur side or through the nitrogen side and providing two linkage isomers.

(iv) Solvate Isomerism: This category of isomerism is just like the ionization isomerism. The solvate isomers are distinguished in a way that the water molecule is present either as a ligand or simply as a free molecule. For example in the case of IUPAC nomenclature of coordination compounds [Cr(H₂O)₆]Cl₃ and [Cr(H₂O)₅Cl]Cl₂.H₂O.

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IUPAC Nomenclature of Coordination Compounds

Standard laws of naming coordination compounds are explained below in detail.

1. The ligands are always written before the central metal ion in the naming complex coordination.

2. When there are different polydentate ligands bonded to the central metal ion, the prefixes are of the type bis-, tris-, and so on.

3. When there are a number of monodentate ligands existing in the coordination compound of complexes, the prefixes that offer insight into many ligands of the type: di-, tri-, tetra-, and so on.

4. The names of the anions in a coordination compound must end with the letter ‘o’, which usually replaces the letter ‘e’. Thus, the chloride anion should be written as ‘chlorido’ and the sulphate anion should be written as ‘sulfato’.

5. When the coordination centre is attached to more than one ligand, the names of the ligands are written in an alphabetical arrangement which is not impacted by the numerical prefixes that should be applied to the ligands.

Ligands:

Ligands are molecules or ions that can form coordinate covalent bonds with metal ions or atoms. They are crucial components of coordination compounds and play a fundamental role in their properties and reactivity. Common ligands include water ($H_2O$), ammonia ($NH_3$), chloride ions ($Cl^-$), and many organic molecules like ethylenediamine (en).

Coordination Number:

The coordination number (CN) is the number of coordinate bonds formed between the central metal ion and the ligands. Different metal ions exhibit different coordination numbers based on the number of empty orbitals available for bond formation. Common coordination numbers include 2, 4, 6, and 8.

Coordination Number 2: In this case, the metal forms two coordinate bonds. For example, $HgCl_2$.

Coordination Number 4: A coordination number of 4 is commonly observed in tetrahedral complexes. For example, $[CoCl_4]^{2-}$.

Coordination Number 6: Many coordination compounds exhibit a coordination number of 6, forming octahedral complexes. For example, $[Fe(CN)_6]^{4-}$.

Coordination Number 8: In some rare cases, a coordination number of 8 is observed in square antiprismatic complexes.

Denticity:

Denticity is a term used to describe the number of donor atoms in a ligand that can form coordinate bonds with a central metal ion. Common ligands have unidentate denticity (one donor atom), while others can be bidentate (two donor atoms) or polydentate (multiple donor atoms).

Chelation:

Chelation is a special type of complex formation in which a polydentate ligand forms more than one coordinate bond with a metal ion. This results in a cyclic structure known as a chelate ring. Chelation is of great importance in chemistry, particularly in the field of medicine and environmental science. Chelating agents are used to treat metal poisoning by forming stable, water-soluble complexes that can be easily excreted from the body.

Significance of Coordination Compounds:

The study of coordination compounds is crucial for various reasons:

• Industrial and Medicinal Applications: Coordination compounds are widely used in industry and medicine, such as in catalysis, dye production, and chemotherapy.

• Color and Luminescence: Many coordination compounds exhibit vibrant colors and luminescence due to the presence of d-electrons. This property is harnessed in the textile and pigment industries.

• Biological Relevance: Metalloenzymes, which are essential for various biological processes, are coordination compounds. Understanding coordination chemistry is vital in comprehending biological systems.

• Environmental Chemistry: Coordination compounds play a role in environmental chemistry, particularly in processes related to metal uptake and pollution control.

• Catalysis: Many industrial processes, such as the Haber process for ammonia production, involve the use of coordination compounds as catalysts.

Bonding: Valence Bond Approach and Crystal Field Theory

Valence Bond Approach:

The valence bond (VB) theory explains the bonding in coordination compounds based on the overlap of atomic orbitals between the metal ion and the ligands. Important points to understand about the valence bond approach include:

Overlapping Orbitals: The central metal atom's vacant orbitals overlap with the filled orbitals of the ligands, forming coordinate covalent bonds.

