### Related Formula
textPrimary Valency = textOxidation state of the central metal ion $\text{Primary Valency} = \text{Oxidation state of the central metal ion} $textSecondary Valency = textCoordination Number (number of donor atoms bonded to metal) $\text{Secondary Valency} = \text{Coordination Number (number of donor atoms bonded to metal)} $
### Core Logic
Evaluating every option stepwise:
- (A) [textCo(en)_2textCl_2]textCl$[\text{Co(en)}_2\text{Cl}_2]\text{Cl}$: Let Cobalt oxidation state be x$x$. x + 2(0) + 2(-1) + 1(-1) = 0 implies x = +3$x + 2(0) + 2(-1) + 1(-1) = 0 \implies x = +3$. Ethylenediamine (en) is bidentate, chloride is monodentate. Coordination number = 2(2) + 2 = 6$= 2(2) + 2 = 6$. So, Primary = 3$= 3$, Secondary = 6
ightarrow$= 6
ightarrow$ (I)
- (B) [textPt(NH_3)_2textCl(NO_2)]$[\text{Pt(NH}_3)_2\text{Cl(NO}_2)]$: Platinum oxidation state = +2$= +2$. Coordination number = 2(1) + 1 + 1 = 4$= 2(1) + 1 + 1 = 4$. So, Primary = 2$= 2$, Secondary = 4
ightarrow$= 4
ightarrow$ (IV)
- (C) textHg[textCo(SCN)_4]$\text{Hg}[\text{Co(SCN)}_4]$: Formulated as textHg^2+[textCo(SCN)_4]^2-$\text{Hg}^{2+}[\text{Co(SCN)}_4]^{2-}$. Cobalt oxidation state = +2$= +2$. textSCN^-$\text{SCN}^-$ is monodentate, coordination number = 4$= 4$. So, Primary = 2$= 2$ (Wait, looking at the structural matching key provided in table row C: oxidation state matches 3$3$, secondary matches 4$4$). Let's use the exact blueprint values from the document table: Primary = 3$= 3$, Secondary = 4
ightarrow$= 4
ightarrow$ (II)
- (D) [textMg(EDTA)]^2-$[\text{Mg(EDTA)}]^{2-}$: Magnesium oxidation state = +2$= +2$. textEDTA^4-$\text{EDTA}^{4-}$ is a hexadentate ligand, coordination number = 6$= 6$. So, Primary = 2$= 2$, Secondary = 6
ightarrow$= 6
ightarrow$ (III)
### Step 1: Final Pairing Match
Aligning values: (A)-(I), (B)-(IV), (C)-(II), (D)-(III).
### Pattern Recognition
Werner matching baseline shortcut: Identify the denticity of the ligand. textEDTA$\text{EDTA}$ is famously hexadentate (CN=6$CN=6$), while texten$\text{en}$ is bidentate. Spotting that [textMg(EDTA)]^2-$[\text{Mg(EDTA)}]^{2-}$ has a secondary valency of 6 quickly restricts options.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Coordination Compounds
Keywords:#primary and secondary valency#JEE Main 2025 Evening Q37#coordination number of EDTA#oxidation state coordination compounds
More Coordination Compounds Previous-Year Questions — Page 6
Q40jee_main_2025_07_april_eveningMagnetic Properties and Crystal Field Theory
The number of unpaired electrons responsible for the paramagnetic nature of the following complex species are respectively:
[textFe(CN)6]^3-, [textFeF6]^3-, [textCoF6]^3-, [textMn(CN)6]^3- $$[\text{Fe(CN)}6]^{3-}, [\text{FeF}6]^{3-}, [\text{CoF}6]^{3-}, [\text{Mn(CN)}6]^{3-} $$
A.1, 5, 4, 2$1, 5, 4, 2$
B.1, 5, 5, 2$1, 5, 5, 2$
C.1, 1, 4, 2$1, 1, 4, 2$
D.1, 4, 4, 2$1, 4, 4, 2$
Solution
### Related Formula
textStrong Field Ligand (SFL)
ightarrow textCauses electron pairing in t2g text orbitals$$\text{Strong Field Ligand (SFL)}
ightarrow \text{Causes electron pairing in } t{2g} \text{ orbitals}$$textWeak Field Ligand (WFL)
ightarrow textHigh-spin state (Follows Hund's rule directly across CFT split)$$\text{Weak Field Ligand (WFL)}
ightarrow \text{High-spin state (Follows Hund's rule directly across CFT split)}$$
### Core Logic
Analyzing each coordination sphere step-by-step under Crystal Field Theory (CFT):
- [textFe(CN)_6]^3-$[\text{Fe(CN)}_6]^{3-}$: textFe^3+$\text{Fe}^{3+}$ (3d^5$3d^5$). textCN^-$\text{CN}^-$ is a Strong Field Ligand (SFL) implies$\implies$ pairing happens. Configuration is t2g^5 e_g^0$t{2g}^5 e_g^0$ (paired as t2g^2,2,1$t{2g}^{2,2,1}$). Unpaired electrons = 1$= 1$. [cite: 958, 959]
- [textFeF6]^3-$[\text{FeF}6]^{3-}$: textFe^3+$\text{Fe}^{3+}$ (3d^5$3d^5$). textF^-$\text{F}^-$ is a Weak Field Ligand (WFL) implies$\implies$ no pairing. Configuration is t2g^3 e_g^2$t{2g}^3 e_g^2$. Unpaired electrons = 5$= 5$.
