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Magazine Chapter 20: Problem 51
Write a structural formula for the coordination compound \(\left[\mathrm{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{I}_{2}\right],\) and give the coordination number for the central \(\mathrm{Cr}^{2+}\) ion.
Short Answer
Expert verified
The coordination number of \( \mathrm{Cr}^{2+} \) in the compound is 6.
Step by step solution
01
Understand the components
The complex given is \( \left[ \mathrm{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{I}_{2}\right] \). Here, \( \mathrm{Cr}^{2+} \) is the central metal ion, \( \mathrm{en} \) stands for ethylenediamine, a bidentate ligand, \( \mathrm{NH}_{3} \) is ammonia, and \( \mathrm{I} \) is iodide. The coordination compound consists of ligands \( \mathrm{en} \), \( \mathrm{NH}_{3} \), and \( \mathrm{I}^{-} \) bonding with the central metal ion \( \mathrm{Cr}^{2+} \).
02
Assign the coordination number
The coordination number of a metal in a complex is the total number of ligand attachment points. Ethylenediamine (\( \mathrm{en} \)) is a bidentate ligand, meaning it forms two bonds, contributing 2 coordination points. \( \mathrm{NH}_{3} \) is a monodentate ligand with each \( \mathrm{NH}_{3} \) contributing one coordination point, totaling 2 from \( (\mathrm{NH}_{3})_{2} \). \( \mathrm{I}^{-} \) is also a monodentate ligand, contributing 1 coordination point per atom, totaling 2 from \( \mathrm{I}_{2} \). Therefore, the coordination number is the sum: \( 2 + 2 + 2 = 6 \).
03
Draw the structural formula
In the structural formula, the central \( \mathrm{Cr}^{2+} \) is connected to each of its ligands based on the determined coordination points. Start by connecting one bidentate \( \mathrm{en} \) (which connects at two sites). Then attach two \( \mathrm{NH}_{3} \) molecules (one connection each), and finally, connect two \( \mathrm{I}^{-} \) ions (one connection each). This ensures the correct coordination number (6) and the structural integrity of the complex.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Understanding the Coordination Number
In coordination chemistry, the coordination number is a crucial concept as it defines the number of ligand attachment points around a metal ion. For instance, in the compound \( \left[ \mathrm{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{I}_{2}\right] \), we have Chromium \( \mathrm{Cr}^{2+} \) as the central metal ion. The coordination number here is calculated by counting the total attachment points provided by all ligands surrounding the metal ion.
- Ethylenediamine (\( \mathrm{en} \)) is a bidentate ligand, meaning it can bond at two points, contributing 2 coordination points.
- Ammonia (\( \mathrm{NH}_{3} \)) is a monodentate ligand, providing 1 point each, summed up to 2 points for two \( \mathrm{NH}_{3} \) molecules.
- Iodide (\( \mathrm{I}^{-} \)) is also monodentate, with one iodide contributing 1 point, totaling to 2 points for two \( \mathrm{I}^{-} \).
Exploring Bidentate Ligands
Bidentate ligands are fascinating because they form two bonds with a central metal atom or ion, contributing significantly to the stability and geometry of coordination complexes.
- Ethylenediamine (\( \mathrm{en} \)) is a classic example of a bidentate ligand. It possesses two donor atoms which can each form a coordinate bond with the metal center.
- This dual bonding ability allows bidentate ligands to "chelate" the metal, forming a more stable ring-like structure due to the chelate effect. This enhances the stability of the coordination compound compared to those with monodentate ligands.
Structural Formula of Coordination Compounds
Creating a structural formula in coordination chemistry involves illustrating the connections between the central metal ion and its surrounding ligands to reflect physical arrangements and bonding.
For the compound \( \left[ \mathrm{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{I}_{2}\right] \), the central \( \mathrm{Cr}^{2+} \) ion is encircled by:
For the compound \( \left[ \mathrm{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{I}_{2}\right] \), the central \( \mathrm{Cr}^{2+} \) ion is encircled by:
- A bidentate ligand \( \mathrm{en} \), connected at two sites, forming a (nearly) closed loop with \( \mathrm{Cr}^{2+} \).
- Two \( \mathrm{NH}_{3} \) molecules, each connecting at one point, represented by simple lines radiating from the chromium ion.
- Two \( \mathrm{I}^{-} \) ions, each making a single connection line to the metal center.
Decoding Metal-Ligand Bonding
Metal-ligand bonding is at the heart of coordination chemistry, and it accounts for the way in which ligands attach to a central metal ion, such as \( \mathrm{Cr}^{2+} \) in our example compound. These bonds form through a mechanism known as coordinate or dative bonding.
- In a coordinate bond, the ligand donates a pair of electrons to the metal ion which can accept those electrons due to its nature.
- Bidentate ligands like ethylenediamine can use two pairs of lone electrons to form coordinate bonds, effectively surrounding the metal ion and stabilizing the complex.
- Monodentate ligands such as \( \mathrm{NH}_{3} \) or \( \mathrm{I}^{-} \) involve a single pair of electrons to form a bond with the metal center.