Hybridization: Hybridization of metal orbitals may occur to maximize the bonding strength.

Directional Bonds: The VB approach explains the directionality of bonds, which can lead to different structural isomers.

What is the Valence Bond Theory?

Valence bond theory is an old theory based on the bonding of chemicals. Linus Pauling first used it. In the experiment, electrostatic linkage generation between two atoms takes place, through the establishment of an electron density between the nuclei of the two atoms. The valence atomic orbital of one atom either shares spaces or converges with the valence atomic orbital of another atom. This convergence of orbitals enables two antithetical rotated electrons to share a room between the two atoms. This process gives birth to a covalent bond.

Bonding and Properties of Coordination Compounds

A coordination compound is made up of a central metal or ion. It is usually a transformation metal with a guard-wall of neutral particles or anion, known as ligands. The transformation metal is a Lewis acid, and a d-block element in the periodic table made up of valence electrons. The neutral particles are generally Lewis bases with at least one electron pair to contribute to the central atom. 

Ligands may contain neutral like water ($H_2O$), ammonia ($NH_3$), and ethylenediamine (en), or anionic like chloride($Cl^-$) and cyanide ($CN^-$).  We can classify ligands upon their strength also: they can be robust or feeble. Let’s discuss with a spectra-chemical series which will help to decide the strength of a ligand:

$O_2^{2-} <I^- <Br^- <S^{2-} <SCN^-$ (S-bonded) $<Cl^- <N_3^- <F^- <NCO^- <OH^- <C_2O_4^{2-} <NCS^- <CH_3CN <$py(pyridine) $< NH_3 <$en (ethylenediamine) <bipy (2,2’-bipyridine) <phen (1,10- phenanthroline) $<NO^{2-} <pph_3 <CN^-$)

Coordination Compounds Hybridization

Coordination compounds are the metals that can transform. Here the transition metal is the atom lies at the center surrounded by other atoms. Our frequent assumption is that atomic on-orbit of the central atom in a compound blend to create hybrid orbitals. The blending process is known as hybridization. 

The metals which are going through the transition have s, p, and d orbitals. This s, p, and d orbitals are gone through hybridization. 

The valence bond theory explains that because of the ligands' impact, the central atom or ion can use its' (n-1) d, ns, nd orbitals and can intersect with the ligand orbitals of specific shape like an octahedral, tetrahedral, square planer, etc. The hybridized orbitals can intersect with the ligand orbitals that can contribute electron pairs for linkage.

Here we will discuss with an example of the diamagnetic octahedral complex [Co (NH3)6]3+; cobalt ion has made up with 3d6 electronic structure. The hybridization can be formulated like 

Orbitals of $Co^{+3}$ ion:

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$d_2sp_3$ hybridized orbitals of $Co^{3+}$ can be seen as,

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$d_2sp_3$  hybrid

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The mixture does not have any unpaired electron, so it is diamagnetic. All six pairs that come from $NH_3$ molecules obtain the six hybridized orbitals. As the inner d orbital is involved in the hybridization process, the complex, $[Co (NH_3)_6]^{3+}$ is called the inner orbitals or low spin or spin-paired complex. The paramagnetic octahedral complex is usually involved in outer orbital (4d) in hybridization $(sp_3d_2)$. It is called the outer orbital or high spin or spin-free complex.

Valence Bond Theory Postulates

The intersection of two almost full valence orbitals of two different atoms results in a covalent bond. The density between two bonded atoms increases because of the intersecting. This provides calmness to the atoms.

Exceptionally, if the atomic orbitals get more than one unpaired electrons, more linkages can be formed. Paired electrons cannot participate in such linkage development.

A covalent bond often has direction. This type of bond is common to the area of overlapping atomic orbitals.

Two types of bonds can be classified as per the design of the intersection. They are pi bond and sigma bond. The covalent bond that is formed by sideways intersecting of atomic orbitals is named as pi bond. A Sigma bond is generated by intersecting an atomic orbital in between the inter nucleus orbits.

