Coordination Compounds: 40 Important MCQs with Explanations

1. What distinguishes coordination compounds from simple ionic or covalent compounds? a) They only contain transition metals. b) They involve electron transfer. c) They form coordinate covalent bonds. d) They are always colored.Answer: c) They form coordinate covalent bonds. Explanation: The “Introduction” states: “This donor-acceptor interaction forms a coordinate covalent bond, distinguishing coordination compounds from simple ionic or covalent compounds.”
2. What is the role of the central metal atom/ion in a coordination compound? a) Lewis base b) Lewis acid c) Proton donor d) Electron pair donorAnswer: b) Lewis acid Explanation: The “Central Metal Atom/Ion” subsection states: “The metal acts as the electron pair acceptor (Lewis acid).”
3. What is the coordination sphere in a complex? a) The counter ions and ligands. b) The central metal and its directly bonded ligands. c) The entire compound, including counter ions. d) The solvent molecules surrounding the complex.Answer: b) The central metal and its directly bonded ligands. Explanation: The “Coordination Sphere” subsection states: “comprising the central metal atom/ion and all ligands directly bonded to it… enclosed in square brackets…”
4. Who developed the foundational theory explaining the nature of bonding and isomerism in coordination compounds? a) Linus Pauling b) Johannes Brønsted c) Alfred Werner d) G.N. LewisAnswer: c) Alfred Werner Explanation: The “Historical Context” section highlights “Alfred Werner, a pioneering Swiss inorganic chemist, revolutionized the field with his ‘coordination theory’…”
5. According to Werner’s postulates, what does “Secondary Valency” refer to? a) The oxidation state of the metal. b) The number of counter ions. c) The coordination number. d) The charge of the complex ion.Answer: c) The coordination number. Explanation: Under “Dual Valencies of Metals” in Werner’s postulates: “Secondary Valency: This refers to the metal’s coordination number…”
6. In the IUPAC naming system, if the complex ion is an anion, what ending is added to the metal name? a) -ium b) -ite c) -ate d) -ylAnswer: c) -ate Explanation: Under “Metal Name and Oxidation State” for “Complex Anion”: “the ending ‘-ate’ is added to the root name of the metal.”
7. What is the name given to the ligand NH3​ in coordination compounds? a) Ammonia b) Hydrazine c) Ammine d) AzidoAnswer: c) Ammine Explanation: Under “Neutral Ligands” in IUPAC nomenclature: “NH3​: ammine (note the double ‘m’).”
8. Which prefix is used for a complex ligand (e.g., ethylenediamine) when there are three such ligands? a) Tri- b) Tetra- c) Tris- d) Tetrakis-Answer: c) Tris- Explanation: Under “Prefixes for Ligand Number”: “For complex ligands… use multiplying prefixes: bis- (2), tris- (3)…”
9. What is the denticity of ethylenediaminetetraacetate (EDTA$^{4-}$)? a) Bidentate b) Tridentate c) Tetradentate d) HexadentateAnswer: d) Hexadentate Explanation: Under “Hexadentate” ligands: “Ethylenediaminetetraacetate (EDTA$^{4-}$)… It binds through two nitrogen atoms and four oxygen atoms, forming six coordinate bonds…”
10. Which type of ligand can bind to the metal through two different donor atoms? a) Polydentate b) Bridging c) Ambidentate d) Monodentate (generally)Answer: c) Ambidentate Explanation: Under “Ambidentate Ligands”: “These are fascinating monodentate ligands that possess two different potential donor atoms, either of which can coordinate to the central metal.”
11. What is the primary explanation for the chelate effect? a) Stronger bond enthalpies b) Increased steric hindrance c) Entropic advantage (increase in number of molecules) d) Formation of pi bondsAnswer: c) Entropic advantage (increase in number of molecules) Explanation: Under “The Chelate Effect: A Thermodynamic Advantage”: “This enhanced stability is predominantly a thermodynamic consequence, primarily driven by entropy (ΔS).”
