Werner's Theory: Introduction, Postulates, Examples, Evidences and Limitations

Werner's Theory A major twist in the comprehension of these coordination compounds was postulated in the year 1893 by the Swiss chemist Alfred Werner. His theory forms the basis for modern coordination chemistry that postulates the existence of two different kinds of valencies for metal atoms namely, the primary and secondary valencies. The complex structure of metal ions and the surrounding molecules or ions in called ligands. Primary valences correspond to the metal's oxidation state and hence are ionizable, whereas secondary valences are nonionizable and only serve to determine the coordination number and sum for the geometry of the compound. Not only did it place Werner among the Nobel laureates of 1913, but it also provided the general principle upon which the behavior, structure, and reactivity of coordination compounds should be rationalized.

But, before Werner's theory at least, they did not have an appreciable understanding of the coordination compounds whose properties they wanted to explain with several models. The older model put forward by most was the valence bond theory, which seemed to emphasize the formation of covalent bonds between atoms, or at least it could not explain the direction and stability of the coordination compounds. The theory of a chain of Jörgensen-Blomstrand chains suggested metal atoms should be attached to ligands by atomic chains; however, this model had a very limited capability to give an explanation for the observed isomerism in coordination compounds.

Werner's Theory: A Primer

Werner's Theory: Werner's Theory about coordination compounds states the reaction that the metallic ions undergo with the ligands to form a complex structure. The theory basically aims at the dual representation of valency, which may be termed primary and secondary. The metal oxidation valency signifies the primary valency of the metal and is ionizable, which is satisfied by the negative ions. In cobalt(III) chloride, CoCl₃, the valency is +3 and is satisfied by three chloride ions. The secondary valency, on the other hand, is that metal coordination number by non-ionizable valency that is satisfied with neutral molecules or negative ions. In this regard, such distinction does allow for the spatial arrangement of the ligands around the central metal center to be different hence endowing a myriad of geometries, most common of which are octahedral, tetrahedral, or square planar.

The main postulates are:

• In coordination compounds, metals show two types of linkages (valences)-primary and secondary.
• The primary valences are normally ionizable and are satisfied by negative ions.
• The secondary valences are non-ionizable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal.
• The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers.

He further postulated that octahedral, tetrahedral, and square planar geometrical shapes are more common in coordination compounds of transition metals. Thus, $[\mathrm{Co}(\mathrm{NH}_3)_6]^{3+}, [\mathrm{CoCl}(\mathrm{NH}_3)_5]^{2+} and [\mathrm{CoCl}_2(\mathrm{NH}_3)_4]^{+}$are octahedral entities, while [Ni(CO)₄] and [PtCl₄]^2– are tetrahedral and square planar, respectively.

Aspects of Werner's Theory

Werner introduced the two kinds of classification for these functional groups that would only furnish coordination compounds at charge numbers much lower in the future. Typical examples of monodentate ligands at these low CN are ammonia or chloride ions that bound to the metal in a coordinate only once. Typical examples of polydentate ligands at these low CN are ethylenediamine, which binds to the metal in more than one site, forming the "chelate complex.". This affects the stability and reactivity of the coordination compounds that are going to be formed.

It has also touched on one of the central ideas in inorganic chemistry: the coordination number that comes out as the total number of ligand attachments to the metallic central ion. Commonly, coordination numbers include

4. For most cases, specific metals favor a range of geometries based on electronic configuration. For example, transition metals generally favor octahedral coordination because they can accommodate six ligands.

So, Werner's Theory helps elaborate the isomerism of the coordination compounds. Isomerism will happen only and only if the two or more compounds differ precisely in only the atomic arrangement but completely in the molecular formula. There can be structural isomerism, in which the arrangement of ligands around the metal, in a coordination compound, is different, and stereoisomerism, in which different spatial arrangements exist. Indeed, these prove important not only to the coordination compounds per se but also in other science fields such as drug designing where a special arrangement of the ligands can have an effect on their biological activity.

Operational Applications of Werner's Theory

Werner's Theory is not only confined to theoretical chemistry but in fact, it is applied in many real-life cases. Case in point, the design of catalysts for industrial applications. Since many times, the coordination compounds are quite efficient for many different reasons in functioning, therefore they are quite often used as catalysts. One such example includes the use of transition metal complexes as catalytic converts in motorcars for the efficient conversion of the transferred polluting gases into less toxic components.

Exterior to Werner's Theory in biochemistry, metal ions have crucial biological systems in terms of functioning enzymes. It is important to almost all the enzymes for the effective and proper operation of a co-factor made of metal. The coordination chemistry of these important metals serves a major role in catalytic activities. These metals are found as important proteins in the blood; for example, iron is a central metal and exists in coordination with various oxygen molecules in order for its efficient transportation.

In this way, the electronic structure is directly responsible for their color properties. In practical applications of the dye and material industry, vivid colors are produced by the use of coordination complexes as part of textile materials or coatings.

In medicine, coordination compounds are also in high usage; this could be because of their potential application as medicinal drugs. For instance, cisplatin is a coordination compound of platinum and is highly heralded in the treatment of cancer due to its interference with deoxyribonucleic acid, hence resulting in a halt in cell division.

