Question

With reference to Valence Bond Theory (VBT), answer the following questions regarding the complex ion [Cr(NH₃)₆]³⁺.

1. What is the oxidation number of chromium in the complex?
2. State the magnetic behaviour of the complex.
3. How many unpaired electrons are there in the complex?
4. State the type of hybridisation of the central metal atom.

Answer 1

(1) The total charge on the complex ([Cr(NH₃)₆]³⁺) is +3. Since ammonia (NH₃) is a neutral ligand with a charge of 0, the equation for the oxidation state (x) of chromium is:

x + 6(0) = +3

x = +3

(2) Chromium (atomic number Z = 24) has a ground state configuration of [Ar]3d⁵ 4s¹. After losing three electrons to form Cr³⁺, its configuration becomes [Ar]3d³. The complex is paramagnetic in nature because it contains unpaired electrons.

(3) Chromium in the +3 oxidation state has the electronic configuration 3d³, hence there are three unpaired electrons in the complex.

(4) The central metal ion undergoes d²sp³ hybridisation, forming an octahedral inner orbital complex.