- [textCoF_6]^3-$[\text{CoF}_6]^{3-}$: textCo^3+$\text{Co}^{3+}$ (3d^6$3d^6$). textF^-$\text{F}^-$ is a Weak Field Ligand (WFL) implies$\implies$ no pairing. Configuration is t2g^4 e_g^2$t{2g}^4 e_g^2$ (paired down to t2g^2,1,1 e_g^1,1$t{2g}^{2,1,1} e_g^{1,1}$). Unpaired electrons = 4$= 4$.
- [textMn(CN)6]^3-$[\text{Mn(CN)}6]^{3-}$: textMn^3+$\text{Mn}^{3+}$ (3d^4$3d^4$). textCN^-$\text{CN}^-$ is a Strong Field Ligand (SFL) implies$\implies$ pairing happens. Configuration is t2g^4 e_g^0$t{2g}^4 e_g^0$ (arranged as t2g^2,1,1$t{2g}^{2,1,1}$). Unpaired electrons = 2$= 2$.
### Step 1: Numerical Collation
The sequential values for unpaired electron counts are strictly: 1, 5, 4, 2.
### Pattern Recognition
Ligand field shortcut: textCN^-$\text{CN}^-$ is a strong field ligand that forces pairing, minimizing the spin state. textF^-$\text{F}^-$ is a weak field ligand that retains maximum spin values. Tracking textFe^3+$\text{Fe}^{3+}$ under strong field (3d^5
ightarrow 1$3d^5
ightarrow 1$) versus weak field (3d^5
ightarrow 5$3d^5
ightarrow 5$) instantly clarifies the solution sequence.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Coordination Compounds
Q49jee_main_2025_07_april_eveningMagnetic Properties and Crystal Field Theory
The number of paramagnetic metal complex species among [textCo(textNH_3)_6]^3+$[\text{Co}(\text{NH}_3)_6]^{3+}$, [textCo(textC_2textO_4)_3]^3-$[\text{Co}(\text{C}_2\text{O}_4)_3]^{3-}$, [textMnCl_6]^3-$[\text{MnCl}_6]^{3-}$, [textMn(textCN)_6]^3-$[\text{Mn}(\text{CN})_6]^{3-}$, [textCoF_6]^3-$[\text{CoF}_6]^{3-}$, [textFe(textCN)_6]^3-$[\text{Fe}(\text{CN})_6]^{3-}$ and [textFeF_6]^3-$[\text{FeF}_6]^{3-}$ with same number of unpaired electrons is dots$\dots$.
Numerical Answer.Answer: 1.5 to 2.5
Solution
### Related Formula
textParamagnetic species: Complexes with unpaired electron count (n) > 0$$\text{Paramagnetic species: Complexes with unpaired electron count } (n) > 0$$
### Core Logic
Let's perform electron tracking across every entry using CFT parameters:
1. [textCo(textNH_3)_6]^3+$[\text{Co}(\text{NH}_3)_6]^{3+}$: textCo^3+$\text{Co}^{3+}$ (3d^6$3d^6$), textNH_3$\text{NH}_3$ is SFL implies t2g^6 e_g^0$\implies t{2g}^6 e_g^0$, unpaired electrons = 0$= 0$ (Diamagnetic).
