Understanding the Naming of Coordination Compounds

The name "dibromidobis(ethylenediamine)chromium(III) bromide" follows IUPAC rules for coordination compounds. Let's break it down step by step:

Step 1: Analyze the Name Components

• "dibromido" indicates 2 bromo (Br⁻) ligands.
• "bis(ethylenediamine)" means 2 ethylenediamine (en) ligands. "bis" is used because "en" is a bidentate ligand.
• "chromium(III)" shows the central metal ion is Cr³⁺.
• "bromide" at the end suggests a counter ion (Br⁻) outside the coordination sphere.

Step 2: Determine the Coordination Entity

The name implies the complex has:

• 2 Br⁻ ligands inside the coordination sphere.
• 2 en ligands (each en is bidentate, so it occupies 2 sites each).
• Cr³⁺ as the central metal.
• One Br⁻ outside as a counter ion to balance charge.

So, the coordination entity is [Cr(en)₂Br₂]⁺ and the counter ion is Br⁻, making the formula [Cr(en)₂Br₂]Br.

Step 3: Verify the Charge

Calculate the charge on the complex ion:

• Charge from Cr³⁺: +3
• Each en is neutral (charge 0).
• Each Br⁻ ligand contributes -1.

So, total charge = +3 + 2*(0) + 2*(-1) = +1. Thus, [Cr(en)₂Br₂]⁺ requires one Br⁻ counter ion to form a neutral compound.

Step 4: Match with Options

Compare with the given options:

• [Cr(en)Br₄]⁻: Has only one en and four Br⁻. Charge = +3 + 0 + 4*(-1) = -1. Doesn't match.
• [Cr(en)Br₂]⁻: Has one en and two Br⁻. Charge = +3 + 0 + 2*(-1) = +1, but only one en, not "bis".
• [Cr(en)₂Br₂]Br: Has two en and two Br⁻ inside, and one Br⁻ outside. Charge on complex = +3 + 0 + 2*(-1) = +1, balanced by Br⁻. Matches the name.
• [Cr(en)₃]Br₃: Has three en ligands. Charge on complex = +3 + 0 = +3, balanced by three Br⁻. Name would be tris(ethylenediamine)chromium(III) bromide.

Final Answer

The correct option is: [Cr(en)₂Br₂]Br

Related Topics

• IUPAC Nomenclature of Coordination Compounds
• Ligands and Denticity (e.g., monodentate, bidentate like ethylenediamine)
• Coordination Number and Geometry
• Charge Calculation in Complex Ions

Key Formulae and Theory

Charge Calculation: For a complex [ML_lX_x]^{n±}, the charge n is given by:

where $q_M$ is metal ion charge, $q_L$ and $q_X$ are ligand charges.

Naming Rules: Ligands are named in alphabetical order, followed by the metal name with oxidation state in Roman numerals. Prefixes like bis, tris are used for multidentate ligands.