NEET Coordination Compounds Previous Year Questions with Solutions

Both ammonia (NH₃) is a neutral ligand (charge = 0) and cyanide (CN⁻) is a uninegative ligand (charge = -1). For [Cr(NH₃)₆][Co(CN)₆], since both metals commonly exhibit +3 states and the overall complex is neutral: Cr³⁺ + 6(0) = +3 for the cation, and Co³⁺ + 6(-1) = -3 for the anion.

• • Oxidation state of Cr = +3.
• • Oxidation state of Co = +3.

Coordination isomerism occurs when both cationic and anionic entities are coordination complexes, and an interchange of ligands occurs between the two different metal centers.

• • Type of isomerism: Coordination isomerism.

1. Name the cation first: hexaamminechromium(III). 2. Name the anion second: hexacyanidocobaltate(III). The anionic metal requires the Latin suffix "-ate". Combined IUPAC name: hexaamminechromium(III) hexacyanidocobaltate(III).

• • Cationic entity: hexaamminechromium(III).
• • Anionic entity: hexacyanidocobaltate(III).

Potash alum is a classic double salt containing potassium sulfate and aluminium sulfate crystallized with 24 molecules of water of hydration.

• • Full crystalline unit: K₂SO₄·Al₂(SO₄)₃·24H₂O.
• • Empirical unit: KAl(SO₄)₂·12H₂O.

Unlike coordination complexes where inner-sphere ligands do not dissociate, double salts dissociate completely into their constituent simple ions in aqueous solution.

• • Total K⁺ ions per formula unit = 2.
• • Total Al³⁺ ions per formula unit = 2.
• • Total SO₄²⁻ ions per formula unit = 1 (from K₂SO₄) + 3 (from Al₂(SO₄)₃) = 4.

Divide the stoichiometric coefficient of potassium ions by that of sulfate ions.

• • Ratio = 1:2.
• • Corresponds to Option (1).

Crystal Field Theory (CFT) treats the interaction between metal d-orbitals and ligands as electrostatic and covalent π-interactions. Ligands that produce large splitting of the d-orbitals are called strong-field ligands, while those that produce small splitting are weak-field ligands.

• • Strong field: Low spin complexes (pairing occurs).
• • Weak field: High spin complexes.

The experimentally determined spectrochemical series arranges ligands in order of increasing field strength: I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < N₃⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO

• • CO (Carbonyl): Strongest known ligand due to synergistic π-backbonding.
• • NH₃: Nitrogen-donor ligand, stronger than oxygen-donors.
• • H₂O: Oxygen-donor ligand, intermediate field strength.
• • Cl⁻: Weak field halide donor due to repulsive π-donor interactions.

Inverting to decreasing order gives: CO > NH₃ > H₂O > Cl⁻

• • Matches Option (1).

Find the d-electron configuration of the central metal ion: 1. [Mn(CN)₆]³⁻: Mn + 6(-1) = -3 ⇒ Mn³⁺. Mn atom is [Ar] 3d⁵ 4s² ⇒ Mn³⁺ is 3d⁴. 2. [Fe(CN)₆]³⁻: Fe + 6(-1) = -3 ⇒ Fe³⁺. Fe atom is [Ar] 3d⁶ 4s² ⇒ Fe³⁺ is 3d⁵. 3. [Co(NH₃)₆]³⁺: Co + 6(0) = +3 ⇒ Co³⁺. Co atom is [Ar] 3d⁷ 4s² ⇒ Co³⁺ is 3d⁶. 4. [Ti(H₂O)₆]³⁺: Ti + 6(0) = +3 ⇒ Ti³⁺. Ti atom is [Ar] 3d² 4s² ⇒ Ti³⁺ is 3d¹.

• • All four complexes possess octahedral geometry.

Examine ligand field strengths: - CN⁻ is a strong field ligand (Δ_o > P), causing electron pairing in t_{2g}. - For Mn³⁺ (d⁴): 4 electrons enter t_{2g} giving configuration t_{2g}⁴ e_g⁰. Number of unpaired electrons n = 2. - For Fe³⁺ (d⁵): 5 electrons enter t_{2g} giving configuration t_{2g}⁵ e_g⁰. Number of unpaired electrons n = 1. - For Co³⁺ (d⁶): NH₃ acts as a strong field ligand with Co³⁺ (high charge density), giving low-spin t_{2g}⁶ e_g⁰. Number of unpaired electrons n = 0 (diamagnetic). - For Ti³⁺ (d¹): configuration is t_{2g}¹ e_g⁰. Number of unpaired electrons n = 1.

