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Ch.21 - Transition Elements and Coordination Chemistry
McMurry - Chemistry 8th Edition
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Tutti i libri di testoMcMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblema 21.140d
Capitolo 21, Problema 21.140d

Cobalt(III) trifluoroacetylacetonate, Co1tfac23, is a sixcoordinate, octahedral metal chelate in which three planar, bidentate tfac ligands are attached to a central Co atom:
(d) Draw a crystal field energy-level diagram for Co1tfac23, and predict its magnetic properties. (In this complex, tfac is a strong-field ligand.)

Guida verificata passo dopo passo
1
Identify the oxidation state of the cobalt ion in the complex. Since the complex is Cobalt(III), the oxidation state of Co is +3.
Determine the electron configuration of the Co(III) ion. Cobalt in its elemental form is [Ar] 3d^7 4s^2. For Co(III), remove three electrons, resulting in [Ar] 3d^6.
Recognize that the complex is octahedral and tfac is a strong-field ligand, which means it will cause a large splitting in the d-orbitals (Δ_oct).
Draw the crystal field splitting diagram for an octahedral complex. The d-orbitals split into two sets: the lower energy t2g (d_xy, d_xz, d_yz) and the higher energy e_g (d_z^2, d_x^2-y^2) orbitals.
Since tfac is a strong-field ligand, the electrons will pair up in the lower energy t2g orbitals before occupying the higher energy e_g orbitals. With 6 electrons, all will pair in the t2g orbitals, resulting in a low-spin configuration, which is diamagnetic (no unpaired electrons).

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Crystal Field Theory

Crystal Field Theory (CFT) explains how the arrangement of ligands around a central metal ion affects its electronic structure and energy levels. In octahedral complexes, the d-orbitals split into two energy levels: the lower-energy t2g and the higher-energy eg orbitals. The extent of this splitting depends on the nature of the ligands, with strong-field ligands causing a larger splitting, which influences the complex's magnetic properties.
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The study of ligand-metal interactions helped to form Ligand Field Theory which combines CFT with MO Theory.

Ligand Field Strength

Ligand field strength refers to the ability of a ligand to influence the energy levels of the d-orbitals in a metal complex. Strong-field ligands, like tfac in this case, cause significant splitting of the d-orbitals, leading to low-spin configurations where electrons pair up in the lower energy orbitals. This property is crucial for predicting the magnetic behavior of the complex, as low-spin complexes tend to be diamagnetic.
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Strong-Field Ligands result in a large Δ and Weak-Field Ligands result in a small Δ.

Magnetic Properties of Complexes

The magnetic properties of coordination complexes are determined by the presence of unpaired electrons in their d-orbitals. A complex with unpaired electrons is paramagnetic, while one with all paired electrons is diamagnetic. In the case of Co1tfac23, the strong-field tfac ligands lead to a low-spin state, resulting in no unpaired electrons and thus a diamagnetic property, which can be predicted from the crystal field energy-level diagram.
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For octahedral complexes, Weak-Field Ligands create High-spin complexes and Strong-Field Ligands create Low-spin complexes.
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For each of the following complexes, draw a crystal field energy-level diagram, assign the electrons to orbitals, and predict the number of unpaired electrons.

(c) [Co(NCS)4]2- (tetrahedral)

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In acidic aqueous solution, the complex trans-[Co(en)2Cl1]2+(aq) undergoes the following substitution reaction:

trans-[Co(en)1Cl2]+(aq) + H2O(l) → trans-[Co(en)2(H2O)Cl]2+(aq) + Cl–(aq)

The reaction is first order in trans-[Co(en)2Cl2]+(aq), and the rate constant at 25°C is 3.2×10–5 s–1.

(d) Is the reaction product chiral or achiral? Explain.

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The complete reaction of 2.60 g of chromium metal with 50.00 mL of 1.200 M H2SO4 in the absence of air gave a blue solution and a colorless gas that was collected at 25°C and a pressure of 735 mm Hg. (e) When an excess of KCN is added to the solution, the color changes, and the paramagnetism of the solution

decreases. Explain.

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Nickel(II) complexes with the formula NiX2L2, where X is Cl- or N-bonded NCS- and L is the monodentate triphenylphosphine ligand P(C6H5)3, can be square planar or tetrahedral.

(c) Draw possible structures for each of the NiX2L2 complexes, and tell which ones have a dipole moment.

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In acidic aqueous solution, the complex trans-[Co(en)2Cl1]2+(aq) undergoes the following substitution reaction:

trans-[Co(en)2Cl1]+(aq) + H2O(l) → trans-[Co(en)2(H2O)Cl]2+(aq) + Cl–(aq)

The reaction is first order in trans-[Co(en)2Cl2]+(aq), and the rate constant at 25°C is 3.2×10–5 s–1.

e. Draw a crystal field energy-level diagram for trans-[Co(en)2Cl2]+ that takes account of the fact that Cl– is a weaker-field ligand than ethylenediamine.

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Spinach contains a lot of iron but is not a good source of dietary iron because nearly all the iron is tied up in the oxalate complex [Fe(C2O4)3]3-.

(d) Draw the structure of [Fe(C2O4)3]3-. Is the complex chiral or achiral?

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