CBSE Class 12 Chemistry (2026–27)
Chapter 6: Coordination Compounds
20 Important Questions and Answers
1. What are coordination compounds? Explain the terms ligand and coordination number.
Answer:
Coordination compounds are chemical substances in which a central metal atom or ion is surrounded by ions or molecules called ligands. These ligands donate one or more lone pairs of electrons to the metal ion, forming coordinate covalent bonds. The entire arrangement is known as a coordination entity. A ligand may be neutral (NH₃, H₂O) or negatively charged (Cl⁻, CN⁻). The coordination number of the metal is the total number of donor atoms directly attached to the central metal atom or ion. For example, in [Co(NH₃)₆]³⁺, ammonia acts as a ligand and cobalt has a coordination number of 6.
2. Differentiate between double salts and coordination compounds.
Answer:
Double salts are compounds formed by crystallization of two simple salts in a fixed ratio. They dissociate completely into constituent ions when dissolved in water. Examples include Mohr’s salt and potash alum. Coordination compounds, on the other hand, contain a central metal atom surrounded by ligands and retain their identity in solution. They do not dissociate completely into simple ions. For example, K₄[Fe(CN)₆] remains as [Fe(CN)₆]⁴⁻ ions in solution. Thus, while double salts lose their identity in water, coordination compounds preserve the coordination entity, resulting in distinct chemical and physical properties.
3. Explain the IUPAC nomenclature of coordination compounds.
Answer:
The naming of coordination compounds follows specific IUPAC rules. Ligands are named before the central metal atom. Anionic ligands end in “o” such as chloro and cyano, while neutral ligands retain their names except water (aqua) and ammonia (ammine). The number of ligands is indicated by prefixes like di-, tri-, tetra-, etc. The oxidation state of the metal is written in Roman numerals within brackets. For anionic complexes, the metal name ends with “ate.” For example, K₄[Fe(CN)₆] is named potassium hexacyanoferrate(II). Proper nomenclature ensures systematic identification and communication of complex compounds.
4. What are ambidentate ligands? Give examples.
Answer:
Ambidentate ligands are ligands that possess two different donor atoms but can coordinate to the central metal ion through only one donor atom at a time. As a result, they can form linkage isomers. One common example is the nitrite ion (NO₂⁻), which can attach through nitrogen forming nitro complexes or through oxygen forming nitrito complexes. Another example is the thiocyanate ion (SCN⁻), which can coordinate through sulfur or nitrogen. Ambidentate ligands are important because they increase the possibility of isomerism in coordination compounds, leading to compounds with different structures and properties despite having the same molecular formula.
5. What are chelating ligands? Explain with an example.
Answer:
Chelating ligands are multidentate ligands that contain two or more donor atoms capable of simultaneously coordinating to a single metal ion, forming ring structures known as chelates. These complexes are generally more stable due to the chelate effect. Ethylenediamine (en) is a common bidentate ligand with two nitrogen donor atoms. In the complex [Co(en)₃]³⁺, each ethylenediamine molecule forms two coordinate bonds with cobalt, creating stable five-membered rings. Chelating ligands are widely used in analytical chemistry, medicine, and industrial processes because they increase the stability and selectivity of metal complexes.
6. Explain Werner’s coordination theory.
Answer:
Werner’s coordination theory was proposed by Alfred Werner to explain the structure of coordination compounds. According to this theory, a metal exhibits two types of valencies: primary and secondary. Primary valencies correspond to the oxidation state of the metal and are ionizable. Secondary valencies correspond to the coordination number and are non-ionizable. Secondary valencies are directed toward fixed positions in space, giving definite geometries to complexes. For example, in CoCl₃·6NH₃, the three chloride ions satisfy primary valencies, while six ammonia molecules satisfy secondary valencies. Werner’s theory successfully explained the structures and properties of many coordination compounds.
