Extra problem set
1. Heating [(η5-C5H5)Fe(CO)3]+ with NaH gives A, having formula FeC7H6O2, plus colourless gas B.
Molecule A reacts rapidly at room temperature to eliminate colourless gas C, forming solid D, which
has empirical formula FeC7H5O2. Compound D has two strong IR bands, one near 1850 cm-1, the other
near 2000 cm-1. Treatment of D with iodine generates solid E of empirical formula FeC7H5O2I. Reaction
of NaC5H5 with E gives solid F of formula FeC12H10O2. On heating F gives off B, leaving a sublimable,
orange solid of formula FeC10H10. Propose structures for A to G.
A: (η5-C5H5)Fe(CO)2H
B: CO
C: H2
D: (η5-C5H5)(CO)Fe(µ-CO)2Fe(CO)(η5-C5H5)
E: (η5-C5H5)Fe(CO)2I
F: (η5-C5H5)Fe(CO)2(η1-C5H5)
G: (η5-C5H5)2Fe
2. Predict the products of the following reactions:
(a) Co2(CO)8 + H2 → A
A is a strong acid, has a molecular weight less than 200, and has a single 1H NMR resonance.
HCo(CO)4
(b) Cr(CO)3(NCMe)3 + cycloheptatriene → C
The number of infrared bands near 2000 cm-1 in the product is the same as in the reactant.
(η6-C7H8)Cr(CO)3
(c) Cobaltacene + O2 → D
The molecular weight of D is more than double that of cobaltacene. D has protons in four
magnetic environments with relative NMR intensities of 5:2:2:1. One half of D is a mirror image
of the other half. D has an infrared band in the C-O single bond region.
[(η5-C5H5)Co(η4-C5H5)]2O (O links the two η4-C5H5 rings). A peroxo (C-O-O-C) link is a
reasonable answer also. Note the new complex is 18e, whereas cobaltacene is 19e.
3. Suggest possible structures for the cation in [Fe2(NO)6][PF6]2 and state how you would distinguish
them experimentally.
[(ON)3FeFe(NO)3]2+; [(ON)2Fe(µ-NO)2Fe(NO)2]2+; [(ON)Fe(µ-NO)4Fe(NO)]2+. IR can
distinguish between bridging and terminal COs, and the number of each. If all the NOs are
terminal, would expect a single Fe-Fe bond (8+3+3+3+1 = 18e).
4. Given that it shows the highest hapticity possible, draw the missing planar, unsaturated and
conjugated carbocyclic hapto ligands in the following compounds, all of which obey the 18 electron
rule.
Cycloheptatrienyl, cyclobutadiene, benzene
5. The reaction of Mo(CO)6 with dicyclopentadiene (C10H12) under microwave conditions yields a
stable compound L with the empirical formula C8H5O3Mo along with evolution of CO and H2 gas. The
infrared spectrum of this compound gives peaks in the range of 1859-1960 cm-1. Compound L on
refluxing in toluene releases CO gas, and on cooling and bubbling acetylene gas through the solution for
12 hours converts to a new compound M with the empirical formula C8H6O2Mo. The infrared spectrum
of this compound shows νCO stretching bands in the range of 1852 to 1997 cm-1. While proton NMR of
L shows a single peak, that of M shows two peaks in the ratio 5:1. Suggest structures for compounds L
and M.