Although the Werner theory represents high significance within the scientific world and the comprehension of coordination chemistry, the theory bears a good effect on several practical applications, from academic research to the real world.

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Some Solved Examples

Example 1

Question: The secondary valency and the number of hydrogen-bonded water molecule(s) in CuSO₄·5H₂O, respectively are:

1) 4 and 1
2) 5 and 1
3) 6 and 5
4) 6 and 4

Solution:

In the case of CuSO₄·5H₂O, the copper ion (Cu²⁺) exhibits a secondary valency of 4. This is because it can coordinate with four water molecules, which are neutral ligands. The number of hydrogen-bonded water molecules in this complex is 1. Therefore, the correct option is (1) 4 and 1.

Example 2

Question: On the treatment of 100 mL of 0.1 M solution of CoCl₃·6H₂O with excess AgNO₃; 1.2 × 10²² ions are precipitated. The complex is:

1) [Co(H₂O)₆]Cl₃
2) [Co(H₂O)₅Cl]Cl₂·H₂O
3) [Co(H₂O₄Cl₂]Cl·2H₂O
4) [Co(H₂O)₃Cl₃]·3H₂O

Solution:

To solve this, we first calculate the moles of the complex:
${Moles of complex} = \frac{{Molarity} \times {Volume (mL)}}{1000} = \frac{0.1 \times 100}{1000} = 0.01{ moles}$

Next, we calculate the moles of ions precipitated:
${Moles of ions precipitated} = \frac{1.2 \times 10^{22}}{6.02 \times 10^{23}} \approx 0.02 { moles}$

The number of chloride ions outside the ionization sphere can be calculated as:
${Number of Cl}^{-} = \frac{0.02}{0.01} = 2$

Thus, the complex is [Co(H₂O)₅Cl]Cl₂·H₂O, corresponding to option (2).

Example 3

Question: For the reaction given below:
${CoCl}_3 \cdot x{NH}_3 + {AgNO}_3({aq}) \rightarrow$

If two equivalents of AgCl precipitate out, then the value of x will be _________.

1) 5
2) 2
3) 3
4) 8

Solution:

Given that 1 equivalent of the complex precipitates 2 equivalents of AgCl, this indicates that the complex has two chloride ions outside of the coordination sphere. Therefore, the complex can be represented as:
${CoCl}_3 \cdot 5{NH}_3 = [{Co}({NH}_3)_5{Cl}] {Cl}_2$

Thus, the value of x is 5, making the correct answer (1).

Example 4

Question: The conductivity of a solution of complex with formula CoCl₃(NH₃)₄ corresponds to 1:1 electrolyte; then the primary valency of the central metal ion is________.

1) 1
2) 4
3) 8
4) 2

Solution:

Since the conductivity corresponds to a 1:1 electrolyte, the complex can be represented as $[{CoCl}_2({NH}_3)_4]^+{Cl}^-$. The primary valency is ionizable and satisfied by negative ions. Therefore, the primary valency will be 1 because there is only one positive charge present. Hence, the answer is option (1).

Example 5

Question: A solution contains 2.675 g of a complex (molar mass = 267.5 g mol⁻¹) passed through a cation exchanger. The chloride ions obtained in the solution were treated with an excess of AgNO₃ to give 4.78 g of AgCl (molar mass = 143.5 g mol⁻¹). The formula of the complex is:

1) [CoCl(NH₃)₅]Cl₂
2) [Co(NH₃)₆]Cl₃
3) [CoCl₂(NH₃)₄]Cl
4) [CoCl₃(NH₃)₃]

Solution:

First, we calculate the moles of the complex:

${Moles of complex} = \frac{2.675}{267.5} \approx 0.01{ moles}$

Next, we calculate the moles of AgCl formed:

${Moles of AgCl} = \frac{4.78}{143.5} \approx 0.0333{ moles}$

Since some Cl⁻ ions are outside of the complex sphere, and they react with AgNO₃ to form AgCl, we find that for every mole of complex, 3 moles of Cl⁻ are ionizable. Therefore, the formula of the compound is [Co(NH₃)_6]Cl₃, which corresponds to option (2).

Summary

In relation to coordination compounds, Werner's theory provides the most systematic framework for several aspects that refer to bonding and structures in general. It explains a lot about geometries and arrangements of metal ions and their interaction with ligands on the basis of primary valencies and coordination numbers. Other important aspects that help in experiencing coordination chemistry are the classification of ligands, coordination number, and isomerism. Proper assistance of this theory is shown by practical examples, ranging from catalysis and biochemistry to materials science.

Before this, coordination compounds were a kind of riddle, in particular, that different theoretical models had been applied to throw light on it. The old valence bond theory and the Jörgensen-Blomstrand chain theory both failed to account for the directionality that was observed in compounds' forming and stability or that of isomerism. Werner's Theory refurbished the subject by the device of a scheme of primary and secondary valencies for an organized study of the behavior, structure, and reactivity of these complicated compounds.

Such an investigation of Werner's Theory, one appreciates even more how the theoretical conception can be transformed into a tool in the service of the guide to the development of science and technology in general. It has enabled the understanding of the nature of coordination compounds and therefore yielded enormous applications both in industrial and medical nicety. As much as we are moving deeper into the heart of coordination chemistry, so much of the Werner Theory remains on our feet upon which further research and innovation are elicited.