1. (a) Sketch the two structures that describe most six-coordinate complexes. (b) Which one of these is rare?
2. Air oxidation of Co(II) carbonate and aqueous ammonium chloride gives a pink chloride salt with a ratio of 4 NH3 : Co. On addition of HCl to a solution of this salt, a gas is rapidly evolved and the solution slowly turns violet on heating. Complete evaporation of the violet solution yields CoCl3·4NH3. When this compound is heated in concentrated HCl, a green salt can be isolated with composition CoCl3·4NH3·HCl. Write balanced equations for all transformations occurring after the air oxidation. Give as much information as possible about the isomerism occurring and the basis for your reasoning. Is it helpful to know that the form of [Co(en)2Cl2]+ that is resolvable into enantiomers is violet?
3. The equilibrium constants for the successive reactions of ethylenediamine with Co2+, Ni2+, and Cu2+ are as follows.
[M(OH2)6]2+(aq) + en(aq) →← [M(en)(OH2)4]2+(aq) + 2 H2O(l) K1
[M(en)(OH2)4]2+(aq) + en(aq) →← [M(en)2(OH2)2]2+(aq) + 2 H2O(l) K2
[M(en)2(OH2)2]2+(aq) + en(aq) →← [M(en)3]2+(aq) + 2 H2O(l) K3
Ionlog K1 log K2log K3
Co2+5.89 4.833.10
Ni2+7.52 6.284.26
Cu2+10.72 9.31–1.0
Discuss whether the data support the generalizations in the text about successive formation constants and the Irving-Williams series.
How do you account for the very low value of K3 for Cu
4. When cobalt(II) salts are oxidized by air in a solution containing ammonia and sodium nitrite, a yellow solid, [Co(NO2)3(NH3)3], can be isolated. In solution it is nonconducting; treatment with HCl gives a complex that, after a series of further reactions, can be identified as trans-[CoCl2(NH3)3(OH2)]+. It requires an entirely different route to prepare cis-[CoCl2(NH3)3(OH2)]+. Is the yellow substance fac or mer? What assumption must you make to arrive at a conclusion?
5. Decide which of the two complexes (a) W(CO)6 or (b) IrCl(PPh3)2(CO) should undergo the fastest exchange with 13CO. Justify your answer.
6. The reaction of [ZrCl4(dppe)] (dppe is a bidentate phosphine ligand) with Mg(CH3)2 gives [Zr(CH3)4(dppe)]. NMR spectra indicate that all the methyl groups are equivalent. Draw octahedral and trigonal prism structures for the complex and show how the conclusion from NMR supports the trigonal prism structure. (P. M. Morse and G. S. Girolami, J.Am. Chem. Soc., 1989, 111, 4114.)
7. For each of the following pairs of complexes, indicate the one that has the larger LFSE:
(a) [Cr(OH2)6]2+ or [Mn(OH2)6]2+
(b) [Mn(OH2)6]2+ or [Fe(OH2)6]3+
(c) [Fe(OH2)6]3+ or [Fe(CN)6]3–
(d) [Fe(CN)6]3– or [Ru(CN)6]3–
(e) tetrahedral [FeCl4]2– or tetrahedral [CoCl4]2–
8. (a) On a chart of the d-block elements in their periodic table arrangement, identify the elements and associated oxidation numbers that form square-planar complexes. (b) Give formulas for three examples of square-planar complexes.
9. Sketch all the possible isomers of [Co(en)2(NO2)2], including optical isomers, in which the en ligands are bidentate. Do not draw different orientations of the same isomer.
10. Give the name and find the ligand field stabilization energy (in terms of Dq) for the following complexes:
a) [Cr(NH3)6]3+
b) [Cu(NH3)4(OH2)2]2+
c) [Ti(OH2)6]3+
d) [Co(CN)6]3– (low spin)
e) [Ni(NH3)4Cl2]
11. Predict which of the following complexes is Jahn-Teller active:
a) [Cr(NH3)6]3+
b) [Cu(NH3)4(OH2)2]2+
c) [Ti(OH2)6]3+
d) [Co(CN)6]3–
e) [Ni(NH3)4Cl2]
12. Predict which of the following complexes will be stable:
a) [Mo(CO)6]
b) [(η6–C6H6)2Cr]
c) [(η3–C3H5)Co(CO)2]
d) [Fe(diphos)(CO)3] (diphos = Ph2PCH2CH2PPh2, Ph = C6H5)
13. Find the LFSE in terms of Dq and calculate the spin-only magnetic moment (in units of Bohr-magnetons) for the following complexes:
a) cis–diaqua–cis–dichloro–cis–difluorocobaltate(II) ion
b) trans–dicyanobis(ethylenediamine)iron(III) ion
c) trans–dichlorotetrakis(triphenylphosphine)nickel(II)
d) tris(bipyridine)ruthenium(II) ion
e) cis–dicyanobis(oxalato)manganate(II) ion
14. Predict whether the following complexes will be stable:
a) hexacarbonylvanadium(0)
b) bis(η5–cyclopentadienyl)bis(η1–cyclopentadienyl)titanium(IV)
c) bis(η4–cyclooctadiene)nickel(0)
d) hexacyanoferrate(III)
e) bis(η6–benzene)chromium(0)
15. Removing the all of the water from hydrates of transition metal complexes is notoriously difficult. Typically, simply heating the complex is ineffective and reaction with something like thionyl chloride is required. For example, cobalt(II) chloride hexahydrate only loses four water molecules to become cobalt(II) chloride dihydrate when heated in an oven at 150 °C. Suggest an explanation for this observation.
16. Find the LFSE (in terms of Dq and P) for: a) tris(oxalato)chromate(III) ion; b) hexacyanoferrate(II) ion; c) tetrachloro–η2–etheneplatinate(II) ion.
17. Predict the stability of the following species: a) bis(η5–cyclopentadienyl)cobalt(II); b) tetracarbonylnickel(0); c) tricarbonyl(η5–cyclopentadienyl)manganese(I).
18. Chromium (III) complexes are nearly all six-coordinate with octahedral geometry. Suggest a reason for this observation.