Coordination Compounds — Class 12 Chemistry Notes

Topics covered in Coordination Compounds

1. 1.Werner’s Theory of Coordination Compounds

   Short Answer

   Werner's theory is a fundamental concept in coordination chemistry that explains how metal ions bond with ligands to form coordination compounds. A key example is the formation of hexaamminecobalt(III) chloride, [(3)6]3[Co(NH3​)6​]Cl3​, where cobalt (Co) forms a complex with six ammonia (NH₃) ligands.

   Long Answer

   Werner's theory, introduced by Alfred Werner in the early 20th century, revolutionized the understanding of coordination compounds. This theory proposes that metal ions exhibit two types of valency: primary and secondary. The primary valency is the oxidation state of the metal, and the secondary valency is the number of ligands the metal ion can bind to, which determines the coordination number of the metal.

   • Primary Valency: Ionic and satisfies the charge on the metal ion.
   • Secondary Valency: Directional and satisfies the coordination number of the metal ion.

   Example: Consider the compound [(3)6]3[Co(NH3​)6​]Cl3​. Here, cobalt (Co) has a primary valency of 3, corresponding to its oxidation state (+3), and it is satisfied by three chloride ions. The secondary valency is 6, which is satisfied by the six ammonia molecules surrounding the cobalt ion, forming a coordination sphere.

   This theory also introduced the concept of isomerism in coordination compounds, predicting the existence of geometrical and optical isomers.

   Real-Life Example: Coordination compounds are used in many areas, including medicine (for example, cisplatin, a chemotherapy drug), catalysis, and materials science.

   Careers and Industries: Understanding Werner's theory is crucial for careers in chemistry, materials science, pharmaceuticals, and environmental science, where coordination compounds play a vital role.

2. 2.Definitions of Some Important Terms Pertaining to Coordination Compounds

   Short Answer

   • Coordination Entity: A metal atom/ion bonded to a set of ligands.
   • Central Atom/Ion: The metal atom/ion in a coordination entity.
   • Ligands: Molecules or ions that bind to the central atom/ion.
   • Coordination Number: The total number of ligand bonds to the central atom/ion.
   • Coordination Sphere: The central atom/ion plus its attached ligands.
   • Coordination Polyhedron: The 3D shape formed by ligands attached to the central atom/ion.
   • Oxidation Number of Central Atom: The charge the central atom would have if all ligands and electron pairs were removed.
   • Homoleptic and Heteroleptic Complexes: Homoleptic complexes have identical ligands, while heteroleptic complexes have different types of ligands.

   Long Answer

   1. Coordination Entity: This is the whole complex that includes a central metal atom or ion and the ligands attached to it. For example, in [(3)4]2+[Cu(NH3​)4​]2+, the coordination entity includes the copper ion and the four ammonia molecules attached to it.

   2. Central Atom/Ion: The metal part of the coordination compound. It is the focus of the complex, like 2+Cu2+ in [(3)4]2+[Cu(NH3​)4​]2+.

   3. Ligands: These are atoms, ions, or molecules that can donate a pair of electrons to the metal to form a coordination bond. Ammonia (3NH3​) in [(3)4]2+[Cu(NH3​)4​]2+ is an example of a ligand.

   4. Coordination Number: This number tells us how many ligand attachments are made to the central metal. In [(3)4]2+[Cu(NH3​)4​]2+, the coordination number is 4 because there are four ammonia molecules attached.

   5. Coordination Sphere: Includes the central ion and its surrounding ligands, denoted within square brackets. In [(3)4]2+[Cu(NH3​)4​]2+, the coordination sphere includes 2+Cu2+ and the four 3NH3​ molecules.

   6. Coordination Polyhedron: The geometric shape formed by the positions of the ligands around the central atom/ion. For tetrahedral or square planar shapes, the polyhedron reflects how ligands are spatially arranged.

   7. Oxidation Number of Central Atom: This is the charge that the central metal atom/ion would have if all bonds to the ligands were ionic. For Cu in [(3)4]2+[Cu(NH3​)4​]2+, the oxidation number is +2.

   8. Homoleptic and Heteroleptic Complexes: Homoleptic complexes contain only one kind of ligand, like [(3)4]2+[Cu(NH3​)4​]2+ which only has 3NH3​ ligands. Heteroleptic complexes have more than one kind of ligand, such as [(3)22][Cu(NH3​)2​Cl2​] which has both 3NH3​ and −Cl− ligands.

