Question

The following question is a case-based question. Read the case carefully and answer the questions that follow:

The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. Similarly the charge on the complex is the sum of the charges of the constituent parts, i.e., the sum of the charges on the central metal ion and its surrounding ligands. Based on this, the complex is called neutral if the sum of the charges of the constituents is equal to zero. However, for an anion or cationic complex, the sum of the charges of the constituents is equal to the charge on the coordination sphere.

Based on the above information, answer the following questions:

(a) What is the secondary valence of \[\ce{Co}\] in \[\ce{[Co(NH3)4Cl2]+}\]?  (1)

(b) What type of isomerism is shown by the complex \[\ce{[Cr(H2O)6]Cl3}\] and \[\ce{[Cr(H2O)5Cl]Cl2 . H2O}\]?  (1)

(c) Write the electronic configuration of d⁴ ion on the basis of crystal field theory when

1. Δ₀ < P
2. Δ₀ > P  (2)

                                            OR

(c) Find the oxidation state and coordination number of the central metal ion in \[\ce{[Co(H2O)(CN)(en)2]^2+]}\].  (2)

Answer 1

(a) The secondary valency indicates the amount of ligands directly linked to the core metal ion. The secondary valency of \[\ce{Co}\] in \[\ce{[Co(NH3)4Cl2]+}\] is 6, indicating it forms six bonds with the surrounding ligands, which include four ammonia \[\ce{(NH3)}\] and two chloride ions \[\ce{(Cl^-)}\].

(b) Resolve isomerism. Chloride ions replace water in these compounds. Therefore, two compounds with the same chemical formula can be distinguished by the presence of free solvent molecules or water or solvent molecules that are directly linked to metal ions.

(c)

1. If Δ₀ < P, the electrons will occupy the higher energy orbitals first. The electron configuration becomes t₂g³eg¹.
2. If Δ₀ > P, the electrons will occupy the lower energy t₂g orbitals first. The electron configuration becomes t₂g⁴eg⁰.

                                                OR

(c) The coordination number of cobalt is 6, and the oxidation state of cobalt \[\ce{(Co)}\] in the complex is +3.