Q.In the complex ion [Co(en)(NH3)2Cl2]+, what is the coordination number of the central cobalt ion?

Showing the 12 most recent of 66 on this concept.

1. CBSE 2026Set 56/3/11 markMCQ
   Q.According to Werner's theory, the primary valencies of the central metal atom : (A) are satisfied by neutral molecules or negative ions. (B) are equal to its coordination number. (C) are satisfied by negative ions. (D) are non-ionisable.

   ›Reveal solutionSolution

   Werner’s theory distinguishes primary valencies (ionisable, satisfied only by negative ions, equal to oxidation state) from secondary valencies (non-ionisable, satisfied by neutral molecules or negative ions, equal to coordination number). The correct answer is (C).

   Werner’s Coordination Theory was a breakthrough because it explained why compounds like $\ce{CoCl3·6NH3}$ (which we now write as $\ce{[Co(NH3)6]Cl3}$) behave so differently from simple salts. The key insight: a metal ion has two kinds of bonding capacity — not just one.

   Primary valency (now called oxidation state) is the metal’s ionic bonding capacity. It is satisfied only by negative ions (anions), because it arises from the metal’s need to neutralise its positive charge. These bonds are ionisable — they break apart in solution, giving the familiar conductivity and precipitation tests.

   Secondary valency (now called coordination number) is the metal’s ability to bind ligands directly. It is satisfied by neutral molecules (like $\ce{NH3}$) or negative ions (like $\ce{Cl-}$). These bonds are non-ionisable — they stay attached to the metal even in solution.

   Now let’s apply this to the options.

   1. Option (A): “Primary valencies are satisfied by neutral molecules or negative ions.”

      This is false. Primary valencies are satisfied only by negative ions. Neutral molecules satisfy secondary valencies. For example, in $\ce{[Co(NH3)6]Cl3}$, the three $\ce{Cl-}$ ions satisfy the primary valency (Co³⁺ needs three negative charges), while six $\ce{NH3}$ molecules satisfy the secondary valency.

   2. Option (B): “Primary valencies are equal to its coordination number.”

      This is false. Primary valency equals the oxidation state of the metal. Coordination number equals the secondary valency. They are often different. In $\ce{[Co(NH3)6]Cl3}$, primary valency = 3, coordination number = 6.

   3. Option (C): “Primary valencies are satisfied by negative ions.” …

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2. CBSE 2026Set 56/1/11 markMCQ
   Q.The secondary valency of Co in the complex $[Co(NH_3)_5(NO_2)]^{2+}$ is (A) 5 (B) 1 (C) 4 (D) 6

   ›Reveal solutionSolution

   In Werner’s coordination theory, secondary valency equals the coordination number — the number of ligand donor atoms directly bonded to the metal. For $[Co(NH_3)_5(NO_2)]^{2+}$, there are 5 NH₃ molecules (each donating one N) and one NO₂⁻ ligand (also donating one N), giving a total of 6 donor atoms. So the secondary valency is 6.

   Werner’s coordination theory is the foundation here. He proposed that metal ions have two kinds of valency: primary (ionisable, corresponding to oxidation state) and secondary (non-ionisable, corresponding to coordination number). The secondary valency is simply the number of ligand donor atoms directly attached to the central metal ion — in other words, the coordination number.

   For the complex $[Co(NH_3)_5(NO_2)]^{2+}$, we need to count how many atoms are actually bonded to cobalt.

   1. Identify the ligands and their denticity.

      • NH₃ (ammine) is a monodentate ligand — it binds through the lone pair on nitrogen. Each NH₃ contributes one donor atom.
      • NO₂⁻ (nitrito or nitro, depending on binding mode) is also monodentate when it binds through nitrogen (the more common nitro form) or through oxygen. In either case, it uses one donor atom per ligand. The problem doesn’t specify the linkage isomer, but that doesn’t change the count — it’s still one donor atom.

   2. Count the total number of donor atoms.

      • Five NH₃ ligands: $5 \times 1 = 5$ donor atoms.
      • One NO₂⁻ ligand: $1 \times 1 = 1$ donor atom.
      • Total = $5 + 1 = 6$ donor atoms.

   3. Relate this to secondary valency.

      Werner defined secondary valency as the number of groups directly coordinated to the metal — exactly the coordination number. So here, secondary valency = 6. …

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3. CBSE 2026Set DZ1 markMCQ
   Q.What is the coordination number of Co in the complex $K_3[Co(C_2O_4)_3]$ ?

   (a) 3

   (b) 4

   (c) 5

   (d) 6

   ›Reveal solutionSolution

   Oxalate is a bidentate ligand; three of them occupy six coordination sites, so the coordination number of Co is 6 — option (d).

   The coordination number is the number of ligand donor atoms directly bonded to the central metal ion (not the number of ligand molecules).

   …

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4. CBSE 2026Set ANNUAL1 markMCQ
   Q.What is the coordination number of Cobalt in [Co(en)3]Cl3 ?

   (a) 6

   (b) 5

   (c) 4

   (d) 3

   ›Reveal solutionSolution

   Each 'en' ligand is bidentate, so [Co(en)3]Cl3 has coordination number 6.

   In $[Co(en)_3]Cl_3$, ethylenediamine (en, $H_2NCH_2CH_2NH_2$) acts as a bidentate ligand — it has two nitrogen donor atoms, each with a lone pair, that can simultaneously coordinate to the central metal ion, forming a five-membered chelate ring with cobalt.