7. What is coordination isomerism?
Answer:
Coordination isomerism occurs in compounds containing both cationic and anionic complex ions. It arises when ligands are exchanged between the positive and negative coordination entities. As a result, compounds have the same molecular formula but different distributions of ligands around the metal centers. For example, [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆] are coordination isomers. Although their compositions are identical, the arrangement of ligands differs, leading to different chemical properties. Coordination isomerism demonstrates the structural diversity of coordination compounds and highlights the importance of ligand arrangement in determining compound behavior.
8. What is ionization isomerism? Give an example.
Answer:
Ionization isomerism occurs when a ligand within the coordination sphere and a counter ion outside the sphere exchange places. The resulting compounds produce different ions when dissolved in water. For example, [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br are ionization isomers. The first compound releases sulfate ions in solution, while the second releases bromide ions. Although both have the same overall composition, they show different chemical reactions due to the different ions produced. Ionization isomerism is useful in understanding the behavior of coordination compounds in aqueous solutions.
9. Explain geometrical isomerism in coordination compounds.
Answer:
Geometrical isomerism arises due to different spatial arrangements of ligands around the central metal ion. It is commonly observed in square planar and octahedral complexes. In square planar complexes such as [Pt(NH₃)₂Cl₂], ligands may occupy adjacent positions (cis form) or opposite positions (trans form). In octahedral complexes like [Co(NH₃)₄Cl₂]⁺, similar cis and trans arrangements occur. These isomers have different physical and chemical properties despite having the same molecular formula. Geometrical isomerism is important in biological and pharmaceutical applications because different isomers can exhibit different activities and reactivities.
10. What is optical isomerism?
Answer:
Optical isomerism occurs when a coordination compound exists as two non-superimposable mirror images called enantiomers. These isomers rotate plane-polarized light in opposite directions. Optical isomerism is commonly found in octahedral complexes containing bidentate ligands. For example, [Co(en)₃]³⁺ exists as a pair of optical isomers. One rotates light to the right (dextro), while the other rotates it to the left (laevo). Although both have identical chemical compositions and most physical properties, they differ in their interaction with polarized light and biological systems. Optical isomerism demonstrates the three-dimensional nature of coordination compounds.
11. Explain Valence Bond Theory (VBT) for coordination compounds.
Answer:
Valence Bond Theory explains bonding in coordination compounds through hybridization of atomic orbitals. The central metal ion provides vacant orbitals, which hybridize to accommodate electron pairs donated by ligands. Depending on the type of hybridization, complexes may have different geometries such as tetrahedral, square planar, or octahedral. VBT also predicts whether a complex is paramagnetic or diamagnetic based on electron pairing. For example, [Co(NH₃)₆]³⁺ involves d²sp³ hybridization and forms an octahedral complex. Although VBT successfully explains shapes and magnetic behavior, it cannot adequately explain the colors and spectra of coordination compounds.
12. What are inner orbital and outer orbital complexes?
Answer:
Inner orbital complexes are formed when electrons pair up in the inner d-orbitals, allowing the use of these orbitals in hybridization. They generally involve d²sp³ hybridization and are often diamagnetic. An example is [Co(NH₃)₆]³⁺. Outer orbital complexes use outer d-orbitals without electron pairing and involve sp³d² hybridization. These complexes are generally paramagnetic. An example is [CoF₆]³⁻. The distinction depends on the strength of the ligand field. Strong ligands tend to form inner orbital complexes, while weak ligands often produce outer orbital complexes. This concept helps explain magnetic properties and bonding characteristics.
13. What is Crystal Field Theory (CFT)?
Answer:
Crystal Field Theory explains the properties of coordination compounds by considering electrostatic interactions between the central metal ion and surrounding ligands. According to CFT, ligands are treated as point charges or dipoles that create an electric field around the metal ion. This field causes the degenerate d-orbitals to split into groups of different energies. The pattern of splitting depends on the geometry of the complex. The theory successfully explains color, magnetic behavior, and stability of complexes. Unlike Valence Bond Theory, CFT provides a better understanding of spectral properties and electronic structures of coordination compounds.