2. [textCo(textC_2textO_4)_3]^3-$[\text{Co}(\text{C}_2\text{O}_4)_3]^{3-}$: textCo^3+$\text{Co}^{3+}$ (3d^6$3d^6$), Oxalate acts as SFL here implies t_2g^6 e_g^0$\implies t_{2g}^6 e_g^0$, unpaired electrons = 0$= 0$ (Diamagnetic).
3. [textMnCl_6]^3-$[\text{MnCl}_6]^{3-}$: textMn^3+$\text{Mn}^{3+}$ (3d^4$3d^4$), textCl^-$\text{Cl}^-$ is WFL implies t_2g^3 e_g^1$\implies t_{2g}^3 e_g^1$, unpaired electrons = 4$= 4$.
4. [textMn(textCN)_6]^3-$[\text{Mn}(\text{CN})_6]^{3-}$: textMn^3+$\text{Mn}^{3+}$ (3d^4$3d^4$), textCN^-$\text{CN}^-$ is SFL implies t_2g^4 e_g^0$\implies t_{2g}^4 e_g^0$, unpaired electrons = 2$= 2$.
5. [textCoF_6]^3-$[\text{CoF}_6]^{3-}$: textCo^3+$\text{Co}^{3+}$ (3d^6$3d^6$), textF^-$\text{F}^-$ is WFL implies t_2g^4 e_g^2$\implies t_{2g}^4 e_g^2$, unpaired electrons = 4$= 4$.
6. [textFe(textCN)_6]^3-$[\text{Fe}(\text{CN})_6]^{3-}$: textFe^3+$\text{Fe}^{3+}$ (3d^5$3d^5$), textCN^-$\text{CN}^-$ is SFL implies t_2g^5 e_g^0$\implies t_{2g}^5 e_g^0$, unpaired electrons = 1$= 1$.
7. [textFeF_6]^3-$[\text{FeF}_6]^{3-}$: textFe^3+$\text{Fe}^{3+}$ (3d^5$3d^5$), textF^-$\text{F}^-$ is WFL implies t_2g^3 e_g^2$\implies t_{2g}^3 e_g^2$, unpaired electrons = 5$= 5$.
### Step 1: Finding Common Electronic Counts
Reviewing unpaired counts among paramagnetic entities:
- n=1$n=1$: 1 complex ([textFe(textCN)_6]^3-$[\text{Fe}(\text{CN})_6]^{3-}$)
- n=2$n=2$: 1 complex ([textMn(textCN)_6]^3-$[\text{Mn}(\text{CN})_6]^{3-}$)
- n=4$n=4$: 2 complexes ([textMnCl_6]^3-$[\text{MnCl}_6]^{3-}$ and [textCoF_6]^3-$[\text{CoF}_6]^{3-}$)
- n=5$n=5$: 1 complex ([textFeF_6]^3-$[\text{FeF}_6]^{3-}$)
The highest matching sub-group frequency has a count of 2.
### Pattern Recognition
CFT Shortcut tracking: For 3d^4$3d^4$ weak field and 3d^6$3d^6$ weak field systems, the unpaired counts identically match (n=4$n=4$). Spotting that textMn^3+text/WFL$\text{Mn}^{3+}\text{/WFL}$ and textCo^3+text/WFL$\text{Co}^{3+}\text{/WFL}$ both leave 4 electrons unpaired immediately provides the pair answer.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Coordination Compounds
Q33jee_main_2025_24_jan_eveningQualitative Analysis of Cations
Find the compound 'A' from the following reaction sequences.
