Solved Practice Questions: Bonding in Metal Carbonyls | Inorganic Chemistry PDF Download

Q.1. The following structure of a carbonyl compound is formed by which transition metal.
(a) Ni
(b) Cr
(c) Mn
(d) Co

Correct Answer is Option (d)
There are 8 carbonyl ligands present. This compound is formed by cobalt metal ion an is named octacarbonyldicobalt(0). It also has Co-Co bond bridged by two carbonyl groups.

Q.2. How is the M-C pi bond formed?
(a) Donation of electron pair of half-filled metal d orbital to empty bonding pi orbital of CO
(b) Donation of electron pair of filled metal d orbital to empty bonding pi orbital of CO
(c) Donation of electron pair of filled metal d orbital to empty antibonding pi orbital of CO
(d) Donation of electron pair of half-filled metal d orbital to empty antibonding pi orbital of CO

Correct Answer is Option (c)
The pi bond involves donation of electrons from filled metal d orbitals into empty antibonding pi orbitals of CO. This is also called a back bond.

Q.3. The donation of lone pair of electrons of CO carbon into the vacant orbital of metal atom results in _________ bond.
(a) sigma
(b) pi
(c) back
(d) synergic

Correct Answer is Option (a)
Synergic bonding is the overall effect of the sigma and pi interactions in metal carbonyl bonds. M-C pi bonds are also known as back bonding.

Q.4. What is the geometry of pentacarbonyliron(0)?
(a) Square planar
(b) Tetrahedral
(c) Trigonal bipyramidal
(d) Octahedral

Correct Answer is Option (c)
The coordination number of pentacarbonyliron(0) is 5 as CO is a unidentate ligand and hence its geometry id trigonal bipyramidal.

Q.5. The metal-carbon bond in metal carbonyls possesses only sigma character.
(a) True
(b) False

Correct Answer is Option (b)
The M-C bond in metal carbonyls has both sigma and pi bonds and this helps in creating a synergic effect and, hence strengthening the bond.

Q.6. Match the following complexes with their v_co stretching frequency 

(a) a - i, b - iv, c - ii, d - iii
(b) a - iii, b - ii, c - i, d - iv
(c) a - iv, b - iii, c - i, d - ii
(d) a - i, b - ii, c - iii, d - iv

Correct Answer is Option (b)
Let's analyze each complex:
(i) (Mo(PF₃)₃(CO)₃ )

• PF₃ is an electron-donating ligand.
• CO is a strong π-acceptor ligand.
• The overall effect is a decrease in vCO frequency.
• The expected range is lower frequencies.

(ii) (Mo{P(OMe)₃}₃(CO)₃)

• P(OMe)₃ is an electron-donating ligand.
• CO is a strong π-acceptor ligand.
• The overall effect is a decrease in V_CO frequency.
• The expected range is lower frequencies.

(iii) (Mo(PPh₃)₃(CO)₃)

• PPh₃ is an electron-donating ligand.
• CO is a strong π-acceptor ligand.
• The overall effect is a decrease in V_CO frequency. 
• The expected range is lower frequencies.

(iv) (Mo(pyridine)₃(CO)₃)

• Pyridine is an electron-donating ligand.
• CO is a strong π-acceptor ligand.
• The overall effect is a decrease in V_CO frequency.
• The expected range is lower frequencies.

Now, let's match these with the provided V_CO stretching frequencies:

 (i) → 1835, 1934 → Lower frequencies, matching with (Mo(PPh3)3(CO)3)
 (ii) → 1888, 1977 → Lower frequencies, matching with (Mo{P(OMe)3}3(CO)3)
 (iii) → 2055, 2090 → Higher frequencies, matching with (Mo(PF3)3(CO)3 )
 (iv) → 1746, 1888 → Lower frequencies, matching with (Mo(pyridine)3(CO)3)

Q.7. The number of carbonyl stretching modes observed in the IR spectra of M(CO)₅X is
(a) 3
(b) 1
(c) 2
(d) 4

Correct Answer is Option (a)

• For the metal carbonyl complex M(CO)5X, the point group is C_4v.
• The symmetries of CO stretching modes for the C_4v point group is

A₁, A₁, B₁, and E.

