Question

Discuss Sidgwick's concept of effective atomic number with the help of suitable example.

Short answer

The concept of Effective Atomic Number suggested by Sidgwick implies that the total number of electrons around the central metal ion (including the electrons provided by ligands) in a complex matches the total number of electrons of the nearest noble gas. This provides the complex's stability. This is demonstrated with the example of the complex ion \([Ni(CN)_4]^{2-}\), which has an EAN of 36, similar to the noble gas Krypton, indicating its stability.

Step-by-step solution

Understanding Sidgwick's Concept of Effective Atomic Number (EAN)

Effective Atomic Number (EAN) is a concept proposed by Nevil Vincent Sidgwick in 1921. According to this concept, the effective atomic number of the central atom in a complex equals the atomic number of the nearest noble gas atom. The EAN is obtained by summing the number of valence electrons of the central atom and the electrons donated by the ligands.

Formula for Effective Atomic Number (EAN)

The Effective Atomic Number (EAN) can be calculated using the following formula: \( EAN = (Number \ of \ Atomic \ Electrons) + (2 \times Number \ of \ Monovalent \ Ligands) + (Number \ of \ Electrons \ Donated \ by \ Polydentate \ Ligands) \)

Illustrating the Concept with a Suitable Example

Consider the \([Ni(CN)_4]^{2-}\) complex ion. The nickel(II) atom (Ni) has 28 electrons, CN- is a monovalent ligand and it donates 1 pair of electrons, hence 4 such CN ligands will contribute 4 pair (8) electrons. So the EAN of Ni in the complex could be calculated as follows: EAN = 28 (Ni electrons) + 2*8(CN- electrons) = 36 (which is the total number of electrons in Krypton, a noble gas). Hence, the complex is stable as it follows Sidgwick's EAN rule.