# Homework 5: Crystal Field Splitting

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These homework problems are suggested and will not be turned in for review. However, answers will be available for them the following week by your class TAs. For more homework feel free to go to the Homework page.

## Q14

The Co2+ ion in aqueous solution is octahedrally coordinated and paramagnetic, with three unpaired electrons. Which one or ones of the following statements follow from this observation:

1. Co(H2O)42+ is square planar
2. Co(H2O)42+ is tetrahedral
3. Co(H2O)62+ has a Δ0 that is larger than the electron-pairing energy;
4. the d levels are split in energy and filled as follows: (t2g)5(eg)2
5. the d levels are split in energy and filled as follows: (t2g)6(eg)1

## Q15

The coordination compound potassium hexafluorochromate(III) is paramagnetic. What is the formula for this compound? What is the configuration of the Cr d electrons?

## Q16

How many unpaired electrons are there in Cr3+, Cr2+, Mn2+, Fe2+, Co3+, Co2+ in

1. a strong octahedral ligand field and
2. a very weak octahedral field?

## Q19

What is the d-orbital electronic configuration of Cr(NH3)63+? How many unpaired electrons are present? If six Br- groups were substituted for the six NH3 groups to give CrBr63-, would you expect Δ0 to increase or decrease?

## Q21

For each of the following, sketch the d-orbita1 energy levels and the distribution of d electrons among them:

1. Ni(CN)42- (square planar)
2. Ti(H2O)62+ (octahedral)
3. NiCl42- (tetrahedra1)
4. CoF63- (high-spin complex)
5. Co(NH3)63+ (low—spin complex)

## Q23

Pt(II) can occur in the complex ion PtCl42-.

1. What is the geometry of this ion? In the valence bond theory, what Pt orbitals are used in making bonds to the Cl- ions?
2. What is the systematic name for the sodium salt of this ion?
3. Using crystal field theory, draw the d-electron configuration for this ion. ls the ion paramagnetic or diamagnetic?
4. Pt(II) can be oxidized to Pt(IV). Draw the d-electron configuration for the chloride complex ion of Pt(IV). Explain the difference between this configuration and that of Pt(II), Is the Pt(IV) chloride complex ion paramagnetic or diamagnetic?

## Q24

A solution is prepared that is 0.025M in tetraamminecopper(II), Cu(NH3)42+. What will be the concentration of Cu2+ hydrated copper ion if the ammonia concentration is 0.10, 0.50, 1.00, and 3.00 M respectively? What ammonia concentration is needed to keep the Cu2+ concentration less than 10-15 M?

## Q27

The ion Co(NH3)63+ is very stable, with Kf = 2.3 x 1034. If the hydrolysis constant for the ammonium ion, Kb, is 5 x 10-10, show that the equilibrium in the reaction

Co(NH3)63+ + 6H+ ↔ Co3+ + 6NH4+

lies far to the right. Then why does Co(NH3)63+ remain intact in hot concentrated sulfuric acid?

## Q29

What is the solubility of Cu(OH)2 in pure water? In buffer at pH 6? Copper(II) forms a complex with NH3, Cu(NH3)42+, with Kf = 1.0 x 1012. What concentration of ammonia must be maintained in a solution to dissolve 0.10 mole of Cu(OH)2 per liter of solution?

## Q31

Predict the electron configuration of an octahedral d4 complex with

1. strong field ligands and
2. weak field ligands, and state the number of unpaired electrons

## Q32

Predict which of the following complexes absorbs light of the shorter wavelength and explain your reasoning: [Co(H2O)6]3+ or [Co(en)3]3+.

## Q33

Compare the magnetic properties of [Fe(H2O)6]2+ and [Fe(CN)6]4-.

## Q34

What change in magnetic properties (if any) can be expected when NO2- ligands in an octahedral complex are replaced by Cl- ligands in a d6 complex?

## Q35

Draw the orbital splitting diagram for the following complex and give its electron configuration: tetrahedral CoCl42-.

## Q36

The complex ion PdCl42- is diamagnetic. Propose a structure for PdCl42-.

## Q37

Explain the following differences in color:

1. For [Cr(H2O)6]Cl3 and [Cr(NH3)6]Cl3, one complex is violet while the other yellow Match the expected color with each complex and explain your identifications.
2. For [Co(H2O)6]2+ and [CoCl4]2-, one complex is blue while the other reddish. Correlate a color with each complex and explain your identifications.
3. One of the following solids is yellow, and the other is green: Fe(NO3)2·6H2O versus K4[Fe(CN)6·3H2O]. Indicate which is which and explain your reasoning.

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