• Among these vibrational modes only A1, A1, and E are IR active.
• Thus for the complex M(CO)5X, the number of carbonyl stretching modes observed in the IR spectra is 3.

Q.8. In [Mo₂(S₂)₆]^2- cluster the number of bridging S₂^2- and coordination number of Mo respectively, are 
(a) 2 and 8
(b) 2 and 6
(c) 1 and 8
(d) 1 and 6

Correct Answer is Option (a)
The coordination number of an atom in a molecule or polyatomic ion is defined as the number of atoms attached to it, excluding any lone pair.
In the [Mo₂(S₂)₆]^2- complex, since it's hypothesized that the disulfide ions (S₂)^2- are acting as bridging ligands, each disulfide ion can link the two molybdenum ions. There are 2 bridging disulfide ions, along with 4 terminal disulfide ions. This results in each molybdenum ion being bonded to 8 sulfurs. So the coordination number of the Mo atom here would be 8.
About the bridging ligands: A ligand that connects two or more atoms is termed a bridging ligand. The bridging ligand can stalk to each atom using one or more of its lone pairs in a μ (mu) mode of bonding , and the electron pairs it uses are referred to as bridging pairs. The disulfide ion (S2)2- here is expected to function as a bridging ligand.
In transition metal sulfur chemistry, the disulfide ion (S₂)^2- can either bind to the metal in a µ (S,S’)-bridging mode, as discussed here with Mo, or it can also bind to the metal via one sulfur atom (in a terminal bonding mode).

Q.9. The number of CO bands for isomers from sets (i) and (ii) in their IR spectra
Set (i): Trigonal bipyramidal isomers, axial‐Fe(CO)₄L (A)and equatorial‐Fe(CO)4L(B)
Set (ii): Octahedral isomers, fac‐Mo(CO)₃L₃ (C) and mer‐Mo(CO)₃L₃(D)
are
(a) A, 4 and B, 3; C, 3 and D, 2
(b) A, 4 and B, 3; C, 2 and D, 3
(c) A, 3 and B, 4; C, 3 and D, 2
(d) A, 3 and B, 4; C, 2 and D, 3

Correct Answer is Option (b)
Set (i): trigonal bipyramidal isomers
(A) axial‐Fe(CO)4L belongs to the point group of C_3V, it shows 3 CO bands in IR spectroscopy.
(B) equitorial‐Fe(CO)4L belongs to C_2v point group. So, it gives 4 CO bands in IR.
Set (ii): Octahedral isomers
(C) fac‐Mo(CO)3L₃ gives 2 CO bands in IR.
(D) mer‐Mo(CO)3L₃ gives 3 CO bands in IR.

Q.10. What is the order of decreasing carbonyl stretching frequencies in the following species (A - D)?
A. [Mn(CO)6]+
B. [Os(CO)6]2+
C. [Ir(CO)6]3+
D. Free CO
(a) B > A > C > D
(b) D > C > B > A
(c) A > B > C > D
(d) C > B > D > A

Correct Answer is Option (d)
A. [Mn(CO)6]+:

• In the complex [Mn(CO)6]+, there is a positive charge on the metal atom and six (CO) ligand. 
• As there is no negative charge on the metal for the delocalization with the carbonyl (CO) group, the CO stretching frequency will be higher in the complex Mn(CO)6+

B. [Os(CO)6]2+:

• In the complex [Os(CO)6]2+, there is two positive charge on the metal atom and six CO ligands.
• Due to the presence of two positive charges, the extent of metal-ligand back donation is low.
• Thus, the CO stretching frequency in [Os(CO)6]2+ will be higher than the complex Mn(CO)6+

C. [Ir(CO)6]3+:

• In the complex [Ir(CO)6]3+, there are three positive charges on the metal atom and six CO ligands.
• Due to the presence of three positive charges, the extent of metal-ligand back donation is the lowest.
• Thus, the CO stretching frequency in [Ir(CO)6]3+ will be highest among all the complexes.

D. Free CO:

• In the free CO, the CO stretching frequency will be more than [Mn(CO)6]+