• • Maximum unpaired electrons n = 2.

Using the spin-only formula μ = √[n(n+2)] BM: 1. [Mn(CN)₆]³⁻: μ = √[2(4)] = √8 ≈ 2.83 BM. 2. [Fe(CN)₆]³⁻: μ = √[1(3)] = √3 ≈ 1.73 BM. 3. [Co(NH₃)₆]³⁺: μ = 0 BM. 4. [Ti(H₂O)₆]³⁺: μ = √[1(3)] = √3 ≈ 1.73 BM.

• • Matches Option (1).

Break down the name "tetraammineaquachloridocobalt(III) chloride": 1. Central metal: Cobalt, oxidation state = +3. 2. tetraammine = 4 NH₃ ligands (neutral). 3. aqua = 1 H₂O ligand (neutral). 4. chlorido = 1 Cl⁻ ligand (charge -1). 5. Counter-ion: chloride (Cl⁻).

• • Coordination number = 4 + 1 + 1 = 6 (octahedral complex).

Sum the oxidation state of the metal and the formal charges of all coordinated ligands.

• • The complex ion is [Co(NH₃)₄(H₂O)Cl]²⁺.

To neutralize the +2 charge of the complex cation, two chloride ions (Cl⁻, each -1) are required outside the coordination sphere.

• • Final formula: [Co(NH₃)₄(H₂O)Cl]Cl₂.
• • Matches Option (4).

Phosphorus has 5 valence electrons, all forming single σ bonds with five chlorine atoms. Steric number = 5 (5 bond pairs + 0 lone pairs). Hybridization = sp³d.

• • Match: A -> III.

Bromine has 7 valence electrons: 5 form single bonds with fluorine, leaving 1 lone pair. Steric number = 5 bond pairs + 1 lone pair = 6. Hybridization = sp³d² with octahedral electron geometry and square pyramidal molecular shape.

• • Match: B -> IV.

Boron has 3 valence electrons + 1 negative charge = 4 electrons, forming four σ bonds with four fluorine atoms. Steric number = 4 bond pairs + 0 lone pairs = 4. Hybridization = sp³.

• • Match: C -> I.

Nickel is in the +2 oxidation state (3d⁸). Cyanide (CN⁻) is a strong field ligand that forces the 3d electrons to pair up, leaving one vacant 3d orbital. Hybridization is dsp² with a square planar geometry.

• • Match: D -> II.
• • Full Match: A-III, B-IV, C-I, D-II (Option 2).

In an octahedral complex of type [M a₃ b₃] containing two sets of three unidentate ligands: 1. Facial (fac) isomer: Three donor atoms of the same ligand occupy adjacent positions at the three corners of an octahedral triangular face. 2. Meridional (mer) isomer: Three donor atoms of the same ligand occupy positions around the meridian of the octahedron (two are trans to each other and one is cis).

• • Fac-mer isomerism is unique to the [M a₃ b₃] stoichiometry.

Pyridine (py) is a neutral monodentate ligand (a). Chloride (Cl⁻) is a uninegative monodentate ligand (b). The complex has the general formula [M a₃ b₃] with chromium in the +3 oxidation state and coordination number 6.

• • It exists as fac-[Cr(py)₃(Cl)₃] and mer-[Cr(py)₃(Cl)₃].

1. [Cr(H₂O)₆]³⁺: Homoleptic [M a₆], no geometrical isomerism. 2. [Co(NH₃)₄(H₂O)₂]³⁺: [M a₄ b₂], exhibits cis-trans isomerism only. 3. [Ni(en)₂(H₂O)₂]²⁺: [M (AA)₂ b₂], exhibits cis-trans isomerism only.

• • Matches Option (1).

An electrolyte of type 1:3 (or A B₃) dissociates in aqueous solution to yield one cation and three anions (total of 4 ions per formula unit).

• • Produces 1 cation and 3 anions.