12. What is the typical geometry for d10 ions with a coordination number of 2? a) Tetrahedral b) Square Planar c) Linear d) OctahedralAnswer: c) Linear Explanation: Under “CN = 2: Linear Geometry”: “Common for d10 ions (e.g., Cu+,Ag+,Au+,Hg2+)…”
13. Which coordination number is highly characteristic of d8 ions (e.g., Pd2+,Pt2+) often leading to square planar geometry? a) 2 b) 4 c) 5 d) 6Answer: b) 4 Explanation: Under “CN = 4” -> “ii. Square Planar”: “Highly Characteristic of: d8 ions (e.g., Ni2+ (often), Pd2+, Pt2+, Au3+).”
14. What is the most common and stable coordination number for transition metals? a) 2 b) 4 c) 5 d) 6Answer: d) 6 Explanation: Under “CN = 6: The Most Prevalent and Stable Coordination Number”: “This is the most common and often the most stable coordination geometry for transition metals.”
15. What type of isomerism occurs when ambidentate ligands bind through different donor atoms? a) Ionization isomerism b) Hydrate isomerism c) Linkage isomerism d) Coordination isomerismAnswer: c) Linkage isomerism Explanation: Under “A. Structural Isomerism” -> “i. Linkage Isomerism”: “This property leads directly to a type of structural isomerism called linkage isomerism.”
16. Which type of isomerism involves an exchange of positions between an anion inside the coordination sphere and an anion outside it? a) Geometric isomerism b) Optical isomerism c) Ionization isomerism d) Coordination isomerismAnswer: c) Ionization isomerism Explanation: Under “ii. Ionization Isomerism”: “This type of isomerism involves an exchange of positions between an anion inside the coordination sphere (acting as a ligand) and an anion outside the coordination sphere (acting as a counter ion).”
17. What kind of isomerism is demonstrated by the distinct colors of [Cr(H2​O)6​]Cl3​ (violet) and [Cr(H2​O)5​Cl]Cl2​⋅H2​O (blue-green)? a) Linkage isomerism b) Hydrate isomerism c) Coordination isomerism d) Geometric isomerismAnswer: b) Hydrate isomerism Explanation: Under “iii. Hydrate Isomerism (Solvate Isomerism)”: This example is explicitly used to illustrate hydrate isomerism.
18. What is the defining characteristic of “Stereoisomerism”? a) Different bonding arrangements. b) Different spatial orientations but same connectivity. c) Different chemical formulas. d) Production of different ions in solution.Answer: b) Different spatial orientations but same connectivity. Explanation: Under “B. Stereoisomerism (Space Isomerism)”: “These isomers have the same connectivity (same bonds between atoms) but differ in the relative spatial arrangement of the atoms or groups.”
19. For a square planar complex of the MA2​B2​ type, what describes the trans-isomer? a) Identical ligands are adjacent (90° apart). b) Identical ligands are opposite (180° apart). c) Ligands occupy one face of the complex. d) Ligands are along a meridian.Answer: b) Identical ligands are opposite (180° apart). Explanation: Under “Square Planar Complexes (MA2​B2​ type)” for geometric isomerism: “trans-isomer: Identical ligands (A or B) are opposite to each other (180° apart).”
20. What type of isomerism involves a complex that is non-superimposable on its mirror image? a) Geometric isomerism b) Linkage isomerism c) Optical isomerism d) Ionization isomerismAnswer: c) Optical isomerism Explanation: Under “ii. Optical Isomerism (Enantiomerism)”: “Arises when a complex is chiral, meaning it is non-superimposable on its mirror image.”
21. What is the core assumption of Valence Bond Theory (VBT) regarding the metal-ligand bond? a) Purely ionic (electrostatic). b) Purely covalent. c) Partially ionic, partially covalent. d) Hydrogen bonding.Answer: b) Purely covalent. Explanation: Under “A. Valence Bond Theory (VBT) – Pauling’s Approach” -> “Core Assumption”: “The metal-ligand bond is essentially purely covalent…”
22. In VBT, if empty (n−1)d orbitals are used for hybridization, the complex is typically classified as: a) High spin b) Outer orbital c) Low spin d) Diamagnetic (always)Answer: c) Low spin Explanation: Under “Inner vs. Outer Orbital Complexes”: “If empty (n−1)d orbitals… are used for hybridization (d2sp3)… This results in fewer or no unpaired electrons, leading to low spin complexes…”
23. What is a significant limitation of Valence Bond Theory (VBT)? a) It cannot predict geometry. b) It does not explain the color of complexes. c) It correctly rationalizes the spectrochemical series. d) It assumes ionic bonding.Answer: b) It does not explain the color of complexes. Explanation: Under “Limitations” of VBT: “No Explanation for Color: It completely fails to explain the characteristic vibrant colors of most transition metal complexes.”
