Question:

Sum of oxidation state (magnitude) and coordination number of cobalt in \(Na[Co(bpy)Cl_4]\) is____ .
(Given : bpy =Sum of oxidation state (magnitude) and coordination number of cobalt)

Updated On: Dec 29, 2025
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Correct Answer: 9

Approach Solution - 1

To determine the sum of the oxidation state (magnitude) and coordination number of cobalt in \( Na[Co(bpy)Cl_4] \), follow these steps:

  1. Identify the Ligands: In the complex \( Na[Co(bpy)Cl_4] \), the ligands are bipyridine (bpy) and chloride ions (Cl-).
  2. Oxidation State Calculation:
  • Sodium (Na) has an oxidation state of +1.
  • Bipyridine (bpy) is a neutral ligand, contributing 0 to the oxidation state.
  • Chloride ions (Cl-) have an oxidation state of -1. There are four chloride ions contributing a total of -4.
  • Let the oxidation state of cobalt (Co) be x. The sum of the oxidation states equals 0:

\[+1 + x + 0 + (-4) = 0\]

Simplifying, we get: \(x - 3 = 0\).

Therefore, \(x = +3\).

  1. Coordination Number: The coordination number of cobalt is determined by the number of ligand donor atoms bonded to it.
  • Each bipyridine ligand binds through two nitrogen atoms, contributing 2 to the coordination number.
  • Each chloride ion (Cl) contributes 1 to the coordination number. With four chloride ions, this adds up to 4.
  • Thus, the total coordination number is \(2 + 4 = 6\).
  1. Sum Calculation:

The sum of the oxidation state magnitude (3) and coordination number (6) is:

\[3 + 6 = 9\]

  1. Verification: The calculated value (9) falls within the provided range (9, 9).

Therefore, the answer is 9.

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Approach Solution -2

\(Na [Co(bpy)Cl_4]\)
Oxidation state of cobalt = \(+ 3\)
Coordination number of cobalt = \(6\)
[As bpy is bidentate]
So, sum = \(9\)
Therefore, the answer is \(9\).

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Concepts Used:

Werner’s Theory of Coordination Compounds

In 1893 Werner produced a theory to explain the structures, formation and nature of bonding in the coordination compounds. This theory is known as Werner’s theory of coordination compounds.

Postulates of Werner's Theory:

The important postulates as observed by Alfred Werner throughout his experiments are as follows:

  • The complex/ coordination compounds contain a central metal atom.
  • The metal atoms in a coordination compound generally show two types of valency: primary valency and secondary valency.
  • The primary valencies denote the oxidation state. They are ionizable and are satisfied by the negative ions.
  • Secondary valencies denote the coordination number. They are non-ionizable and are fixed for every metal atom. The secondary valency is generally satisfied by the neutral molecules or negative ions.
  • The metal atoms should satisfy both primary and secondary valencies.
  • The secondary valency of the atom basically shows the geometry/ polyhedra of the particular coordination compound.

Limitations of Werner’s Theory:

  • Though Werner explained some properties of the coordination compound,  he failed to explain the colour of the coordinate compound.
  • He could not explain the magnetic and optical properties of coordination compounds.
  • He could not answer the question, why does the coordination sphere have a definite geometry.