   These concepts are fundamental in inorganic chemistry, especially in the synthesis and analysis of coordination compounds used in catalysis, material science, and medicine. Understanding these terms helps in fields like pharmaceuticals, where coordination compounds are used in MRI contrast agents and chemotherapy drugs, and in materials science for creating new materials with specific properties.

3. 3.Nomenclature of Coordination Compounds

   Short Answer

   Nomenclature is a system for naming chemical compounds based on their structure and composition. In coordination compounds, this involves naming the central metal and the ligands attached to it.

   Formulas of Mononuclear Coordination Entities represent the composition of a single metal ion complexed with one or more ligands. The formula starts with the central metal followed by the ligands in square brackets.

   Naming of Mononuclear Coordination Compounds involves stating the names of the ligands first (in alphabetical order), followed by the metal ion. The oxidation state of the metal is given in Roman numerals in parentheses.

   Long Answer

   What is Nomenclature?

   Nomenclature in chemistry is the methodical process of naming chemical substances. For coordination compounds, this involves a specific set of rules that help in systematically identifying and communicating the structure of these compounds. It helps chemists to convey a lot of information about the compound's composition and structure through its name.

   Formulas of Mononuclear Coordination Entities

   A mononuclear coordination entity consists of a single central metal atom or ion bonded to several ligands (molecules or ions that donate electrons to the metal). The formula is written with the central metal first, followed by the ligands within square brackets. For example, in the complex [(3)4]2+[Cu(NH3​)4​]2+, Cu is the central metal ion, and 3NH3​ is the ligand, with four such ligand molecules attached to copper.

   Naming of Mononuclear Coordination Compounds

   The process of naming these compounds follows a sequence:

   1. Ligands are named first, in alphabetical order. Neutral ligands are usually named as the molecule (e.g., ammonia is named as "ammonia"), while anionic ligands have names ending in 'o' (e.g., chloride becomes "chlorido").
   2. The metal’s name is stated next. If the compound is a complex ion, the name of the metal ends in '-ate' if the metal has a different oxidation state than its common state (e.g., ferrate for iron in a state other than +2 or +3).
   3. The oxidation state of the metal is given in Roman numerals in parentheses immediately after the metal's name.

   For example, the compound [(3)4]4[Cu(NH3​)4​]SO4​ is named as "tetraamminecopper(II) sulfate", indicating the copper ion is in a +2 oxidation state and bonded to four ammonia ligands, and the compound is a sulfate salt.

4. 4.Isomerism in Coordination Compounds

   Short Answer

   Isomerism in Coordination Compounds refers to the phenomenon where two or more coordination compounds have the same formula but different arrangements of atoms or bonds. This can lead to different properties even though the compounds are made of the same elements.

   • Geometric Isomerism occurs when the same atoms or groups of atoms are arranged differently around the central atom.
   • Optical Isomerism happens when a compound can exist in two forms that are mirror images of each other but cannot be superimposed.
   • Linkage Isomerism is observed when a ligand can bind to the central metal atom in different ways.
   • Coordination Isomerism occurs when the composition of the coordination sphere varies between isomers.
   • Ionisation Isomerism arises when the isomers give different ions in solution.
   • Solvate Isomerism (also known as hydrate isomerism) happens when the solvent molecule is either inside or outside the coordination sphere.

   Long Answer

   Isomerism in Coordination Compounds

   Isomerism in coordination compounds is a key concept in coordination chemistry, leading to the existence of compounds that appear similar but have significant differences in chemical and physical properties.

   1. Geometric Isomerism This type of isomerism arises due to different spatial arrangements of ligands around the central metal. For example, in [(3)22][Pt(NH3​)2​Cl2​], there can be a cis-isomer (where the two Cl ligands are next to each other) and a trans-isomer (where the Cl ligands are opposite each other).

   2. Optical Isomerism Optical isomers are non-superimposable mirror images of each other, similar to left and right hands. A classic example is [3]3+[Co(en)3​]3+, where en stands for ethylenediamine. The two forms rotate plane-polarized light in opposite directions.