1. [CoCl₂(NH₃)₄]Cl: [CoCl₂(NH₃)₄]Cl → [CoCl₂(NH₃)₄]⁺ + Cl⁻ (1 cation + 1 anion = 1:1 electrolyte, total 2 ions). 2. [CoCl(NH₃)₅]Cl₂: [CoCl(NH₃)₅]Cl₂ → [CoCl(NH₃)₅]²⁺ + 2 Cl⁻ (1 cation + 2 anions = 1:2 electrolyte, total 3 ions). 3. [Co(NH₃)₆]Cl₃: [Co(NH₃)₆]Cl₃ → [Co(NH₃)₆]³⁺ + 3 Cl⁻ (1 cation + 3 anions = 1:3 electrolyte, total 4 ions). 4. [Co(NH₃)₃(NO₂)₃]: Neutral coordination compound, does not dissociate (non-electrolyte, 0 ions).

• • Option (3) is a 1:3 electrolyte.

[Co(NH₃)₆]Cl₃ is a 1:3 electrolyte.

• • Matches Option (3).

Cations of analytical Group-I form insoluble chlorides upon the addition of dilute hydrochloric acid (dil. HCl). Lead (Pb²⁺), along with Ag⁺ and Hg₂²⁺, belongs to Group-I:

• • Thus C matches I (C-I).

Cations of Group-III are precipitated as their hydroxides by ammonium hydroxide (NH₄OH) in the presence of ammonium chloride (NH₄Cl). Aluminium (Al³⁺), Fe³⁺, and Cr³⁺ belong to Group-III:

• • Thus D matches II (D-II).

Group-IV cations (Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺) precipitate as sulphides in basic medium (H₂S + NH₄OH + NH₄Cl). Magnesium (Mg²⁺) does not precipitate in Groups I–V and belongs to Group-VI:

• • Co²⁺ is in Group-IV ⟹ A matches III (A-III).
• • Mg²⁺ is in Group-VI ⟹ B matches IV (B-IV).
• • Complete match: A-III, B-IV, C-I, D-II, corresponding to Option (2).

In [NiCl₄]²⁻, nickel is in +2 oxidation state with electronic configuration [Ar] 3d⁸. Chloride (Cl⁻) is a weak field ligand, so the crystal field splitting Δ_t is less than the pairing energy P:

• • There are 2 unpaired electrons (n = 2).
• • Magnetic moment μ = √(2(2+2)) = √8 ≈ 2.83 BM.
• • Therefore, [NiCl₄]²⁻ is PARAMAGNETIC.

Examine the low-spin and zero-valent complexes with strong π-acceptor ligands:

• • [Ni(CN)₄]²⁻: Ni²⁺ (3d⁸). CN⁻ causes pairing into four d-orbitals, leaving one d-orbital empty for dsp² square planar hybridization (n = 0, Diamagnetic).

In the octahedral complex [Ni(H₂O)₆]²⁺, nickel is in +2 oxidation state (3d⁸). In octahedral symmetry, the d⁸ configuration is uniquely determined regardless of ligand field strength:

• • Two unpaired electrons reside in the eg orbitals (dx²-y² and dz²).
• • Therefore, [Ni(H₂O)₆]²⁺ is PARAMAGNETIC (μ ≈ 2.83 BM).
• • Conclusion: A and D only are paramagnetic, matching Option (3).

When a coordination entity absorbs a photon, an electron is promoted across the crystal field splitting gap (d-d transition):

• • A larger crystal field splitting energy corresponds to a shorter absorbed wavelength.

Analyze central metal oxidation states and ligand field strengths from the spectrochemical series:

• • Complex B [Co(CN)₆]³⁻: Co³⁺ with the extremely strong field ligand CN⁻ produces the highest splitting (absorbs UV/violet, λ ≈ 310 nm).
• • Complex A [Co(NH₃)₆]³⁺: Co³⁺ with strong field ligand NH₃ has large Δ_o (absorbs blue-green, λ ≈ 475 nm).
• • Complex D [Ti(H₂O)₆]³⁺: Ti³⁺ (+3 state) with H₂O has Δ_o = 20,300 cm⁻¹ (absorbs yellow-green, λ ≈ 498 nm).
• • Complex C [Cu(H₂O)₄]²⁺: Cu²⁺ (+2 state) with H₂O has much lower splitting (absorbs red/near-IR, λ ≈ 600–800 nm, transmitting blue).

Since absorbed wavelength is inversely proportional to splitting energy:

• • Thus the correct order of absorbed wavelength is B < A < D < C, matching Option (2).

The molar conductance of an aqueous solution of a coordination compound is directly proportional to the number of free ions produced per formula unit upon dissolution:

• • Ligands inside the coordination sphere [ ] are covalently/coordinatively bonded and do not dissociate in water.