   Since there are three 'en' ligands attached to the cobalt centre, and each contributes 2 donor atoms, the total number of coordinate bonds (and hence the coordination number) is:

   $3 \times 2 = 6$

   …

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5. CBSE 2026Set ANNUAL1 markMCQ
   Q.Ambidentate ligand is

   (a) H2O

   (b) NH3

   (c) NO2-

   (d) Cl-

   ›Reveal solutionSolution

   An ambidentate ligand has two different donor atoms but binds through only one of them at a time, depending on conditions.

   • H2O, NH3 and Cl- each have only one type of donor atom, so they are simple monodentate ligands, not ambidentate. …

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6. CBSE 2026Set ANNUAL1 markMCQ
   Q.Coordination number of Pt in [Pt(NH3)2 Cl (NO2)] complex is

   (a) 3

   (b) 4

   (c) 5

   (d) 6

   ›Reveal solutionSolution

   Coordination number = the total number of ligand donor atoms directly bonded to the central metal atom/ion.

   …

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7. CBSE 2026Set ANNUAL1 markMCQ
   Q.Identify the homoleptic complex among the following compounds:

   (a) [Co(NH3)4Cl2]+

   (b) [Ni(CN)4]2-

   (c) [Cr(NH3)2Cl2(en)]+

   (d) Both (A) and (C)

   ›Reveal solutionSolution

   A homoleptic complex is one in which the metal is bonded to only a single kind of donor ligand; a heteroleptic complex has more than one kind.

   Checking each option:

   • [Co(NH3)4Cl2]+: contains two different ligands, NH3 and Cl- (heteroleptic).
   • [Ni(CN)4]2-: contains only CN- ligands, all of the same type (homoleptic). …

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8. CBSE 2026Set ANNUAL1 markMCQ
   Q.Which of the following is a bidentate ligand?

   (a) CN-

   (b) NH3

   (c) C2O4^2-

   (d) CO3^2-

   ›Reveal solutionSolution

   A bidentate ligand has two donor atoms that simultaneously coordinate to the same central metal atom, forming a ring (chelate).

   • CN- has one donor atom (C), so it is monodentate.
   • NH3 has one donor atom (N), so it is monodentate.
   • C2O4^2- (oxalate ion) has two oxygen atoms, one from each carboxylate group, that can each donate a lone pair to the metal simultaneously - this makes it bidentate, forming a five-membered chelate ring. …

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9. CBSE 2026Set ANNUAL1 markMCQ
   Q.The co-ordination number of Cr in K3[Cr(C2O4)3] is: (as printed; the paper's option values are +3/+4/+2/+6)

   (a) +3

   (b) +4

   (c) +2

   (d) +6

   ›Reveal solutionSolution

   As printed, the options ($+3,+4,+2,+6$) are oxidation-state values, so this question is really testing the oxidation number of Cr, which is $+3$. (For completeness: the true coordination number — the count of donor atoms bonded to Cr — is 6, not among the given options, since oxalate is bidentate and there are three oxalate ligands.)

   Oxidation number of Cr: In $K_3[Cr(C_2O_4)_3]$, potassium contributes $+1$ each (three K$^+$ ions), and each oxalate ion $C_2O_4^{2-}$ carries a charge of $-2$. Let the oxidation number of Cr be $x$:

   $3(+1) + x + 3(-2) = 0$

   $3 + x - 6 = 0 \implies x = +3$

   …

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10. CBSE 2026Set ANNUAL1 mark
    Q.What is the oxidation number of Fe in $[Fe(CN)_6]^{4-}$?

    ›Reveal solutionSolution

    Balancing the ligand charges against the overall complex-ion charge gives the oxidation state of iron.

    Working it out

    Let the oxidation number of Fe be $x$. Cyanide is an anionic ligand, $CN^-$, contributing $-1$ each; there are 6 of them:

    $x + 6(-1) = -4\ (\text{overall charge of the complex ion})$ …

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11. CBSE 2026Set ANNUAL1 mark
    Q.What is the coordination number of Fe in [Fe(EDTA)]⁻?

    ›Reveal solutionSolution

    EDTA is a hexadentate chelating ligand (2 N + 4 O donor atoms), so a single EDTA fills all six coordination sites — coordination number of Fe = 6.

    Concept. The coordination number of a metal ion is the number of ligand donor atoms directly bonded to it (the number of coordinate bonds), not the number of ligand molecules.

    …

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12. CBSE 2026Set ANNUAL1 markMCQ
    Q.Assertion (A): Glycinate ion H₂NCH₂COO⁻ is a chelating ligand. Reason (R): It can link through either nitrogen or anionic oxygen.

    (a) Both A and R are true and R is the correct explanation of A.

    (b) Both A and R are true but R is not the correct explanation of A.

    (c) A is true but R is false.

    (d) A is false but R is correct.

    ›Reveal solutionSolution

    A (glycinate is chelating) is true, but the reason given describes ambidentate behaviour (binding through one OR the other), not chelation (binding through both at once), so R is false — option (C).

    Assertion (A): The glycinate ion $H_2N-CH_2-COO^-$ is a chelating ligand. This is true: it is a bidentate ligand that binds a metal through the nitrogen of the $-NH_2$ group and the anionic oxygen of the $-COO^-$ group at the same time, forming a stable five-membered chelate ring.

    …

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