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Coordination Compounds - Werner's Theory of Coordination Compounds

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Alfred Werner proposed that in coordination compounds, central metal atoms exhibit two types of valencies: Primary Valency and Secondary Valency.

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Primary Valency: This is ionizable and corresponds to the oxidation state of the metal ion. It is satisfied by negative ions. For example, in [Co(NH3)6]Cl3[Co(NH_3)_6]Cl_3, the primary valency is 33.

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Secondary Valency: This is non-ionizable and corresponds to the Coordination Number (CNCN). It is satisfied by either neutral molecules or negative ions. In [Co(NH3)6]Cl3[Co(NH_3)_6]Cl_3, the secondary valency is 66.

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Coordination Sphere: The central metal ion and the ligands satisfying the secondary valency are enclosed in square brackets [...][...] and do not ionize in solution.

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Counter Ions: The ions present outside the square brackets satisfy the primary valency and are ionizable in aqueous solution.

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Geometry (Coordination Polyhedron): Secondary valencies are directed in fixed positions in space, which determines the stereochemistry of the complex (e.g., Octahedral for CN=6CN=6, Tetrahedral or Square Planar for CN=4CN=4).

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Experimental Verification: The number of ionizable chloride ions can be determined by precipitation with excess silver nitrate (AgNO3AgNO_3). For example, [Co(NH3)6]Cl3[Co(NH_3)_6]Cl_3 yields 33 moles of AgClAgCl.

📐Formulae

Total Valency=Primary Valency (Oxidation State)+Secondary Valency (Coordination Number)\text{Total Valency} = \text{Primary Valency (Oxidation State)} + \text{Secondary Valency (Coordination Number)}

Complex Representation: [M(L)n]Xm\text{Complex Representation: } [M(L)_n]X_m

Moles of AgCl precipitated=Number of ionizable Cl− ions outside the bracket\text{Moles of } AgCl \text{ precipitated} = \text{Number of ionizable } Cl^- \text{ ions outside the bracket}

Molar Conductivity∝Number of ions in solution\text{Molar Conductivity} \propto \text{Number of ions in solution}

💡Examples

Problem 1:

A coordination compound with the empirical formula PdCl2⋅4NH3PdCl_2 \cdot 4NH_3 precipitates 22 moles of AgClAgCl when treated with excess AgNO3AgNO_3. What is the structural formula and the secondary valency of the metal?

Solution:

Structural Formula: [Pd(NH3)4]Cl2[Pd(NH_3)_4]Cl_2. Secondary Valency: 44.

Explanation:

Since 22 moles of AgClAgCl are precipitated, there must be 22 chloride ions outside the coordination sphere. Thus, the 44 NH3NH_3 molecules must be inside the coordination sphere, making the coordination number (secondary valency) 44.

Problem 2:

Arrange the following complexes in increasing order of molar conductivity: (i) [Co(NH3)6]Cl3[Co(NH_3)_6]Cl_3, (ii) [Co(NH3)5Cl]Cl2[Co(NH_3)_5Cl]Cl_2, (iii) [Co(NH3)4Cl2]Cl[Co(NH_3)_4Cl_2]Cl.

Solution:

(iii) < (ii) < (i)

Explanation:

The number of ions produced in solution for (i) is 44 ([Co(NH3)6]3++3Cl−[Co(NH_3)_6]^{3+} + 3Cl^-), for (ii) is 33 ([Co(NH3)5Cl]2++2Cl−[Co(NH_3)_5Cl]^{2+} + 2Cl^-), and for (iii) is 22 ([Co(NH3)4Cl2]++Cl−[Co(NH_3)_4Cl_2]^+ + Cl^-). Higher the number of ions, higher the molar conductivity.

Problem 3:

In the complex PtCl4⋅2HClPtCl_4 \cdot 2HCl, no precipitate is formed with AgNO3AgNO_3. What is the coordination number of PtPt?

Solution:

The coordination number of PtPt is 66.

Explanation:

Since no AgClAgCl is precipitated, all Cl−Cl^- ions must be inside the coordination sphere. The formula is H2[PtCl6]H_2[PtCl_6]. Thus, the secondary valency (coordination number) is 66.

Werner's Theory of Coordination Compounds Revision - Class 12 Chemistry CBSE