Examine the ionization behavior of each given option:

• • Option (1): [Co(NH₃)₃Cl₃] is a neutral coordination complex with zero counter ions, giving essentially zero electrical conductance (minimum).
• • Option (2): Printed as [Co(NH₃)₄Cl₂]. Without an external counter-ion, it represents a neutral entity yielding 0 ions in solution.
• • Option (3): [Co(NH₃)₆]Cl₃ ⟶ [Co(NH₃)₆]³⁺ + 3 Cl⁻ (Total 4 ions) ⟹ Maximum conductance.
• • Option (4): [Co(NH₃)₅Cl]Cl ⟶ [Co(NH₃)₅Cl]⁺ + Cl⁻ (Total 2 ions).

Standard textbook problems list [Co(NH₃)₄Cl₂]Cl (which yields 2 ions). However, the examination paper printed [Co(NH₃)₄Cl₂] with no external chlorine atom, making both (1) and (2) neutral non-electrolytes:

• • NTA recognized this printing discrepancy and awarded full marks to candidates selecting either Option (1) or Option (2).

Examine transition metal catalysts in major industrial processes from NCERT Chapter 8:

• • A matches I (A-I).
• • Wacker Oxidation: Conversion of ethene into ethanal using aqueous PdCl₂ catalyst in the presence of CuCl₂ and air:
• • \text{C}_2\text{H}_4 + \text{PdCl}_2 + \text{H}_2\text{O} \longrightarrow \text{CH}_3\text{CHO} + \text{Pd} + 2\text{HCl} \implies \text{B matches II (B-II)}.

Examine organometallic and coordination complexes used in homogeneous and heterogeneous polymerizations:

• • Wilkinson’s catalyst is a square planar rhodium(I) complex used for selective homogeneous hydrogenation of double bonds.
• • Ziegler-Natta Catalyst: A mixture of titanium tetrachloride and trialkylaluminium (TiCl₄ + Al(CH₃)₃ or Al(C₂H₅)₃), used for stereospecific polymerization of ethene into high-density polyethylene (HDPE): D matches IV (D-IV).

Combine all four matches into the final answer:

• • Matches Option (3) perfectly.

Linkage isomerism arises in coordination compounds containing ambidentate ligands (ligands possessing two distinct donor atoms). In [Co(NH3)5(NO2)]Cl2, the ligand can bind via nitrogen (nitro, yellow) or oxygen (nitrito, -ONO, red):

Ionization isomerism occurs when the counterion in a complex salt is itself a potential ligand and can displace a ligand from the coordination sphere, producing different ions upon dissolution:

This type of isomerism occurs in compounds where both the cation and anion are complex ions, and ligands interchange between the two different metal centers:

Solvate isomerism (hydrate isomerism when water is involved) differs based on whether a solvent molecule is directly bonded to the metal ion as a ligand or merely present as crystal lattice water:

Both [Co(NH3)6]³⁺ and [CoF6]³⁻ possess a central cobalt ion surrounded by six monodentate ligands (coordination number = 6). Coordination number 6 uniquely dictates an Octahedral coordination polyhedral geometry for both complexes:

Cobalt(III) has a 3d⁶ valence electron configuration. NH3 occupies a high position in the spectrochemical series and behaves as a strong field ligand with Co³⁺. Because Δo > P (pairing energy), electrons fully pair in the lower energy t2g orbitals:

Fluoride ion (F⁻) lies at the weak field end of the spectrochemical series. The crystal field splitting is smaller than the pairing energy (Δo < P), so electrons follow Hund’s rule of maximum multiplicity across both t2g and eg sets:

Statement I correctly asserts that both are octahedral but differ in magnetic behavior. Statement II correctly specifies that [Co(NH3)6]³⁺ is diamagnetic and [CoF6]³⁻ is paramagnetic. Both statements are factually and theoretically correct.

Homoleptic complexes are coordination entities in which a central metal ion is coordinated to only one chemical type of donor atom or ligand. In [Co(NH3)6]³⁺, all six coordination sites are occupied exclusively by neutral ammonia (ammine) molecules:

Heteroleptic complexes are coordination entities in which the central metal ion is bound to two or more distinct types of ligands. In [Co(NH3)4Cl2]⁺, cobalt is coordinated simultaneously to two different ligand species: four neutral ammine ligands and two anionic chlorido ligands:

Statement I correctly identifies [Co(NH3)6]³⁺ as homoleptic and [Co(NH3)4Cl2]⁺ as heteroleptic. Statement II accurately provides the foundational definition: [Co(NH3)6]³⁺ has only one kind of ligand while [Co(NH3)4Cl2]⁺ has more than one kind. Both statements are true.