   3. Linkage Isomerism This occurs with ambidentate ligands, which can attach to the central metal through different atoms. An example is [(3)5(2)]2+[Co(NH3​)5​(NO2​)]2+ which can form [(3)5]2+[Co(NH3​)5​(ONO)]2+ depending on whether the nitrite ligand binds through the nitrogen or oxygen atom.

   4. Coordination Isomerism Seen in compounds containing more than one metal center, where the ligands can switch between metal centers. For example, [(3)6][6][Cr(NH3​)6​][Co(CN)6​] can swap ligands with [(3)6][6][Co(NH3​)6​][Cr(CN)6​].

   5. Ionisation Isomerism This is observed when compounds give different ions in solution, affecting the compound's chemical behavior. For example, [(3)42]2[Pt(NH3​)4​Cl2​]Br2​ can form [(3)42]2[Pt(NH3​)4​Br2​]Cl2​ in solution.

   6. Solvate Isomerism Occurs when solvent molecules such as water are involved in isomerism. For example, [(2)6]3[Cr(H2​O)6​]Cl3​ and [(2)5]2⋅2[Cr(H2​O)5​Cl]Cl2​⋅H2​O are solvate isomers, differing by the position of the water molecules.

   Real-Life Examples

   • Geometric isomers can have different biological activities. For instance, only one form of a drug might be biologically active due to the specific arrangement of its atoms.
   • Optical isomers are crucial in the pharmaceutical industry, as the two forms of a drug can have very different effects on the human body.
   • Solvate isomers have implications in hydration processes and can affect the solubility and stability of compounds in different solvents.

   These isomerisms illustrate the complexity and versatility of coordination compounds, impacting their synthesis, applications, and behavior in biological systems.

5. 5.Bonding in Coordination Compounds

   Short Answer

   Werner's theory was an early attempt to understand coordination compounds, but it couldn't explain why only some elements form them, why their bonds are directional, or their unique magnetic and optical properties. To address these questions, several theories like Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT), and Molecular Orbital Theory (MOT) have been developed. VBT and CFT are particularly useful for explaining the nature of bonding in coordination compounds at an elementary level.

   Long Answer

   Werner's theory provided a foundational understanding of coordination compounds, introducing concepts like coordination number and primary and secondary valences. However, as science progressed, it became clear that Werner's theory lacked explanations for several phenomena observed in coordination compounds. This led to the development of more sophisticated theories.

   1. Valence Bond Theory (VBT): VBT explains coordination compounds by suggesting that metal atoms or ions form bonds with ligands through the donation of electron pairs from the ligands to the metal. It accounts for the formation of coordination compounds, their shapes, and magnetic properties by using the concept of hybridization of atomic orbitals.

   2. Crystal Field Theory (CFT): CFT offers an explanation for the colors and magnetic properties of coordination compounds. It describes the interaction between the metal ion and the ligands as an electrostatic field effect, leading to the splitting of d-orbitals into different energy levels. This splitting explains the absorption of specific wavelengths of light, leading to the characteristic colors of many coordination compounds.

   3. Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT): While not covered in elementary treatments, these theories further refine our understanding of the bonding in coordination compounds. LFT is an extension of CFT that incorporates quantum mechanics, and MOT describes the bonding in terms of molecular orbitals formed from the atomic orbitals of the metal and the ligands.

   Applications and Careers: Understanding the bonding in coordination compounds is crucial for various applications and careers in chemistry, including:

   • Medicinal Chemistry: Designing drugs that can selectively target and bind to specific biological molecules.
   • Material Science: Creating materials with specific magnetic and optical properties for use in technology and manufacturing.
   • Environmental Chemistry: Developing compounds that can capture and remove pollutants from the environment.

6. 6.Valence Bond Theory

   • Short Answer

   Valence Bond Theory (VBT) explains how atoms bond together to form molecules. It says that a chemical bond forms when the atomic orbitals of two atoms overlap and share electron pairs. This theory helps us understand the shapes of molecules, their bond strengths, and how they are formed.

   Long Answer

   Valence Bond Theory (VBT) is a fundamental theory in chemistry that describes how atoms form chemical bonds in molecules. Here's a step-by-step explanation:

   1. Atomic Orbitals and Electron Sharing: According to VBT, atoms bond by sharing electrons. This sharing occurs when the atomic orbitals (the regions around an atom where electrons are most likely to be found) of two atoms overlap. The shared electron pair creates a covalent bond between the atoms.

