Problem 92 - Chemistry The Molecular Science

Question

Determine the coordination number of the central metal ion in (a) \(\left[\mathrm{Ni}(\mathrm{en}) \mathrm{Cl}_{2}\right]\) (b) \(\left[\mathrm{Mo}(\mathrm{CO})_{4} \mathrm{Br}_{2}\right]\) (c) \(\left[\mathrm{Cd}(\mathrm{CN})_{4}\right]^{2-}\). (d) \(\left[\mathrm{Co}(\mathrm{CN})_{5}(\mathrm{OH})\right]^{3-} .\)

Step-by-Step Solution

Answer

(a) 4, (b) 6, (c) 4, (d) 6.

Step 1: Understanding Coordination Number

The coordination number refers to the number of ligand atoms that are directly bonded to the central metal atom or ion in a complex.

Step 2: Analyzing (a) \([\mathrm{Ni}(\mathrm{en}) \mathrm{Cl}_{2}]\)

The complex contains "en," ethylenediamine, which is bidentate, meaning it binds through two atoms. Hence, one \(\mathrm{en}\) contributes 2 and two chloride ions contribute 2 to the coordination number. Coordination number: 2 (from \(\mathrm{en}\)) + 2 (from \(\mathrm{Cl}\)) = 4.

Step 3: Analyzing (b) \([\mathrm{Mo}(\mathrm{CO})_{4} \mathrm{Br}_{2}]\)

The ligands \(\mathrm{CO}\) and \(\mathrm{Br}\) are monodentate, each contributing 1 to the coordination number. Therefore, we have 4 \(\mathrm{CO}\) and 2 \(\mathrm{Br}\), totaling to a coordination number of 6.

Step 4: Analyzing (c) \([\mathrm{Cd}(\mathrm{CN})_{4}]^{2-}\)

The \(\mathrm{CN}^{-}\) is a monodentate ligand, meaning each ligand bonds through a single atom. With 4 \(\mathrm{CN}^{-}\) ligands, the coordination number is 4.

Step 5: Analyzing (d) \([\mathrm{Co}(\mathrm{CN})_{5}(\mathrm{OH})]^{3-}\)

Here, \(\mathrm{CN}\) is a monodentate ligand contributing 5, and \(\mathrm{OH}\) is another monodentate ligand contributing 1. The total coordination number is 5 + 1 = 6.

Key Concepts

Coordination Chemistry

Coordination chemistry is the study of compounds that feature metals at their core surrounded by atoms, ions, or molecules known as ligands. These metals are typically transition metals, which have the capacity to form numerous kinds of bonds with surrounding entities.

In coordination chemistry, the central metal atom or ion forms a complex by bonding with ligands through coordinate covalent bonds. Coordinate covalent bonds are special bonds where both electrons in the bonding pair come from the same atom, which is usually the ligand in coordination complexes. These complexes exhibit varied shapes and arrangements, providing fascinating insights into chemical bonding and molecular geometry.

The characteristics of these complexes, such as their reactivity and stability, are vastly influenced by the coordination number, which defines the number of ligands directly attached to the central metal atom or ion.

Metal Complexes

A metal complex is an assembly composed of a central metal atom or ion surrounded by molecules or ions called ligands. Ligands donate electron pairs to the metal, resulting in a stable entity.

Metal complexes can exhibit various geometries based on their coordination number and the spatial orientation of the ligands.

• Tetrahedral and square planar geometries are common for coordination numbers of four.
• Octahedral shapes are prevalent for coordination numbers of six.

These complexes have significant applications in various fields like catalysis, medicine, and bioinorganic chemistry due to their unique properties. For instance, the effectiveness of platinum-based drugs like cisplatin in cancer treatment stems from their ability to form stable complexes.

Ligands

Ligands are ions or neutral molecules that donate a pair of electrons to a central metal atom or ion to form a coordination complex.

Ligands can range from simple ions like chloride (Cl⁻) to complex organic molecules such as ethylenediamine (en).

• Monodentate ligands offer one pair of electrons to the metal atom, forming one bond.
• Bidentate ligands can attach to the central atom at two points simultaneously, providing two pairs of electrons.

In the complex \([\mathrm{Ni}(\mathrm{en}) \mathrm{Cl}_{2}]\), ethylenediamine acts as a bidentate ligand, effectively doubling its contribution to the coordination number compared to monodentate ligands.

Chemical Bonding

Chemical bonding in coordination compounds primarily involves coordinate covalent bonds between the central metal and ligands.

These bonds occur when both electrons in the bond are supplied by one atom, typically a ligand. Understanding these bonds is critical for grasping how coordination compounds achieve stability and particular geometric configurations. The nature of the donor atoms in the ligands can significantly affect the properties of the metal complex, influencing reactivity and color.

• The strength of these bonds can vary; for example, ligands like \(\mathrm{CN}^-\) create very stable bonds with metals due to their high electronegativity.

Additionally, through the concept of crystal field theory, we can predict and explain the splitting of a metal atom's orbitals in a complex, which informs us about the color and magnetic properties of the complex.