According to IUPAC coordination chemistry conventions, complexes are classified based on the diversity of their coordinating donor ligands: (i) Homoleptic complexes: complexes in which the central metal atom or ion is coordinated to only ONE type of ligand species; (ii) Heteroleptic complexes: complexes in which the metal ion is coordinated to two or more chemically distinct types of ligands.

Potassium trioxalatoaluminate (III) has the chemical formula K₃[Al(C₂O₄)₃]. In this complex, the central aluminum(III) ion is coordinated exclusively to three bidentate oxalate ligands (C₂O₄²⁻):

Writing the coordination formulas for the remaining choices reveals multiple distinct ligand types:

When a multidentate (polydentate) ligand binds to a central metal ion to form a ring structure, the resulting complex possesses significantly higher thermodynamic stability than an analogous complex containing only unidentate ligands. This phenomenon is known as the chelate effect:

Examine each coordination sphere: [Co(NH₃)₆]³⁺, [Co(NH₃)₄(H₂O)Br]²⁺, and [Co(NH₃)₃(NO₃)₃] contain only unidentate ligands. [CoCl₂(en)₂]⁺ contains two bidentate chelating ethylenediamine ligands:

Chelate formation is thermodynamically favored primarily due to a positive entropy change (ΔS° > 0). Displacement of multiple monodentate solvent molecules by a chelating ligand increases disorder:

Ethylenediaminetetraacetate ion ($EDTA^{4-}$) is an important polychelating agent derived from ethylenediaminetetraacetic acid by loss of four carboxylate protons. Its molecular structure is:

When $EDTA^{4-}$ binds to a central metal cation ($M^{n+}$) to form an octahedral complex, it wraps around the metal using six donor atoms: - Two Nitrogen Atoms: The two tertiary nitrogen atoms of the ethylenediamine core, each containing an unshared pair of electrons ($2\text{ N}$). - Four Oxygen Atoms: One oxygen atom from each of the four deprotonated carboxylate groups ($-COO^-$) ($4\text{ O}$).

Because it coordinates simultaneously through six donor sites, it is classified as a hexadentate ligand with four "O" and two "N" donor atoms.

Under an octahedral ligand field, the five degenerate $d$-orbitals split into lower $t_{2g}$ (triply degenerate) and higher $e_g$ (doubly degenerate) sets separated by crystal field splitting energy $\Delta_o$. Strong field ligands ($CN^-$) induce high $\Delta_o > P$ (pairing energy), producing low-spin paired configurations. Weak field ligands ($H_2O$) have $\Delta_o < P$, producing high-spin configurations obeying Hund's rule.

Iron is in the $+3$ oxidation state ($Fe^{3+}: [Ar]3d^5$). With strong-field cyanide ligands ($CN^-$), pairing occurs: $t_{2g}^5 e_g^0$. Number of unpaired electrons $n = 1$: $$\mu = \sqrt{1(1+2)} = \sqrt{3} \approx 1.73\text{ BM} \implies \mathbf{\text{(a)-(iv)}}$$

Iron is in the $+3$ oxidation state ($Fe^{3+}: [Ar]3d^5$). With weak-field aqua ligands ($H_2O$), electrons occupy orbitals singly: $t_{2g}^3 e_g^2$. Number of unpaired electrons $n = 5$: $$\mu = \sqrt{5(5+2)} = \sqrt{35} \approx 5.92\text{ BM} \implies \mathbf{\text{(b)-(i)}}$$

Iron is in the $+2$ oxidation state ($Fe^{2+}: [Ar]3d^6$). With strong-field cyanide ligands ($CN^-$), all 6 electrons completely pair in $t_{2g}$: $t_{2g}^6 e_g^0$. Number of unpaired electrons $n = 0$ (diamagnetic): $$\mu = 0\text{ BM} \implies \mathbf{\text{(c)-(ii)}}$$

Iron is in the $+2$ oxidation state ($Fe^{2+}: [Ar]3d^6$). With weak-field aqua ligands ($H_2O$), the configuration is $t_{2g}^4 e_g^2$. Number of unpaired electrons $n = 4$: $$\mu = \sqrt{4(4+2)} = \sqrt{24} \approx 4.90\text{ BM} \implies \mathbf{\text{(d)-(iii)}}$$