24. What is the core assumption of Crystal Field Theory (CFT) regarding the metal-ligand bond? a) Purely covalent. b) Purely ionic (electrostatic). c) Metallic. d) Covalent with some ionic character.Answer: b) Purely ionic (electrostatic). Explanation: Under “B. Crystal Field Theory (CFT) – Purely Electrostatic Model” -> “Core Assumption”: “CFT treats the metal-ligand bond as purely ionic (electrostatic).”
25. In octahedral complexes, which set of d-orbitals experiences greater repulsion from ligands and is raised in energy? a) t2g​ (dxy​,dxz​,dyz​) b) eg​ (dx2−y2​,dz2​) c) All d-orbitals are raised equally. d) No specific set is raised.Answer: b) eg​ (dx2−y2​,dz2​) Explanation: Under “d-Orbital Splitting Patterns (Crystal Field)” -> “Octahedral Complexes (Oh​)”: “The lobes of the dx2−y2​ and dz2​ orbitals (collectively known as the eg​ orbitals) point directly along these axes, experiencing maximum repulsion. Consequently, their energy is significantly raised.”
26. What is the approximate relationship between octahedral splitting (Δo​) and tetrahedral splitting (Δt​)? a) Δt​≈(9/4)Δo​ b) Δt​≈(4/9)Δo​ c) Δt​=Δo​ d) Δt​=2Δo​
    Answer: b) Δt​≈(4/9)Δo​ Explanation: Under “Tetrahedral Complexes (Td​)” in CFT: “the magnitude of splitting is generally smaller: Δt​≈(4/9)Δo​.”
27. For octahedral d4−d7 and tetrahedral d3−d6 configurations, what determines whether a complex is high spin or low spin? a) The overall charge of the complex. b) The metal’s oxidation state only. c) The relative magnitude of crystal field splitting energy (Δ) and pairing energy (P). d) The temperature.Answer: c) The relative magnitude of crystal field splitting energy (Δ) and pairing energy (P). Explanation: Under “High Spin vs. Low Spin”: “two distinct spin states are possible, depending on the relative magnitude of the crystal field splitting energy (Δo​ or Δt​) and the pairing energy (P)…”
28. Which ligand is at the strong-field end of the spectrochemical series? a) I− b) Cl− c) H2​O d) CO
    Answer: d) CO Explanation: The “Spectrochemical Series” lists CO at the far strong-field end.
29. What is a key limitation of Crystal Field Theory (CFT)? a) It fails to explain magnetic properties. b) It assumes significant covalent character in bonding. c) It cannot fully account for the position of certain ligands (like CO) in the spectrochemical series. d) It is only applicable to d0 complexes.Answer: c) It cannot fully account for the position of certain ligands (like CO) in the spectrochemical series. Explanation: Under “Limitations of CFT” -> “No π-Bonding Explanation”: “It cannot fully account for the position of certain ligands (like CO,CN−,NO2−​) at the strong-field end of the spectrochemical series, as their strong field nature involves π-bonding (backbonding) which is not considered in a purely electrostatic model.”
30. Which bonding theory is considered the most comprehensive and rigorous for coordination compounds? a) Valence Bond Theory (VBT) b) Crystal Field Theory (CFT) c) Ligand Field Theory (LFT) / Molecular Orbital (MO) Theory d) Electrostatic TheoryAnswer: c) Ligand Field Theory (LFT) / Molecular Orbital (MO) Theory Explanation: Under “C. Ligand Field Theory (LFT) / Molecular Orbital (MO) Theory Approach” -> “Most Comprehensive and Rigorous”: “LFT represents the most sophisticated and accurate approach to describing bonding in coordination compounds.”
