Problem 31 - Chemistry: The Central Science in SI Units, Global Edition

Question

For each of the following pairs, identify the molecule or ion that is more likely to act as a ligand in a metal complex: (a) carbonic acid \(\left(\mathrm{H}_{2} \mathrm{CO}_{3}\right)\) or carbonate \(\left(\mathrm{CO}_{3}^{2-}\right),(\mathbf{b})\) water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\) or hydronium ion \(\left(\mathrm{H}_{3} \mathrm{O}^{+}\right),(\mathbf{c})\) phosphine \(\left(\mathrm{PH}_{3}\right)\) or phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\).

Step-by-Step Solution

Answer

(a) Carbonate ion \(\left(\mathrm{CO}_{3}^{2-}\right)\) is more likely to act as a ligand. (b) Water \(\left(\mathrm{H}_{2}\mathrm{O}\right)\) is more likely to act as a ligand. (c) Phosphine \(\left(\mathrm{PH}_{3}\right)\) is more likely to act as a ligand.

Step 1: Analyze Pair (a) - Carbonic Acid and Carbonate Ion

In carbonic acid (H2CO3), there are two O-H bonds and one C=O bond. Both oxygen atoms are electron-rich with lone pairs, but they are involved in O-H bonding, which makes them less likely to act as ligands. For the carbonate ion (CO3^2-), the three oxygen atoms are doubly bonded to the carbon atom, and each one has lone pairs available for bonding. As an anionic species, the carbonate ion is also more likely to interact with metal cations to form metal complexes. Therefore, the carbonate ion (CO3^2-) is more likely to act as a ligand in a metal complex than carbonic acid (H2CO3).

Step 2: Analyze Pair (b) - Water and Hydronium Ion

In water (H2O), there are two O-H bonds, resulting in two lone pairs available on the oxygen atom, which can potentially bond to metal centers. In the hydronium ion (H3O^+), there are three O-H bonds, leaving no lone pairs available on the oxygen atom. Additionally, the positive charge on the hydronium ion makes it less likely to act as a ligand. Therefore, water (H2O) is more likely to act as a ligand in a metal complex than the hydronium ion (H3O^+).

Step 3: Analyze Pair (c) - Phosphine and Phosphoric Acid

In phosphine (PH3), there is a lone pair on the phosphorus atom, available for bonding to metal centers. In phosphoric acid (H3PO4), there are three O-H bonds and one P=O bond. The oxygen atoms are involved in O-H and P=O bonding, leaving no available lone pairs for interaction with metal centers. Moreover, potential steric hindrance around phosphorus introduces challenges for neighboring metal atoms to bond with it. Consequently, phosphine (PH3) is more likely to act as a ligand in a metal complex than phosphoric acid (H3PO4). To summarize the results: - Pair (a): The carbonate ion (CO3^2-) is more likely to act as a ligand in a metal complex. - Pair (b): Water (H2O) is more likely to act as a ligand in a metal complex. - Pair (c): Phosphine (PH3) is more likely to act as a ligand in a metal complex.

Key Concepts

Ligands

In coordination chemistry, ligands are vital players. They are atoms, ions, or molecules that donate electron pairs to a central metal atom or ion, forming a metal complex. Ligands can vary in size, charge, and type of atoms involved. They create coordinate bonds by providing their lone pairs of electrons to the metal. The ability of a ligand to donate electrons is due to the presence of these lone pairs.

A good example lies in the carbonate ion, which has multiple lone pairs available on its oxygen atoms, making it an excellent candidate for donating electrons. Ligands are responsible for the properties and reactivity of metal complexes. Their interactions with metal centers can significantly affect the overall geometry and electronic structure of the complex.

• Ligands donate lone electron pairs to form coordinate covalent bonds.
• They influence the geometry and reactivity of metal complexes.
• Ligands vary in size, charge, and donor atoms.

Metal Complexes

Metal complexes form when ligands bond to central metal atoms. The central metal ion or atom typically has vacant orbitals, which can accept electron pairs from ligands. This electron acceptance leads to the formation of a coordination bond, resulting in a stable structure. The nature of the metal complex depends on both the metal and its ligand.

For example, a carbonate ion bonding with metal results in a metal carbonate complex. Metal complexes are crucial in various chemical processes and industries, including catalysis and biological systems. Their formation can affect the color, solubility, and magnetism of the compounds.

• Formed by coordination of ligands to central metal atoms.
• Coordination bonds are stronger due to shared electron pairs.
• Affects compound properties and reactions.

Electron Pairs

The concept of electron pairs is foundational in the formation of ligands and metal complexes. Electron pairs can be classified into bonding pairs and lone pairs. Bonding pairs are shared between atoms, forming covalent bonds, whereas lone pairs are not involved in bonding and remain as non-shared electrons.

Lone pairs play a crucial role in forming coordination bonds. They are the "donors" which are provided by the ligand to the metal in a metal complex. An example is the lone pairs on the oxygen in water which enable it to function as a ligand. Understanding electron pairs is essential to predicting and explaining chemical behavior of complexes.

• Lone pairs are non-bonding electron pairs, vital for coordination.
• Bonding pairs form the basis of covalent bonds.
• Lone pairs allow ligands to donate electrons to metals.