Application of Valence Bond Theory

With the theory's application to a coordination compound, the real electrons from the d axis of the transition metal turn into unhybridized d-orbitals. The electrons contributed by the ligand changed into hybridized orbitals with more energy. Then they are filled with electron pairs added by the ligand.

Valence bond theory is involved in stating covalent bond creation in many particles when it passes through the condition of most intersect; it generates the possibility of the creation of the possibly most robust linkages.

Crystal Field Theory:

The crystal field theory (CFT) focuses on the interaction between metal d-orbitals and ligands in coordination compounds. Key concepts of CFT for JEE Main students include:

Splitting of d-Orbitals: When ligands approach the central metal atom, the degenerate (same energy) d-orbitals split into higher-energy t2g and lower-energy eg orbitals. This energy splitting is known as crystal field splitting.

Color: The color of coordination compounds is attributed to the energy difference between the split d-orbitals. Transitions between these energy levels absorb certain wavelengths of light, resulting in colored compounds.

Magnetic Properties: The number of unpaired electrons in the split d-orbitals determines the magnetic properties of a coordination compound. High-spin complexes have more unpaired electrons and are paramagnetic, while low-spin complexes are diamagnetic.

Importance of Coordination Compounds

Coordination compounds play a significant role in various aspects of chemistry, including qualitative analysis, extraction of metals, and biological systems. Understanding their importance is crucial for both academic and practical purposes. Here's an overview of the relevance of coordination compounds in these fields:

1. Qualitative Analysis:

Coordination compounds find extensive use in qualitative analysis, which involves identifying the presence of specific ions or elements in a given sample. Some important aspects of their importance in qualitative analysis include:

1. Selective Complex Formation: Many metal ions in solution can be selectively complexed with certain ligands to form distinct coordination compounds. These compounds often have characteristic colors or other properties that aid in the identification of the metal ions.

2. Separation and Group Testing: Qualitative analysis frequently involves the separation of cations into different groups for systematic testing. Complex formation helps in grouping and identifying ions based on their behavior with specific reagents.

3. Formation of Precipitates: In some cases, coordination compounds lead to the formation of precipitates that are indicative of the presence of certain ions. This is crucial for the qualitative identification of various elements in a sample.

4. Complexometric Titrations: Complexometric titrations are widely used in qualitative analysis to determine the concentration of metal ions in a sample. The formation of stable complexes between metal ions and suitable ligands serves as the basis for these titrations.

2. Extraction of Metals:

In the field of metallurgy, coordination compounds play a vital role in the extraction of metals from their ores. Their importance in this context includes:

1. Solubility Enhancement: Coordination compounds can increase the solubility of metal ions from ores, making it easier to extract them using suitable leaching agents.

2. Selective Extraction: Certain ligands can selectively complex with specific metal ions, allowing for the separation of metals from complex ore mixtures.

3. Redox Reactions: Some coordination compounds participate in redox reactions, which are essential in the extraction of metals from their ores. These reactions involve the conversion of metal ions into soluble forms that can be further processed.

4. Cyanide Process: The use of coordination compounds, such as the complex $[Au(CN)_2]^-$, is instrumental in the extraction of gold through the cyanide process. This complexation facilitates the dissolution of gold from its ore.

3. Biological Systems:

Coordination compounds are prevalent in biological systems and play a crucial role in various biological processes:

1. Metalloenzymes: Many enzymes in biological systems are metalloenzymes, which contain metal ions coordinated to specific ligands. These metal centers are essential for the enzyme's catalytic activity and are involved in processes like oxygen transport, electron transfer, and DNA replication.

2. Hemoglobin: Hemoglobin, the oxygen-carrying molecule in blood, contains an iron atom coordinated with a porphyrin ligand. This coordination compound enables the reversible binding and release of oxygen, facilitating its transport in the body.

3. Vitamin B12: Vitamin B12, an essential nutrient, contains a cobalt ion coordinated with a corrin ligand. This coordination compound is vital for various metabolic reactions in the body.

4. Oxygen Transport: Myoglobin, found in muscle tissue, contains an iron-porphyrin complex that binds and releases oxygen during muscle activity.

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