31. In LFT, how do π-donor ligands (e.g., Cl−) affect Δo​? a) They increase Δo​. b) They decrease Δo​. c) They have no effect on Δo​. d) They cause d-orbital splitting to be inverted.Answer: b) They decrease Δo​. Explanation: Under “π-Bonding” -> “π-Donor Ligands (Weak Field)”: “By pushing the t2g​ orbitals higher in energy, π-donor ligands effectively decrease Δo​…”
32. What is the phenomenon where complexes possess one or more unpaired electrons and are weakly attracted to a magnetic field? a) Diamagnetism b) Ferromagnetism c) Paramagnetism d) SuperconductivityAnswer: c) Paramagnetism Explanation: The “Paramagnetism” section defines it: “Exhibited by complexes that possess one or more unpaired electrons… These substances are weakly attracted into an external magnetic field.”
33. What is the formula for the spin-only magnetic moment (μs​)? a) μs​=n(n+2) BM b) μs​=n(n−2)​ BM c) μs​=n(n+2)​ BM d) μs​=n2+2 BMAnswer: c) μs​=n(n+2)​ BM Explanation: The “Spin-Only Magnetic Moment (μs​)” section provides the formula.
34. What is the primary cause of color in many transition metal coordination compounds? a) Charge transfer transitions b) Vibrational transitions c) d-d electronic transitions d) Nuclear magnetic resonanceAnswer: c) d-d electronic transitions Explanation: The “Color in Coordination Compounds” section states: “This phenomenon is primarily attributed to d-d electronic transitions.”
35. If a complex absorbs green light, what color will it appear to our eyes? a) Green b) Blue c) Red d) YellowAnswer: c) Red Explanation: Under “Observed Color (Complementary Color)”: “For example, if a complex absorbs light in the green region of the spectrum, it will appear red.”
36. Which factor generally leads to a larger Δ for a complex? a) Lower oxidation state of the metal ion. b) Presence of weak-field ligands. c) Higher oxidation state of the metal ion. d) Tetrahedral geometry (compared to octahedral for same metal/ligands).Answer: c) Higher oxidation state of the metal ion. Explanation: Under “Factors Affecting Color” -> “Oxidation State of the Metal Ion”: “Generally, as the oxidation state of the central metal ion increases… leading to a larger Δ.”
37. What type of transition causes the intense purple color of permanganate ion (MnO4−​)? a) d-d transition b) Spin-forbidden transition c) Ligand-to-Metal Charge Transfer (LMCT) d) Metal-to-Ligand Charge Transfer (MLCT)Answer: c) Ligand-to-Metal Charge Transfer (LMCT) Explanation: Under “Charge Transfer (CT) Transitions”: “The intensely purple color of the permanganate ion (MnO4−​) is a classic example of an LMCT transition.”
38. Which of Werner’s postulates was crucial for explaining the varying conductivities of complexes? a) Metals exhibit dual valencies. b) Ligands are directed in specific fixed positions. c) Quantitative precipitation of counter ions. d) Formation of isomers.Answer: c) Quantitative precipitation of counter ions. Explanation: Under “Experimental Validation” -> “Quantitative Precipitation”: Werner’s theory “accurately predicted the number of chloride ions that would precipitate upon adding silver nitrate (AgNO3​)…” This directly correlates to conductivity as well since different numbers of ions are present outside the sphere.
39. What is the correct IUPAC name for [Co(NH3​)6​]Cl3​? a) Hexaamminechlorocobalt(III) b) Triamminedichlorocobalt(III) chloride c) Hexaamminecobalt(III) chloride d) Cobalt(III) hexaammine chlorideAnswer: c) Hexaamminecobalt(III) chloride Explanation: Following IUPAC rules: ligands named first (hexaammine), then metal (cobalt), then oxidation state (III), then counter ion (chloride).
40. What distortion in octahedral complexes is explained by the Jahn-Teller effect? a) Linear elongation b) Tetragonal elongation or compression c) Trigonal bipyramidal distortion d) Square planar distortionAnswer: b) Tetragonal elongation or compression Explanation: Under “Distorted Octahedral Geometries” -> “Jahn-Teller Effect”: “This often results in tetragonal elongation (stretching along one axis) or compression.”