Problem 69 - General Chemistry: Principles and Modern Applications

Question

Explain the important distinction between each of the following pairs: (a) coordination number and oxidation number; (b) monodentate and polydentate ligands; (c) cis and trans isomers; (d) dextrorotatory and levorotatory compounds; (e) low-spin and highspin complexes.

Step-by-Step Solution

Answer

A: Coordination number refers to the total number of ligands attached to a central atom, oxidation number indicates the degree of oxidation or reduction of an atom. B: Monodentate ligands bind at a single point, polydentate ligands bind at multiple points. C: In cis isomers, functional groups are on the same side; in trans, on opposite sides. D: Dextrorotatory compounds rotate light clockwise, levorotatory counterclockwise. E: High spin complexes maximize total spin by occupying all orbitals singly first, low spin complexes minimize total spin by pairing electrons in lower energy orbitals first.

Step 1: Distinction A

Coordination number refers to the total number of ligands that are directly attached to a metal ion in a complex ion or a compound, while the oxidation number is a concept in chemistry that indicates the degree of oxidation or reduction of an atom in a chemical compound.

Step 2: Distinction B

Monodentate ligands are ligands that bind to a central atom at a single point, usually through one atom. Polydentate ligands, on the other hand, bind to a central atom at multiple points. They are often referred to as chelating ligands due to the way they 'hold' the central atom like a claw.

Step 3: Distinction C

Cis and trans isomers are terms used to describe the orientation of functional groups within a molecule. In cis isomers, functional groups are on the same side of the molecule. In trans isomers, functional groups are on opposite sides of the molecule.

Step 4: Distinction D

Dextrorotatory and levorotatory compounds refer to the direction in which these compounds rotate plane-polarized light. Dextrorotatory substances rotate light clockwise, and are denoted by a positive rotation angle. Levorotatory substances rotate light counter clockwise and are denoted by a negative rotation angle.

Step 5: Distinction E

Low-spin and high-spin complexes refer to the differences in electron configuration in transition metal complexes. A high-spin complex is one where the electrons prefer to occupy all orbitals singly before any pairing occurs, maximizing the total spin. A low-spin complex is one where electron pairing occurs in lower energy orbitals before the higher energy ones are occupied, minimizing total spin.

Key Concepts

Oxidation Number

The oxidation number is a fundamental concept in coordination chemistry, providing insight into the electron distribution of an atom within a chemical compound. It indicates the degree of oxidation or reduction, essentially showing how many electrons an atom either gains or loses during chemical bonding.

• An increase in oxidation number signifies oxidation, meaning the atom loses electrons.
• A decrease indicates reduction, where the atom gains electrons.

In coordination complexes, the oxidation number of the central metal atom is deduced from the charge of the complex and the known charges of the ligands attached. It's crucial in determining the chemical behavior and reactivity of the complex.

Ligands

Ligands are ions or molecules that can donate a pair of electrons to a central atom to form a coordination complex. Understanding how they bind is essential:

• Monodentate Ligands: These ligands possess a single atom that mounts a lone pair of electrons to the metal center. Examples include Cl⁻, H₂O, and NH₃.
• Polydentate Ligands: They have multiple atoms capable of binding to the metal. These are sometimes called chelating agents, as they 'claw' onto the metal using several attachments. Ethylenediamine (en) is a classic polydentate ligand.

The binding strength of polydentate ligands is usually greater due to the chelate effect, which stabilizes the complex more than monodentate ligands do.

Isomerism

Isomerism involves compounds with the same formula but different structures or arrangements. In coordination compounds, isomerism is pivotal:

• Cis Isomers: These occur when two identical ligands are next to each other, on the same side of the metal center.
• Trans Isomers: Here, identical ligands are situated opposite each other, providing a different spatial arrangement.

This distinction is crucial because the properties of cis and trans isomers can vary significantly, impacting the reactivity and interaction of complexes with biological systems and catalysts.

Optical Activity

Optical activity is a fascinating property of chiral molecules, which can rotate the plane of polarized light. This is a hallmark of stereochemistry in coordination chemistry:

• Dextrorotatory Compounds: These rotate plane-polarized light clockwise, denoted by the prefix 'd-' or '+'.
• Levorotary Compounds: In contrast, these rotate light counterclockwise, identified by 'l-' or '-'.

Understanding the optical activity of compounds is essential, especially in pharmaceuticals, where different enantiomers can have drastically different effects.

Spin States in Complexes

Spin states in coordination complexes are determined by the distribution of electrons among a metal's d-orbitals and affect the magnetic properties of the compound.

• High-Spin Complexes: These have unpaired electrons and usually form when weak-field ligands are present, causing maximum unpaired spins. The 3d electrons occupy all orbitals singly before any pairing occurs.
• Low-Spin Complexes: Occurring with strong-field ligands, these complexes have paired electrons in lower-energy orbitals. This minimizes unpaired spins and total spin.

The distinction between high-spin and low-spin affects the color, magnetism, and reactivity of the complexes, making it a fundamental concept in predicting and understanding their behavior.