Coordination Number & Oxidation State of Cobalt in [Co(en)2Cl2]+

Concept Overview

This question tests your understanding of fundamental concepts in coordination chemistry, specifically how to determine the oxidation state of the central metal ion and its coordination number within a complex. You'll need to recognize the denticity of ligands and apply charge balance principles to solve for these properties. This is crucial for understanding the structure and reactivity of coordination compounds.

Step 1: Identify the ligands and their charges. The ligands in the complex $[Co(en)_2Cl_2]^+$ are ethylenediamine ($en$) and chloride ($Cl$). Ethylenediamine ($en$) is a neutral ligand, meaning it has a charge of 0. Chloride ($Cl$) is a halide ion, and in coordination complexes, it typically carries a charge of -1.

Step 2: Determine the oxidation state of the central metal ion (Cobalt). Let the oxidation state of cobalt ($Co$) be $x$. The overall charge of the complex is +1. We can set up an equation based on the sum of the oxidation states of the metal and the ligands, which must equal the overall charge of the complex. $x + 2 \times (\text{charge of } en) + 2 \times (\text{charge of } Cl) = +1$ Substituting the known charges: $x + 2 \times (0) + 2 \times (-1) = +1$ $x + 0 - 2 = +1$ $x = +1 + 2$ $x = +3$ Therefore, the oxidation state of cobalt in this complex is +3.

Step 3: Determine the coordination number of the central metal ion. The coordination number is the total number of donor atoms directly bonded to the central metal ion. Ethylenediamine ($en$) is a bidentate ligand, meaning each $en$ molecule can donate two donor atoms (the nitrogen atoms) to the metal ion. Chloride ($Cl$) is a monodentate ligand, meaning it donates one donor atom. In the complex $[Co(en)_2Cl_2]^+$, there are two ethylenediamine ligands and two chloride ligands. Number of donor atoms from $en$ ligands = $2 \times (\text{denticity of } en) = 2 \times 2 = 4$. Number of donor atoms from $Cl$ ligands = $2 \times (\text{denticity of } Cl) = 2 \times 1 = 2$. The total coordination number is the sum of the donor atoms from all ligands: Coordination Number = (Donor atoms from $en$) + (Donor atoms from $Cl$) Coordination Number = $4 + 2 = 6$. Thus, the coordination number of cobalt in this complex is 6.

Key Takeaways:

• The oxidation state of a metal ion in a complex is determined by balancing the charges of the ligands and the overall complex charge.
• The coordination number is the total count of donor atoms from all ligands attached to the central metal ion.
• Bidentate ligands like ethylenediamine ($en$) contribute two donor atoms each, while monodentate ligands like chloride ($Cl^-$) contribute one donor atom each.

Answer: Coordination number = 6, Oxidation state = +3