24. Transition Metals and Coordination Compounds

Multiple Choice

Determine the spin and number of unpaired electrons in the following complex ion: [Cd(H₂O)₄]²⁺.

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Multiple Choice

Which of the following complex ions is/are diamagnetic in nature?

I. [Mn(Br₄)]^2– II. [V(NO₂)₄]^4– III. [Zn(NH₃)₄]²⁺ IV. [Sc(H₂O)₆]³⁺

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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-.

(c) Draw a crystal field energy-level diagram for [Fe(C₂O₄)₃]^3-, and predict the number of unpaired electrons. (C₂O₄^2- is a weak-field bidentate ligand.)

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.

(a) [CrF₆]^3-

(b) [V(H₂O)₆]³⁺

Draw a crystal field energy-level diagram, assign the electrons to orbitals, and predict the number of unpaired electrons for each of the following.

(a) [Cu(en)₃]²⁺

(b) [FeF₆]^2-

Which of the following complexes are paramagnetic?

(a) [Mn(CN)₆]^3-

(b) [Zn(NH₃)₄]²⁺ (tetrahedral)

(d) [FeF₆]^4-

Which of the following complexes are diamagnetic?

(a) [Ni(H₂O)₆]²⁺

(b) [Co(CN)₆]^3-

(d) [Cu(NH₃)₄]²⁺ (square planar)

Draw a crystal field energy-level diagram, and predict the number of unpaired electrons for each of the following:

(a) [Mn(H2O)6]2+

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.

The percent iron in iron ore can be determined by dissolving the ore in acid, then reducing the iron to Fe2+, and finally titrating the Fe2+ with aqueous KMnO4. The reaction products are Fe2+ and Mn2+.

(c) Draw a crystal field energy-level diagram for the reactants and products, MnO4-, 3Fe1H2O2642+, 3Fe1H2O2643+, and 3Mn1H2O2642+, and predict the number of unpaired electrons for each.

Cobalt(III) trifluoroacetylacetonate, Co1tfac23, is a sixcoordinate, octahedral metal chelate in which three planar, bidentate tfac ligands are attached to a central Co atom: <IMAGE>

(d) Draw a crystal field energy-level diagram for Co1tfac23, and predict its magnetic properties. (In this complex, tfac is a strong-field ligand.)

Although Cl^- is a weak-field ligand and CN^- is a strong field ligand, [CrCl₆]^3- and [Cr(CN)₆]^3- exhibit approximately the same amount of paramagnetism. Explain.

The amount of paramagnetism for a first-series transition metal complex is related approximately to its spin-only magnetic moment. The spin-only value of the magnetic moment in units of Bohr magnetons (BM) is given by sqrt(n(n + 2)), where n is the number of unpaired electrons. Calculate the spin-only value of the magnetic moment for the 2+ ions of the first-series transition metals (except Sc) in octahedral complexes with (a) weak-field ligands and (b) strong-field ligands. For which electron configurations can the magnetic moment distinguish between high-spin and low-spin electron configurations?

In octahedral complexes, the choice between high-spin and low-spin electron configurations arises only for d4 - d7 complexes. Explain.

For each of the following, (i) give the systematic name of the compound and specify the oxidation state of the transi-tion metal, (ii) draw a crystal field energy-level diagram and assign the d electrons to orbitals, (iii) indicate whether the complex is high-spin or low-spin (for d4 - d7 complexes), and (iv) specify the number of unpaired electrons.

(a) (NH4)[Cr(H2O)6](SO4)2

Open Question

How many electrons will be unpaired in the complex ion [Co(OH)₃]³⁻ ?