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Chapter 23: Problem 27

(a) What is the difference between a monodentate ligand and a bidentate ligand? (b) How many bidentate ligands are necessary to fill the coordination sphere of a six-coordinate complex? (c) You are told that a certain molecule can serve as a tridentate ligand. Based on this statement, what do you know about the molecule?

Short Answer

Expert verified
(a) A monodentate ligand binds to a central metal atom/ion in a coordination complex through one donor atom, while a bidentate ligand binds through two donor atoms. (b) Three bidentate ligands are necessary to fill the coordination sphere of a six-coordinate complex. (c) A tridentate ligand can bind to a central metal atom/ion through three donor atoms, meaning its structure enables these atoms to form simultaneous bonds with the metal ion, potentially creating a more stable complex.

Step by step solution

01

(a) Definition of Monodentate and Bidentate Ligands

A monodentate ligand is a species that can bind to a central metal atom/ion in a coordination complex through only one donor atom, while a bidentate ligand can bind to a central metal atom/ion through two donor atoms. Usually, bidentate ligands have a flexible structure that allows both donor atoms to attach simultaneously to the metal center.
02

(b) Number of Bidentate Ligands for a Six-Coordinate Complex

A six-coordinate complex is a coordination complex in which the central metal atom/ion is surrounded by six ligand species. Since each bidentate ligand can attach to the metal center using two donor atoms, we would need: \[\frac{6 \text{ coordination sites}}{2 \text{ donor atoms per bidentate ligand}} = 3 \text{ bidentate ligands}\] So, three bidentate ligands are necessary to fill the coordination sphere of a six-coordinate complex.
03

(c) Information about a Tridentate Ligand Molecule

A tridentate ligand is a molecule that can bind to a central metal atom/ion in a coordination complex through three donor atoms. This means that the molecule has a structure that enables the three donor atoms to form bonds with the central metal ion simultaneously. The molecule can occupy three coordination sites on the central metal ion, potentially creating a more stable complex compared to some monodentate or bidentate ligands, due to the chelate effect.

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Most popular questions from this chapter

Explain the lanthanide contraction, and describe how it affects the properties of the transition-metal elements.

Generally speaking, for a given metal and ligand, the stability of a coordination compound is greater for the metal in the +3 rather than in the +2 oxidation state (for metals that form stable +3 ions in the first place). Suggest an explanation, keeping in mind the Lewis acid-base nature of the metal-ligand bond.

The molecule methylamine \(\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)\) can act as a monodentate ligand. The following are equilibrium reactions and the thermochemical data at \(298 \mathrm{~K}\) for reactions of methylamine and en with \(\mathrm{Cd}^{2+}(a q):\) $$ \begin{array}{c} \mathrm{Cd}^{2+}(a q)+4 \mathrm{CH}_{3} \mathrm{NH}_{2}(a q) \rightleftharpoons\left[\mathrm{Cd}\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)_{4}\right]^{2+}(a q) \\ \Delta H^{\circ}=-57.3 \mathrm{~kJ} ; \quad \Delta S^{\circ}=-67.3 \mathrm{~J} / \mathrm{K} ; \quad \Delta G^{\circ}=-37.2 \mathrm{~kJ} \\\ \mathrm{Cd}^{2+}(a q)+2 \mathrm{en}(a q) \rightleftharpoons\left[\mathrm{Cd}(\mathrm{en})_{2}\right]^{2+}(a q) \\ \Delta H^{\circ}=-56.5 \mathrm{~kJ} ; \quad \Delta S^{\circ}=+14.1 \mathrm{~J} / \mathrm{K} ; \quad \Delta G^{\circ}=-60.7 \mathrm{~kJ} \end{array} $$ (a) Calculate \(\Delta G^{\circ}\) and the equilibrium constant \(K\) for the following ligand exchange reaction: \(\left[\mathrm{Cd}\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)_{4}\right]^{2+}(a q)+2 \operatorname{en}(a q) \rightleftharpoons\) $$ \left[\mathrm{Cd}(\mathrm{en})_{2}\right]^{2+}(a q)+4 \mathrm{CH}_{3} \mathrm{NH}_{2}(a q) $$ Based on the value of \(K\) in part (a), what would you conclude about this reaction? What concept is demonstrated? (b) Determine the magnitudes of the enthalpic \(\left(\Delta H^{\circ}\right)\) and the entropic \(\left(-T \Delta S^{\circ}\right)\) contributions to \(\Delta G^{\circ}\) for the ligand exchange reaction. Explain the relative magnitudes. (c) Based on information in this exercise and in the "A Closer Look" box on the chelate effect, predict the sign of \(\Delta H^{\circ}\) for the following hypothetical reaction: $$ \begin{aligned} \left[\mathrm{Cd}\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)_{4}\right]^{2+}(a q) &+4 \mathrm{NH}_{3}(a q) \rightleftharpoons \\ \left[\mathrm{Cd}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}(a q)+4 \mathrm{CH}_{3} \mathrm{NH}_{2}(a q) \end{aligned} $$

No compounds are known in which scandium is in the +2 oxidation state. Suggest an explanation.

Give the number of (valence) \(d\) electrons associated with the central metal ion in each of the following complexes: (a) \(\mathrm{K}_{3}\left[\mathrm{TiCl}_{6}\right]\) (b) \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{6}\right],\) (c) \(\left[\mathrm{Ru}(\mathrm{en})_{3}\right] \mathrm{Br}_{3},\) (d) \([\mathrm{Mo}(\mathrm{EDTA})] \mathrm{ClO}_{4},(\mathrm{e}) \mathrm{K}_{3}\left[\mathrm{ReCl}_{6}\right] .\)
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