Problem 20 - Chemistry Central Science

Question

Indicate the likely coordination number of the metal in each of the following complexes: (a) \(\left[\mathrm{Rh}(\mathrm{bipy})_{3}\right]\left(\mathrm{NO}_{3}\right)_{3}\) (b) \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]\) (c) \(\left[\mathrm{Cr}(\mathrm{o} \text { -phen })_{3}\right]\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{3}\) (d) \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA}) \mathrm{Br}]\)

Step-by-Step Solution

Answer

The coordination numbers for the metal ions in the given complexes are: (a) Rhodium: 6 (b) Cobalt: 6 (c) Chromium: 6 (d) Cobalt: 7

Step 1: Identify the central metal atom and ligands

In this complex, Rhodium (Rh) is the central metal atom. The bipyridine (bipy) ligands are coordinated to Rhodium, while the nitrate (NO3) groups are counterions and not directly bonded to Rh.

Step 2: Count the ligands bonded to the central metal atom

There are 3 bipyridine (bipy) ligands attached to the central Rhodium atom. Each bipy ligand acts as a bidentate ligand, meaning it can form two bonds with the central metal atom.

Step 3: Calculate the coordination number

Since each bipy ligand can form two bonds with Rhodium and there are 3 bipy ligands, the coordination number of Rhodium is 6. #b) Determine the coordination number of Cobalt in \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]\)

Step 4: Identify the central metal atom and ligands

In this complex, Cobalt (Co) is the central metal atom. The oxalate (C2O4) and chloride (Cl) ligands are coordinated to Cobalt. Sodium ions (Na) are present as counterions and not directly bonded to Co.

Step 5: Count the ligands bonded to the central metal atom

There are 2 oxalate (C2O4) and 2 chloride (Cl) ligands attached to the central Cobalt atom. Each oxalate ligand acts as a bidentate ligand, which can form two bonds with the central metal atom.

Step 6: Calculate the coordination number

Since each oxalate ligand can form two bonds with Cobalt and there are 2 oxalate ligands, along with 2 chloride ligands, the coordination number of Cobalt is 6. #c) Determine the coordination number of Chromium in \(\left[\mathrm{Cr}(\mathrm{o} \text { -phen })_{3}\right]\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{3}\)

Step 7: Identify the central metal atom and ligands

In this complex, Chromium (Cr) is the central metal atom. The ortho-phenanthroline (o-phen) ligands are coordinated to Chromium, while the acetate (CH3COO) groups are counterions and not directly bonded to Cr.

Step 8: Count the ligands bonded to the central metal atom

There are 3 ortho-phenanthroline (o-phen) ligands attached to the central Chromium atom. Each o-phen ligand acts as a bidentate ligand, meaning it can form two bonds with the central metal atom.

Step 9: Calculate the coordination number

Since each o-phen ligand can form two bonds with Chromium and there are 3 o-phen ligands, the coordination number of Chromium is 6. #d) Determine the coordination number of Cobalt in \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA}) \mathrm{Br}]\)

Step 10: Identify the central metal atom and ligands

In this complex, Cobalt (Co) is the central metal atom. The EDTA (ethylenediaminetetraacetic acid) and bromide (Br) ligands are coordinated to Cobalt. Sodium ions (Na) are present as counterions and not directly bonded to Co.

Step 11: Count the ligands bonded to the central metal atom

There is 1 EDTA ligand and 1 bromide (Br) ligand attached to the central Cobalt atom. The EDTA ligand acts as a hexadentate ligand, which can form six bonds with the central metal atom.

Step 12: Calculate the coordination number

Since the EDTA ligand can form six bonds with Cobalt and there is 1 bromide ligand, the coordination number of Cobalt is 7.

Key Concepts

Coordination Number

In coordination chemistry, the coordination number is a crucial concept. It represents the number of ligand donor atoms directly bonded to the central metal ion in a complex.
This number can affect the shape and properties of the metal complex it forms.
For example, in a complex with a coordination number of six, the geometry might be octahedral.

• The coordination number is not always equal to the number of ligands because some ligands can bind through multiple donor atoms.
• Hence, bidentate and polydentate ligands significantly influence this count.
• By analyzing the coordination number, we can infer the type and structure of the metal complex.

Understanding coordination numbers helps in predicting reactivity and stability of metal complexes.

Ligands

Ligands are molecules or ions that attach to a metal atom to form a coordination complex.
They donate electrons to the metal, establishing a coordinate bond.

• Ligands can be classified based on their size and the number of bonding sites.
  • Monodentate ligands has one donor atom to bind with the metal.
  • Bidentate ligands possess two donor atoms, allowing them to form two bonds.
  • Multidentate ligands, like hexadentate, have multiple bonding sites.
• They influence the properties and stability of the metal complex they form.

Ligands determine the geometry and driving force behind forming metal complexes.

Metal Complexes

A metal complex consists of a core metal atom or ion bonded to groups of atoms known as ligands.
These complexes can take on various geometric shapes, primarily dictated by the coordination number of the metal center.

• Metal complexes can be charged or neutral, depending on the ligands and counterions bonded to the metal.
• They play vital roles in fields like biochemistry, for instance, in hemoglobin and chlorophyll structures.
• The nature of the metal and ligands impacts the chemical reactivity of the complex.

Understanding metal complexes aids in grasping the fundamentals of transition metal chemistry.

Bidentate Ligands

Bidentate ligands are special types of ligands that can form two bonds to a central metal atom or ion.
This occurs because they possess two donor atoms, which can simultaneously attach to a metal.

• Common examples include ethylenediamine and oxalate.
• Bidentate ligands increase the stability of complexes due to the chelate effect, where multiple bonds make the complex more secure.
• They play a significant role in determining the coordination number and geometric configuration of the complex.

Bidentate ligands are crucial in creating more stable and robust metal complexes.

Hexadentate Ligands

Hexadentate ligands possess six donor sites, enabling them to form six bonds with a central metal atom. This makes them incredibly powerful in stabilizing metal complexes.

• EDTA (ethylenediaminetetraacetic acid) is a prime example of a hexadentate ligand.
• The multiple bonding sites allow hexadentate ligands to fully encompass metal ions, effectively shielding them from surrounding environments.
• The extensive bonding enhances the chelating effect, rendering the complex exceedingly stable.

Hexadentate ligands are essential in applications like water treatment, where they capture metal ions effectively.