Homework Problems Chapter 10

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Homework Problems

Section 1

Exercise 1

Predict the charge n of cobaltocene (Co(Cp)₂)ⁿ. Cp=cyclopentadienyl. Explain briefly.

Answer: +1, because the 18 electron rule is fulfilled.

Exercise 2

Bis-benzene chromium (Ƞ⁴-C₆H₆)Cr is a sandwich compound having two parallel benzene rings in an eclipsed conformation. Sketch the ligand group orbitals for this molecule.

Answer

Section 2

Exercise 1

The MO diagram of the CO molecule is given below. Identify the molecular orbitals that can act

a) As sigma-donors

b) Pi-acceptors in carbonyls.

Answer

a) 5 σ, 4 σ, bonding π orbitals

b) anti-bonding π orbitals

Exercise 2

Which carbonyl will most likely exhibit the highest frequency for the C-O stretch vibration? Give a brief explanation for your decision.

a) V(CO)₆^-

b) Cr(CO)₆

c) Mn(CO)₆⁺

Answer

c) Mn(CO)₆⁺

Carbonyl cations have the weakest pi-backbonding, therefore the bond order for the C-O bond is the highest and thus the frequency of the C-O stretch vibration is the highest.

Exercise 3

Predict the right charge of the most stable pentacarbonyl of

a) Iron

b) Mn

c) Cr

Answer

a) 0

b) 1-

c) 2-

Section 3

Exercise 1

What structure would you expect for a carbonyl made of the following fragments according to the concept of isolobality? Predict if the structure would likely be stable.

a) Cr(CO)₅⁺

b) Cr(CO)₄^-

c) Mn(CO)₄^-

Answer

a) 15VE -> tetrahedral cluster (not stable b/c CN too high)

b) 15 VE -> tetrahedral cluster (not stable b/c CN too high)

c) 16 VE -> trimer (stable b/c CN = 6, charge not too high).

Exercise 2

Which of the following fragments are isolobal to the CH₂ fragment?

a) NH₂

b) NH

c) OH

d) Fe(CO)₃

e) Co(CO)₄⁺

Answer: b) NH

Exercise 3

Which of the following fragments are isolobal to the Mn(CO)₅ fragment?

a) CH₃

b) CH₂CH₃

c) OH

d) F

e) NH

Answer

a) CH₃

b) CH₂CH₃

c) OH

d) F