   2. Hybridization: VBT introduces the concept of hybridization, where atomic orbitals mix to form new, hybrid orbitals. This mixing allows atoms to form bonds in specific geometries, such as linear, tetrahedral, and trigonal planar, explaining the shapes of molecules.

   3. Bond Strength and Overlap: The theory suggests that the strength of a bond depends on the extent of overlap between atomic orbitals. Greater overlap results in stronger bonds because more energy is required to break them.

   4. Sigma (σ) and Pi (π) Bonds: VBT explains two types of covalent bonds: sigma (σ) and pi (π) bonds. Sigma bonds result from head-on overlap of orbitals, while pi bonds result from the side-on overlap of two p orbitals. Sigma bonds are stronger and more common in molecules.

   5. Magnetic Properties of Coordination Compounds

      Explanation: The magnetic properties of coordination compounds stem from the electronic configuration of the central metal ion, especially the presence or absence of unpaired electrons in its d orbitals. These properties are crucial for understanding how these compounds interact with magnetic fields. Compounds with unpaired electrons are paramagnetic and are attracted to magnetic fields, whereas those without unpaired electrons are diamagnetic and repel magnetic fields.

      Real-life Example: In medical imaging, particularly MRI, paramagnetic compounds like gadolinium complexes are used as contrast agents. They enhance the contrast of images by affecting the relaxation times of protons in water molecules within the body, improving the visibility of internal structures.

      Reaction Example: Consider the complex [6]3−[FeF6​]3−. Iron in this complex is in the +3 oxidation state and has 5 unpaired electrons in its d orbitals, making the complex paramagnetic. When placed in a magnetic field, it will align with the field, demonstrating its paramagnetic nature.

      Limitations of Valence Bond Theory

      Explanation: Valence Bond Theory (VBT) explains the formation of bonds in coordination compounds as the result of orbital overlaps between the central metal ion and ligands. However, it falls short in explaining phenomena like the exact color, magnetic properties, and the thermodynamic or kinetic stability of coordination compounds. VBT also struggles with predicting the strength of metal-ligand bonds.

      Example: VBT can't accurately explain why the complex [(3)6]3+[Cr(NH3​)6​]3+ is violet, as it does not fully account for the electronic transitions responsible for the absorption of light that results in color.

      Crystal Field Theory

      Explanation: Crystal Field Theory (CFT) offers a more detailed explanation of the behavior of metal ions in a crystal field, which is created by the approach of ligands. CFT focuses on how the spatial arrangement of ligands around the central metal ion leads to the splitting of the metal's d orbitals into different energy levels. This splitting can predict the magnetic properties and colors of coordination compounds by determining the electronic transitions between these split d orbitals.

      Example: In the complex [(2)6]2+[Ni(H2​O)6​]2+, the octahedral arrangement of water molecules around the nickel ion causes a specific splitting of the Ni's d orbitals. This splitting explains the compound's magnetic properties and why it absorbs certain wavelengths of light, giving it a characteristic color.

      Colour in Coordination Compounds

      Explanation: The color observed in coordination compounds arises from the electronic transitions between different energy levels of the d orbitals, split by the crystal field effect. When these compounds absorb visible light, electrons are excited from lower to higher energy d orbitals, and the compound exhibits the complementary color of the absorbed light.

      Example: The complex [(3)4]2+[Cu(NH3​)4​]2+ appears deep blue because it absorbs light in the red region of the visible spectrum. The absorption corresponds to an electronic transition from a lower to a higher energy d orbital in the copper ion.

      Limitations of Crystal Field Theory

      Explanation: Crystal Field Theory significantly advances our understanding of coordination compounds, yet it has limitations. It treats the interactions between the central metal ion and its ligands as purely electrostatic, ignoring covalent aspects of bonding. CFT also fails to explain the spectral properties of complexes with ligands other than simple point charges, and it doesn't consider the repulsion between electrons in the ligands and the central ion.

      Example: CFT cannot fully describe the bonding and color properties of the [6]+[Mn(CO)6​]+ complex. The theory's electrostatic approach is insufficient to account for the covalent character of the metal-carbon monoxide bonding, which significantly influences the complex's stability and spectral properties.

