What is the Difference Between Primary and Secondary Valency

Primary and secondary valency are two important concepts in coordination chemistry that describe different aspects of how metal ions interact with ligands. Understanding both types of valencies helps to identify the structure and stability of coordination compounds.

What is the difference between primary and secondary valency? Primary valency deals with the metal’s ionic bond and oxidation state, whereas secondary valency focuses on the number and types of ligands attached.

Key Areas Covered

1. What is Primary Valency
      – Definition, Features 
2. What is Secondary Valency
      – Definition, Features
3. Similarities Between Primary and Secondary Valency
      – Outline of Common Features
4. Difference Between Primary and Secondary Valency
      – Comparison of Key Differences
5. FAQ: Primary and Secondary Valency
      – Answers to Frequently Asked Questions

Key Terms

Primary Valency, Secondary Valency

What is Primary Valency

Primary valency is the number of negative ions the central metal ion can attract. It mirrors its oxidation state. The metal ion needs a set number of negative charges to balance its own positive charge. For example, iron (Fe) in the +3 oxidation state (Fe³⁺) would exhibit a primary valency of 3, signifying its ability to attract three negative ions. Primary valency does not affect the spatial orientation of the surrounding atoms. It shows the ionic bonding capacity of metals.

Primary valency could be said to form the bridge (connection) between coordination complexes and ionic compounds. Primary value may not always provide a proper answer sometimes in complex scenarios. The size of the metal ion and surrounding ligands can influence the actual number of anions involved.

What is Secondary Valency

Secondary valency is the number of atoms or molecules (ligands) that are directly bonded to the central metal atom in a coordination compound. These ligands can be ions (positive or negative) or neutral molecules. Secondary valency is involved in the determination of the 3D geometry of the coordination complex. This is not a fixed number for a particular metal ion. This can vary according to the metal and the ligands involved.

Furthermore, coordination compound’s shape is determined by the secondary valency. For example, a metal with a coordination number of 6 will likely form an octahedral complex, while a coordination number of 4 will lead to the formation of a square planar geometry.

Consider the complex ion, [Cr(NH₃)₆]³⁺. Here, chromium (Cr) acts as the central metal ion. The six ammonia (NH₃) molecules are the ligands, each donating an electron pair to chromium. The coordination number is 6, signifying the secondary valency. This complex exhibits an octahedral geometry due to the six ligands arranged around the central chromium ion.

Similarities Between Primary and Secondary Valency

• Both describe the bonding behavior of a central metal ion.
• Moreover, both are important for understanding the structure of coordination compounds.

Difference Between Primary and Secondary Valency

Definition

• Primary valency is the number of negative ions the central metal ion can attract, while secondary valency is the number of atoms or molecules (ligands) that are directly bonded to the central metal atom in a coordination compound.

Represents

• Primary valency represents the oxidation state of the metal ion while secondary valency represents the coordination number of the central metal ion.

Nature

• Primary valency determines the number of ionic bonds formed with negative ions (anions) surrounding the metal. Meanwhile, secondary valency determines the total number of ligands attached to the metal ion (including both anions and neutral molecules).

Ligands

• Primary valency is not involved in the spatial arrangement of ligands, while secondary valency specifies the specific positions occupied by ligands around the metal ion.

Bonds

• In the primary valency concept, bonds can be separated in solution, while in the secondary valency concept, bonds do not easily dissociate in solution.

Charge

• The charge of primary valency is positive or zero, while the secondary valency does not have a charge.

Conclusion

In conclusion, primary valency deals with the metal’s ionic character, while secondary valency focuses on the coordination aspect and the complex’s structure. Thus, this is the basic difference between primary and secondary valency.

FAQ: Primary and Secondary Valency

1. What is the secondary valency of [Cr(NH₃)₆]³⁺?

The secondary valency of the complex is 6. It is the number of ligands participating in the formation of the coordination complex.

2. What is the primary and secondary valency of PT (NH3 )2Cl2?

PT (NH3 )2Cl2 does not have any primary valency and has 4 secondary valency. Secondary valency is the number of ligands participating in the formation of the complex.

3. What is the valency of magnesium?

The valency of magnesium is 2. This means that magnesium atoms tend to lose 2 electrons from their outermost shell to achieve a stable electron configuration.

Reference:

1. “Primary and Secondary Valency.” UnAcademy.

Image Courtesy:

1. “Potassium octachlorodimolybdate” By LittleWhole – Own work (CC BY-SA 4.0) via Commons Wikimedia