14. Explain crystal field splitting in octahedral complexes.
Answer:
In an octahedral complex, six ligands approach the metal ion along the coordinate axes. The five d-orbitals split into two sets due to ligand repulsion. The dxy, dyz, and dxz orbitals form the lower-energy t₂g set, while dx²–y² and dz² form the higher-energy eg set. The energy difference between these sets is called crystal field splitting energy (Δ₀). Electrons occupy these orbitals according to Hund’s rule and pairing energy considerations. The magnitude of Δ₀ determines whether a complex is high spin or low spin. This splitting explains the magnetic and color properties of octahedral complexes.
15. What are strong field and weak field ligands?
Answer:
Strong field ligands produce a large crystal field splitting energy (Δ), causing electrons to pair in lower-energy orbitals. Examples include CN⁻, CO, and NH₃. These ligands often form low-spin complexes. Weak field ligands produce a small splitting energy, allowing electrons to occupy higher-energy orbitals before pairing. Examples include F⁻, Cl⁻, and Br⁻. These ligands usually form high-spin complexes. The nature of the ligand significantly influences the magnetic properties and stability of coordination compounds. The spectrochemical series arranges ligands according to their field strength and ability to cause d-orbital splitting.
16. Why are coordination compounds colored?
Answer:
Most coordination compounds are colored because of electronic transitions between split d-orbitals. In the presence of ligands, d-orbitals split into different energy levels. When visible light falls on the complex, electrons absorb specific wavelengths corresponding to the energy gap between these levels. The remaining transmitted or reflected light gives the compound its observed color. For example, [Ti(H₂O)₆]³⁺ appears purple due to d–d transitions. The color depends on factors such as the metal ion, oxidation state, geometry, and nature of ligands. Complexes with completely filled or empty d-orbitals are usually colorless.
17. Explain the magnetic properties of coordination compounds.
Answer:
The magnetic behavior of coordination compounds depends on the presence of unpaired electrons in the d-orbitals of the central metal ion. Complexes with one or more unpaired electrons are paramagnetic and are attracted by a magnetic field. Complexes with all electrons paired are diamagnetic and are weakly repelled by a magnetic field. The number of unpaired electrons can be calculated using electronic configuration and crystal field splitting. For example, [Fe(CN)₆]⁴⁻ is diamagnetic due to electron pairing, while [FeF₆]⁴⁻ is paramagnetic because it contains unpaired electrons. Magnetic studies help determine electronic structures.
18. What is the spectrochemical series?
Answer:
The spectrochemical series is an arrangement of ligands based on their ability to split d-orbitals in coordination compounds. Ligands producing small crystal field splitting are placed at the weak-field end, while those causing large splitting are placed at the strong-field end. A common sequence is:
I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < NO₂⁻ < CN⁻ < CO.
The series helps predict whether a complex will be high spin or low spin and whether it will be paramagnetic or diamagnetic. It is an important concept in Crystal Field Theory and coordination chemistry.
19. Discuss the importance of coordination compounds in biological systems.
Answer:
Coordination compounds play vital roles in biological processes. Hemoglobin contains iron in a coordination environment and transports oxygen in blood. Chlorophyll contains magnesium and is responsible for photosynthesis in plants. Vitamin B₁₂ contains cobalt and is essential for normal metabolic functions. Many enzymes also contain metal ions coordinated to ligands, which help catalyze biochemical reactions. These complexes are crucial for respiration, energy production, and nutrient utilization. The study of bioinorganic chemistry highlights how coordination compounds support life processes and contribute to health and disease management.
20. Mention important applications of coordination compounds.
Answer:
Coordination compounds have numerous applications in chemistry, industry, medicine, and agriculture. EDTA forms stable complexes with metal ions and is used in water softening and analytical chemistry. Cisplatin, a platinum coordination compound, is widely used in cancer treatment. Coordination compounds are employed in electroplating, extraction of metals, photography, and dye manufacturing. Complexes of transition metals serve as catalysts in industrial reactions. In biological systems, they perform essential functions such as oxygen transport and photosynthesis. Their diverse applications arise from their stability, unique electronic properties, and ability to form selective interactions with various substances.