Question

The formation of the complex ion \(\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) is described as a Lewis acid-base reaction. Explain. (a) What are the formulas of the Lewis acid and the Lewis base in this reaction? (b) What is the formula of the ligand? (c) What is the name of the species that provides the donor atom? (d) What atom is the donor atom, and why is it so designated? (e) What is the name of the species that is the acceptor?

Short answer

The Lewis acid is \(\mathrm{Cu}^{2+}\); the Lewis base is \(\mathrm{H}_{2}O\). The formula of the ligand is \(\mathrm{H}_{2}O\). The species providing the donor atom is water, with oxygen being the donor atom. The species that is the acceptor is the copper(II) ion, \(\mathrm{Cu}^{2+}\).

Step-by-step solution

Identify the Lewis acid and Lewis base

In the formation of the complex ion \(\left[\mathrm{Cu}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+}\), the Lewis acid is the species that accepts electron pairs, and the Lewis base is the species that donates electron pairs. Copper(II) ion, \(\mathrm{Cu}^{2+}\), is the electron pair acceptor, making it the Lewis acid. The water molecules, \(\mathrm{H}_{2}O\), are the electron pair donors, making them the Lewis bases.

Specify the formula of the ligand

A ligand is a molecule or ion that binds to a central metal atom to form a coordination complex. In this reaction, the ligand is water, \(\mathrm{H}_{2}O\).

Name the species providing the donor atom

The species that provides the donor atom is water (\(\mathrm{H}_{2}O\)). In the context of the complex ion, water is acting as a ligand that provides a donor atom to bond with the metal ion.

Identify the donor atom and its designation

The donor atom is the oxygen atom in the water molecule. It is designated as the donor atom because it has lone pairs of electrons that can be donated to the copper ion to form coordinate bonds.

Name the species that is the acceptor

The species that is the acceptor in this Lewis acid-base reaction is the copper(II) ion, \(\mathrm{Cu}^{2+}\). It is the acceptor because it has empty orbitals that can accept electron pairs from the water molecules.

Key concepts

Complex Ion Formation

Complex ion formation is a fundamental concept in coordination chemistry, where a central metal ion is surrounded by a series of molecules or ions known as ligands. These ligands can be neutral or charged species that possess at least one donor atom with a pair of electrons available for bonding. In the given exercise, the complex ion in question is \(\left[\mathrm{Cu}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+}\), where the central metal ion is copper(II), \(\mathrm{Cu}^{2+}\).

The process of forming a complex ion involves a Lewis acid-base reaction, where the Lewis acid (electron acceptor) is the metal ion, and the Lewis base (electron donor) consists of the ligands. When \(\mathrm{Cu}^{2+}\) encounters six water molecules, each water molecule donates a pair of electrons from its oxygen atom to the copper ion. This donation results in a coordinate bond, and the whole structure forms an octahedral shape, which is common for complex ions with six ligands. The formation of such complex ions has significant implications in various fields, including biochemistry and metallurgy.

In studying complex ion formation, it's essential to recognize the nature and role of both the metal ion and the ligands involved. Through this interaction, the metal ion typically achieves a more stable electronic configuration, and the ligands satisfy their tendency to pair up their electrons.

Coordinate Bonds

Coordinate bonds, also known as dative covalent bonds, are a type of chemical bond where both electrons in the bond originate from the same atom, typically the ligand in a complex ion. This is contrary to the traditional covalent bond, where each atom contributes one electron to the bond's formation. The distinguishing feature of a coordinate bond is that one atom, the donor, provides a lone (unshared) pair of electrons to an acceptor, typically a metal with empty orbitals.

In our exercise example, the \(\mathrm{Cu}^{2+}\) ion acts as the acceptor and the \(\mathrm{H}_{2}O\) molecules act as donors. Specifically, oxygen, being more electronegative, has a pair of unshared electrons that it can offer to form a bond with copper, which lacks electrons.

When the coordinate bond is formed, the electron density from the oxygen's lone pair is shared with the copper ion, resulting in a bond that holds the ligand and the metal ion together in the complex. Understanding the concept of coordinate bonding is key to grasping the overarching principles of molecular geometry, reactivity, and the characteristics of the metals and ligands involved.

Ligand Chemistry

Ligand chemistry centers around the molecules or ions that bind to central atoms or ions in a complex ion. A ligand must have at least one donor atom with a lone electron pair available for bond formation. In the context of our exercise, the water molecule \(\mathrm{H}_{2}O\) serves as the ligand, with oxygen being the donor atom.

Ligands are classified by their denticity, or the number of donor atoms they possess that can bond to the metal center. The water molecule is a monodentate ligand, meaning it has a single donor atom. There are also polydentate ligands, which have multiple bonding sites, and can form several bonds with a single metal ion, creating a more stable complex.

Moreover, ligands can dramatically influence the properties of the metal ion center, such as its color, magnetic properties, and solubility. Ligands like water are quite common, but others can include amines, halides, and sulfur-containing molecules, each bringing distinct characteristics to the resulting complex ion. The study of ligand chemistry involves not just the identification of the ligand and donor atoms, but also an understanding of how different ligands affect the behavior and stability of complex ions in a variety of chemical environments.