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Coordination Compounds - Werner's theory

Grade 12ICSEChemistry

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: Corresponds to the oxidation state of the metal. It is ionizable, non-directional, and satisfied only by negative ions (Cl−Cl^-, NO3−NO_3^-, etc.). In structural diagrams, it is represented by dotted lines (......).

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Secondary Valency: Corresponds to the coordination number (CNCN) of the metal. It is non-ionizable and directional in space, determining the geometry of the complex. It is satisfied by ligands (neutral molecules like NH3NH_3, H2OH_2O or negative ions like Cl−Cl^-). In structural diagrams, it is represented by solid lines (—).

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Every metal atom has a fixed number of secondary valencies. For example, Co3+Co^{3+} and Pt4+Pt^{4+} usually have a coordination number of 66.

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The species present within the square brackets [M(L)n][M(L)_n] are non-ionizable and constitute the coordination sphere, while those outside are ionizable and satisfy the primary valency.

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The geometry of the complex is governed by the spatial arrangement of secondary valencies: CN=6CN=6 results in an Octahedral geometry, while CN=4CN=4 results in Tetrahedral or Square Planar geometries.

📐Formulae

Total number of ions=Ions from coordination sphere+Ionizable counter ions\text{Total number of ions} = \text{Ions from coordination sphere} + \text{Ionizable counter ions}

[Co(NH3)6]Cl3→H2O[Co(NH3)6]3++3Cl− (4 ions total)[Co(NH_3)_6]Cl_3 \xrightarrow{H_2O} [Co(NH_3)_6]^{3+} + 3Cl^- \text{ (4 ions total)}

[Co(NH3)5Cl]Cl2→H2O[Co(NH3)5Cl]2++2Cl− (3 ions total)[Co(NH_3)_5Cl]Cl_2 \xrightarrow{H_2O} [Co(NH_3)_5Cl]^{2+} + 2Cl^- \text{ (3 ions total)}

[Co(NH3)4Cl2]Cl→H2O[Co(NH3)4Cl2]++Cl− (2 ions total)[Co(NH_3)_4Cl_2]Cl \xrightarrow{H_2O} [Co(NH_3)_4Cl_2]^{+} + Cl^- \text{ (2 ions total)}

💡Examples

Problem 1:

A coordination compound has the empirical formula CoCl3⋅4NH3CoCl_3 \cdot 4NH_3. When 1 mole of this compound is treated with excess AgNO3AgNO_3 solution, 1 mole of AgClAgCl is precipitated. Formulate the complex according to Werner's theory and identify the primary and secondary valencies of Cobalt.

Solution:

The formula of the complex is [Co(NH3)4Cl2]Cl[Co(NH_3)_4Cl_2]Cl. The primary valency (oxidation state) of CoCo is +3+3 and the secondary valency (coordination number) is 66.

Explanation:

Since 1 mole of AgClAgCl is precipitated, only 1 Cl−Cl^- ion must be outside the coordination sphere (ionizable). To maintain a coordination number of 66 for Co3+Co^{3+}, 4 NH3NH_3 molecules and 2 Cl−Cl^- ions must be inside the coordination sphere as ligands. Thus, the structure is [Co(NH3)4Cl2]Cl[Co(NH_3)_4Cl_2]Cl.

Problem 2:

Compare the molar conductivity of 1.0M1.0 M solutions of PdCl2⋅4NH3PdCl_2 \cdot 4NH_3 and PtCl4⋅2NH3PtCl_4 \cdot 2NH_3.

Solution:

PdCl2⋅4NH3PdCl_2 \cdot 4NH_3 has higher molar conductivity because it dissociates into 3 ions: [Pd(NH3)4]Cl2→[Pd(NH3)4]2++2Cl−[Pd(NH_3)_4]Cl_2 \rightarrow [Pd(NH_3)_4]^{2+} + 2Cl^-. PtCl4⋅2NH3PtCl_4 \cdot 2NH_3 exists as [Pt(NH3)2Cl4][Pt(NH_3)_2Cl_4], which is a non-electrolyte (0 ions).

Explanation:

According to Werner's theory, molar conductivity depends on the number of ions produced in solution. The complex [Pd(NH3)4]Cl2[Pd(NH_3)_4]Cl_2 releases two chloride ions, whereas [Pt(NH3)2Cl4][Pt(NH_3)_2Cl_4] does not ionize as all chloride ions satisfy the secondary valency.