   Applications in Real Life and Careers:

   • Pharmaceuticals: Designing drugs by understanding how molecules bond and interact.
   • Material Science: Developing new materials with desired properties by manipulating molecular structures.
   • Environmental Science: Creating molecules that can capture pollutants or break down harmful substances.

7. 7.Chart: Number of Orbitals and Types of Hybridisations

   Here's a chart for the number of orbitals involved and the corresponding types of hybridizations is a great way to visualize how atomic orbitals combine to form hybrid orbitals in different geometries. Here's a detailed explanation:
   

   Chart: Number of Orbitals and Types of Hybridisations

   Hybridization Type	Number of Orbitals Involved	Geometry	Example Molecules
   sp	2 (1 s orbital + 1 p orbital)	Linear	BeCl2, CO2
   sp2	3 (1 s orbital + 2 p orbitals)	Trigonal Planar	BF3, NO3-
   sp3	4 (1 s orbital + 3 p orbitals)	Tetrahedral	CH4, NH3
   sp3d	5 (1 s orbital + 3 p orbitals + 1 d orbital)	Trigonal Bipyramidal	PCl5, SF4
   sp3d2	6 (1 s orbital + 3 p orbitals + 2 d orbitals)	Octahedral	SF6, [Co(NH3)6]3+
   sp3d3	7 (1 s orbital + 3 p orbitals + 3 d orbitals)	Pentagonal Bipyramidal	Not commonly encountered in simple molecules

   Explanation:

   • sp Hybridization: Involves the mixing of one s and one p orbital, forming two sp hybrid orbitals. This results in a linear geometry with a bond angle of 180°.
   • sp2 Hybridization: Involves the mixing of one s and two p orbitals, forming three sp2 hybrid orbitals. This results in a trigonal planar geometry with bond angles of 120°.
   • sp3 Hybridization: Involves the mixing of one s and three p orbitals, forming four sp3 hybrid orbitals. This results in a tetrahedral geometry with bond angles of 109.5°.
   • sp3d Hybridization: Involves the mixing of one s, three p, and one d orbital, forming five sp3d hybrid orbitals. This results in a trigonal bipyramidal geometry with bond angles of 90° and 120°.
   • sp3d2 Hybridization: Involves the mixing of one s, three p, and two d orbitals, forming six sp3d2 hybrid orbitals. This results in an octahedral geometry with bond angles of 90°.
   • sp3d3 Hybridization: This hybridization involves the mixing of one s, three p, and three d orbitals, forming seven sp3d3 hybrid orbitals. This leads to a pentagonal bipyramidal geometry, although it's rare and not typically encountered in simpler molecules.

   This chart provides a snapshot of how the mixing of different orbitals can result in various molecular geometries and hybridizations, which are foundational concepts in understanding molecular structure and bonding.

8. 8.Importance and Applications of Coordination Compounds

   Short Answer

   Coordination compounds are important in various fields like medicine, analysis, and industry. They are used in cancer treatment, water purification, and as catalysts in industrial processes.

   Long Answer

   Coordination compounds, which consist of a central metal atom or ion bonded to one or more ligands, play a vital role in various scientific and industrial fields due to their unique properties and behavior. Here are some key areas where they are applied:

   1. Medicine: Certain coordination compounds are used as drugs to treat diseases. For example, cisplatin, a platinum-based coordination compound, is used in chemotherapy to treat cancer.

   2. Analysis: Coordination compounds are used in qualitative and quantitative chemical analysis. They can help identify the presence of metals in a sample or measure their concentration.

   3. Industry: These compounds are used as catalysts to speed up industrial chemical reactions without being consumed themselves. For example, in the Haber process for ammonia production, iron-based coordination compounds are used as catalysts.

   4. Color and Dyes: Many coordination compounds exhibit vibrant colors and are used in dyes and pigments. They are used in paints, inks, and textiles to provide a wide range of colors.

   5. Water Purification: Some coordination compounds can remove harmful metals from water, making them useful in water purification systems.

   Real-Life Example: In everyday life, the vibrant colors of gems are often due to the presence of coordination compounds. For instance, the deep blue color of sapphires is due to the presence of iron and titanium coordination compounds.

   Careers and Industries: Knowledge of coordination compounds is essential in fields like pharmaceuticals, environmental science, materials science, and chemical engineering. Professionals in these areas often work with coordination compounds to develop new drugs, create more efficient catalysts, and design better materials for various applications.