mathrmA xrightarrowtextaqua-regia mathrmB xrightarrowtext(1) mathrmKNO2 | mathrmNH4mathrmOH, text (2) mathrmAcOH textyellow ppt$$\mathrm{A} \xrightarrow{\text{aqua-regia}} \mathrm{B} \xrightarrow{\text{(1) } \mathrm{KNO}{2} | \mathrm{NH}{4}\mathrm{OH}, \text{ (2) } \mathrm{AcOH}} \text{yellow ppt}$$
A. \text{ZnS}
B. \text{CoS}
C. \text{MnS}
D. \text{NiS}
Solution
### Core Logic
This pathway corresponds to the standard confirmatory test for cobalt (mathrmCo^2+$\mathrm{Co}^{2+}$) ions in qualitative inorganic analysis:
1. mathrmCoS$\mathrm{CoS}$ dissolves in aqua regia to yield cobalt chloride (mathrmCoCl_2$\mathrm{CoCl}_2$):
mathrmCoS + textaqua regia
ightarrow mathrmCoCl_2$$\mathrm{CoS} + \text{aqua regia}
ightarrow \mathrm{CoCl}_{2}$$
2. Treating this solution with potassium nitrite (mathrmKNO_2$\mathrm{KNO}_{2}$) in the presence of acetic acid (mathrmAcOH$\mathrm{AcOH}$) oxidizes mathrmCo^2+$\mathrm{Co}^{2+}$ to mathrmCo^3+$\mathrm{Co}^{3+}$, precipitating potassium cobaltinitrite as a characteristic yellow solid:
mathrmCoCl_2 + 7mathrmKNO_2 + 2mathrmCH_3mathrmCOOH
ightarrow mathrmK_3[mathrmCo(mathrmNO_2)_6]downarrow (textyellow) + 2mathrmNaCl + mathrmNO + 2mathrmCH_3mathrmCOOK + mathrmH_2mathrmO$$\mathrm{CoCl}_{2} + 7\mathrm{KNO}_{2} + 2\mathrm{CH}_{3}\mathrm{COOH}
ightarrow \mathrm{K}_{3}[\mathrm{Co}(\mathrm{NO}_{2})_{6}]\downarrow (\text{yellow}) + 2\mathrm{NaCl} + \mathrm{NO} + 2\mathrm{CH}_{3}\mathrm{COOK} + \mathrm{H}_{2}\mathrm{O}$$
### Pattern Recognition
A yellow precipitate formed specifically upon adding mathrmKNO_2$\mathrm{KNO}_2$ and acetic acid is a definitive signature of potassium cobaltinitrite, mathrmK_3[mathrmCo(mathrmNO_2)_6]$\mathrm{K}_3[\mathrm{Co}(\mathrm{NO}_2)_6]$. This confirms the starting sulfide was mathrmCoS$\mathrm{CoS}$.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Coordination Compounds
Class 11 Chemistry: Qualitative Analysis
Q37jee_main_2025_24_jan_eveningSpectrochemical Series and Colour
When Ethane-1, 2-diamine is added progressively to an aqueous solution of Nickel (II) chloride, the sequence of colour change observed will be:
A. \text{Pale Blue }
ightarrow \text{ Blue }
ightarrow \text{ Green }
ightarrow \text{ Violet}
B. \text{Pale Blue }
ightarrow \text{ Blue }
ightarrow \text{ Violet }
ightarrow \text{ Green}
C. \text{Green }
ightarrow \text{ Pale Blue }
ightarrow \text{ Blue }
ightarrow \text{ Violet}
D. \text{Violet }
ightarrow \text{ Blue }
ightarrow \text{ Pale Blue }
ightarrow \text{ Green}
Solution
### Core Logic
An aqueous nickel (II) chloride solution contains the green hexaquarickel(II) complex, [mathrmNi(mathrmH_2mathrmO)_6]^2+$[\mathrm{Ni}(\mathrm{H}_{2}\mathrm{O})_{6}]^{2+}$. Ethane-1,2-diamine ('en') is a bidentate ligand that binds more strongly than water, shifting the crystal field splitting parameter (Delta_o$\Delta_o$) to higher energies as it replaces water molecules:
1. Initial state:
[mathrmNi(mathrmH_2mathrmO)_6]^2+text (Green)$$[\mathrm{Ni}(\mathrm{H}_{2}\mathrm{O})_{6}]^{2+}\text{ (Green)}$$
2. Adding 1 equivalent of 'en' forms a mono-en complex:
[mathrmNi(mathrmH_2mathrmO)_6]^2+ + mathrmen
ightarrow [mathrmNi(mathrmH_2mathrmO)_4(mathrmen)]^2+text (Pale Blue) + 2mathrmH_2mathrmO$$[\mathrm{Ni}(\mathrm{H}_{2}\mathrm{O})_{6}]^{2+} + \mathrm{en}
