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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 \(\left(\mathrm{H}_{3} \mathrm{O}^{+}\right)\) (c) phosphine \(\left(\mathrm{PH}_{3}\right)\) or phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\)

Short Answer

Expert verified
(a) Carbonate, (b) Water, (c) Phosphine.

Step by step solution

01

Define Ligand Characteristics

A ligand is a molecule or ion that donates at least one pair of electrons to a metal ion to form a coordination complex. Ligands are usually Lewis bases, meaning they provide an electron pair for bonding.
02

Compare Pair (a) - Carbonic Acid vs. Carbonate

Carbonate ion \ \(\left(\mathrm{CO}_{3}^{2-}\right)\ \) has a -2 charge, meaning it contains extra electrons and can easily donate electrons, making it a good ligand. Carbonic acid \ \(\left(\mathrm{H}_{2}\mathrm{CO}_{3}\right)\ \) is a neutral molecule and less likely to donate electrons. Therefore, carbonate is more likely to act as a ligand.
03

Compare Pair (b) - Water vs. Hydronium Ion

Water \ \(\left(\mathrm{H}_{2}\mathrm{O}\right)\ \) is neutral but has two lone pairs of electrons on the oxygen, making it an effective ligand. In contrast, the hydronium ion \ \(\left(\mathrm{H}_{3}\mathrm{O}^{+}\right)\ \) is positively charged and less likely to donate electrons. Hence, water is more likely to act as a ligand.
04

Compare Pair (c) - Phosphine vs. Phosphoric Acid

Phosphine \ \(\left(\mathrm{PH}_{3}\right)\ \) is a neutral molecule with a lone pair on the phosphorus atom, readily available for donation, making it a potential ligand. Phosphoric acid \ \(\left(\mathrm{H}_{3}\mathrm{PO}_{4}\right)\ \) donates protons and is less likely to donate electrons. Thus, phosphine is more likely to act as a ligand.

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Key Concepts

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

Lewis Bases
In the realm of chemistry, Lewis bases are substances that can donate a pair of electrons. This fundamental property allows them to form bonds with Lewis acids, which are typically electron-pair acceptors. The ability to donate an electron pair makes Lewis bases crucial components in many chemical interactions, including the formation of coordination complexes.

When analyzing potential ligands in metal complexes, understanding the Lewis base property becomes essential. Ligands function as Lewis bases, donating their electron pairs to metal ions. This electron donation is key to stabilizing the metal in a coordination complex. To determine which substance is more likely to act as a ligand, look for its willingness to donate electrons. Generally, molecules or ions with lone pairs of electrons stand out as excellent candidates. Under this lens, you can see how negatively charged ions, which possess extra electrons, tend to be strong Lewis bases and effective ligands.
Coordination Complex
A coordination complex is an entity consisting of a central metal atom or ion bonded to surrounding molecules or ions called ligands. These ligands attach to the metal through coordinate covalent bonds, where both electrons in the bond originate from the ligand. This structure is pivotal in fields such as inorganic chemistry and biochemistry.

The central metal ion, often a transition metal, acts as a Lewis acid by accepting electron pairs. On the other hand, the surrounding ligands, acting as Lewis bases, donate these electron pairs, creating the intricate structure of a coordination complex. These complexes showcase unique properties, including variations in color, reactivity, and magnetic properties, depending on the metal center and the type of ligands attached. Moreover, the geometric arrangement of the ligands around the metal can greatly influence the characteristics and functionality of the complex.
Ligand Characteristics
Ligands are indispensable to the formation and stabilization of coordination complexes. Their characteristics dictate how they interact with metal ions. Typically, efficient ligands are also Lewis bases, known for their electron-donating capabilities. Two primary traits define effective ligands: the presence of lone pairs and the charge they carry.

Lone pairs are unshared valence electrons that are readily available for forming coordinate covalent bonds with metal ions. This means that molecules or ions containing lone pairs are prime candidates for acting as ligands. Moreover, charged ions, specifically negatively charged ones, are often more potent ligands. This is because they have additional electrons, making them eager to stabilize positive metal centers through electron donation. Understanding these characteristics helps identify which substances are more likely to successfully form strong coordination complexes.

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Most popular questions from this chapter

One of the more famous species in coordination chemistry is the Creutz-Taube complex: It is named for the two scientists who discovered it and initially studied its properties. The central ligand is pyrazine, a planar six-membered ring with nitrogens at opposite sides. (a) How can you account for the fact that the complex, which has only neutral ligands, has an odd overall charge? (b) The metal is in a low-spin configuration in both cases. Assuming octahedral coordination, draw the \(d\) -orbital energy-level diagram for each metal. (c) In many experiments the two metal ions appear to be in exactly equivalent states. Can you think of a reason that this might appear to be so, recognizing that electrons move very rapidly compared to nuclei?

A complex is written as \(\mathrm{NiBr}_{2} \cdot 6 \mathrm{NH}_{3} .\) (a) What is the oxidation state of the \(\mathrm{Ni}\) atom in this complex? (b) What is the likely coordination number for the complex? (c) If the complex is treated with excess \(\mathrm{AgNO}_{3}(a q)\), how many moles of AgBr will precipitate per mole of complex?

Write the names of the following compounds, using the standard nomenclature rules for coordination complexes: (a) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right] \mathrm{NO}_{3}\) (b) \(\mathrm{Hg}\left[\mathrm{Co}(\mathrm{SCN})_{4}\right]\) (c) \(\left[\mathrm{Ru}\left(\mathrm{PPh}_{3}\right)_{3} \mathrm{Cl}_{3}\right]\) (d) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{CO}_{3}\right]_{2} \mathrm{SO}_{4}\)

Two Co(III) complexes are both low spin but have different ligands. A solution of one is orange and a solution of the other is yellow. Which solution is likely to contain the complex that has the stronger-field ligand? [Section 23.6]

(a) A compound with formula \(\mathrm{RuCl}_{3} \cdot 5 \mathrm{H}_{2} \mathrm{O}\) is dissolved in water, forming a solution that is approximately the same color as the solid. Immediately after forming the solution, the addition of excess \(\mathrm{AgNO}_{3}(a q)\) forms \(2 \mathrm{~mol}\) of solid \(\mathrm{AgCl}\) per mole of complex. Write the formula for the compound, showing which ligands are likely to be present in the coordination sphere. (b) After a solution of \(\mathrm{RuCl}_{3} \cdot 5 \mathrm{H}_{2} \mathrm{O}\) has stood for about a year, addition of \(\mathrm{AgNO}_{3}(a q)\) precipitates 3 mol of AgCl per mole of complex. What has happened in the ensuing time?

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