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Chapter 23: Problem 31

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 \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\).

Short Answer

Expert verified
(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 by step solution

01

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).
02

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^+).
03

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.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

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.

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Most popular questions from this chapter

Indicate the coordination number and the oxidation number of the metal for each of the following complexes: (a) \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA})]\) (b) \(\mathrm{KMnO}_{4}\) (c) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{2}\) (d) \(\mathrm{K}_{3} \mathrm{Fe}(\mathrm{CN})_{6}\) (e) \(\mathrm{Rh}\left(\mathrm{PPh}_{3}\right)_{3} \mathrm{Cl}\) (f) \(\mathrm{Zn}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\left(\mathrm{NH}_{3}\right)_{2}\)

Determine if each of the following complexes exhibits geometric isomerism. If geometric isomers exist, determine how many there are. (a) tetrahedral \(\left[\mathrm{Cd}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2} \mathrm{Cl}_{2}\right],(\mathbf{b})\) square-pla- \(\operatorname{nar}\left[\operatorname{IrCl}_{2}\left(\mathrm{PH}_{3}\right)_{2}\right]^{-},(\mathbf{c})\) octahedral \(\left[\mathrm{Fe}(o \text { -phen })_{2} \mathrm{Cl}_{2}\right]^{+} .\)

The lobes of which \(d\) orbitals point directly between the ligands in (a) octahedral geometry, (b) tetrahedral geometry?

Consider the tetrahedral anions \(\mathrm{VO}_{4}^{3-}\) (orthovanadate ion), \(\mathrm{CrO}_{4}^{2-}\) (chromate ion), and \(\mathrm{MnO}_{4}^{-}\) (permanganate ion). (a) These anions are isoelectronic. What does this statement mean? (b) Would you expect these anions to exhibit d-d transitions? Explain. (c) As mentioned in "A Closer Look" on charge-transfer color, the violet color of \(\mathrm{MnO}_{4}\) is due to a ligand-to-metal charge transfer (LMCT) transition. What is meant by this term? (d) The LMCT transition in \(\mathrm{MnO}_{4}^{-}\) occurs at a wavelength of \(565 \mathrm{nm}\). The \(\mathrm{CrO}_{4}^{2-}\) ion is yellow. Is the wavelength of the LMCT transition for chromate larger or smaller than that for \(\mathrm{MnO}_{4}^{-}\) ? Explain. (e) The \(\mathrm{VO}_{4}^{3-}\) ion is colorless. Do you expect the light absorbed by the LMCT to fall in the UV or the IR region of the electromagnetic spectrum? Explain your reasoning.

Metallic elements are essential components of many important enzymes operating within our bodies. Carbonic anhydrase, which contains \(\mathrm{Zn}^{2+}\) in its active site, is responsible for rapidly interconverting dissolved \(\mathrm{CO}_{2}\) and bicarbonate ion, \(\mathrm{HCO}_{3}^{-}\). The zinc in carbonic anhydrase is tetrahedrally coordinated by three neutral nitrogencontaining groups and a water molecule. The coordinated water molecule has a \(\mathrm{p} K_{a}\) of \(7.5,\) which is crucial for the enzyme's activity. (a) Draw the active site geometry for the \(\mathrm{Zn}(\mathrm{II})\) center in carbonic anhydrase, just writing "N" for the three neutral nitrogen ligands from the protein. (b) Compare the \(\mathrm{p} K_{a}\) of carbonic anhydrase's active site with that of pure water; which species is more acidic? (c) When the coordinated water to the \(\mathrm{Zn}(\mathrm{II})\) center in carbonic anhydrase is deprotonated, what ligands are bound to the \(\mathrm{Zn}(\mathrm{II})\) center? Assume the three nitrogen ligands are unaffected. \((\mathbf{d})\) The \(\mathrm{p} K_{a}\) of \(\left[\mathrm{Zn}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) is \(10 .\) Suggest an explanation for the difference between this \(\mathrm{p} K_{a}\) and that of carbonic anhydrase. (e) Would you expect carbonic anhydrase to have a deep color, like hemoglobin and other metal-ion-containing proteins do? Explain.
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