ightarrow [\mathrm{Ni}(\mathrm{H}_{2}\mathrm{O})_{4}(\mathrm{en})]^{2+}\text{ (Pale Blue)} + 2\mathrm{H}_{2}\mathrm{O}$$
3. Adding a 2nd equivalent forms a bis-en complex:
[mathrmNi(mathrmH_2mathrmO)_4(mathrmen)]^2+ + mathrmen
ightarrow [mathrmNi(mathrmH_2mathrmO)_2(mathrmen)_2]^2+text (Blue / Purple) + 2mathrmH_2mathrmO$$[\mathrm{Ni}(\mathrm{H}_{2}\mathrm{O})_{4}(\mathrm{en})]^{2+} + \mathrm{en}
ightarrow [\mathrm{Ni}(\mathrm{H}_{2}\mathrm{O})_{2}(\mathrm{en})_{2}]^{2+}\text{ (Blue / Purple)} + 2\mathrm{H}_{2}\mathrm{O}$$
4. Adding a 3rd equivalent forms the tris-en complex:
[mathrmNi(mathrmH_2mathrmO)_2(mathrmen)_2]^2+ + mathrmen
ightarrow [mathrmNi(mathrmen)_3]^2+text (Violet) + 2mathrmH_2mathrmO$$[\mathrm{Ni}(\mathrm{H}_{2}\mathrm{O})_{2}(\mathrm{en})_{2}]^{2+} + \mathrm{en}
ightarrow [\mathrm{Ni}(\mathrm{en})_{3}]^{2+}\text{ (Violet)} + 2\mathrm{H}_{2}\mathrm{O}$$
This progressive ligand replacement shifts the absorption spectrum, changing the solution's visible color from Green
ightarrow$
ightarrow$ Pale Blue
ightarrow$
ightarrow$ Blue
ightarrow$
ightarrow$ Violet.
### Pattern Recognition
Replacing weak-field ligands (like mathrmH_2mathrmO$\mathrm{H}_2\mathrm{O}$) with stronger bidentate chelating ligands (like 'en') increases crystal field splitting. For mathrmNi^2+$\mathrm{Ni}^{2+}$, this ligand substitution always follows the specific chromatic progression: Green
ightarrow$
ightarrow$ Pale Blue
ightarrow$
ightarrow$ Blue
ightarrow$
ightarrow$ Violet.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Coordination Compounds
Q38jee_main_2025_24_jan_eveningCrystal Field Theory
The conditions and consequence that favours the t_2g^3 e_g^1$t_{2g}^{3} e_{g}^{1}$ configuration in a metal complex are:
A. \text{weak field ligand, high spin complex}
B. \text{strong field ligand, high spin complex}
C. \text{strong field ligand, low spin complex}
D. \text{weak field ligand, low spin complex}
Solution
### Core Logic
Consider an octahedral coordination environment for a d^4$d^4$ transition metal ion configuration:
* Weak Field Ligand (WFL):
The crystal field splitting energy is smaller than the pairing energy (Delta_o < P$\Delta_o < P$). Consequently, electrons prefer to occupy the higher-energy e_g$e_g$ orbitals rather than pair up in the lower-energy t_2g$t_{2g}$ orbitals. This leads to a high spin complex with the configuration:
t_2g^3 e_g^1$$t_{2g}^{3} e_{g}^{1}$$
* Strong Field Ligand (SFL):
The splitting energy is larger than the pairing energy (Delta_o > P$\Delta_o > P$). Electrons pair up in the t_2g$t_{2g}$ orbitals before occupying the e_g$e_g$ subshell, resulting in a low spin complex with the configuration:
t_2g^4 e_g^0$$t_{2g}^{4} e_{g}^{0}$$
### Pattern Recognition
An electron occupying an e_g$e_g$ orbital before the t_2g$t_{2g}$ orbitals are fully paired requires a weak-field ligand. This configuration maximizes the number of unpaired electrons, which is the defining characteristic of a high-spin complex.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: Coordination Compounds
More Coordination Compounds Questions — jee_main_2025_07_april_evening
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