Question

Consider the following complexes: (P) $[Mn (CO)_6]^+$ (Q) $[Cr (CO)_6]$ (R) $[V(CO)]^-$ Choose correct statement

• A. Order of M - C bond strength : R > P
• B. Order of C - O bond order: P > Q
• C. Order of M - C bond length : P > R
• D. Order of C - O bond length : Q > R

Answer 1

Answer: (A), (B), (C), (D)

A, B, C, and D are all correct statements.

Answer 2

The strength of $\pi$-backbonding in metal carbonyls is directly proportional to the electron density on the metal atom, which increases with decreasing oxidation state. The oxidation states are:

• (P) $[Mn (CO)_6]^+$: Mn is +1
• (Q) $[Cr (CO)_6]$: Cr is 0
• (R) $[V(CO)]^-$: Assuming this is $[V(CO)_6]^-$, V is -1.

The order of electron density is V(-1) > Cr(0) > Mn(+1), leading to the order of $\pi$-backbonding strength: R > Q > P.

Increased $\pi$-backbonding results in:

1. Increased M-C bond strength: Stronger backbonding leads to a stronger M-C bond.
2. Decreased C-O bond order: Electron donation into CO $\pi^*$ orbitals weakens the C-O bond.
3. Decreased M-C bond length: Stronger bonds are shorter.
4. Increased C-O bond length: Weaker bonds are longer.

Analyzing the options: A. Order of M - C bond strength : R > P: Since backbonding strength is R > P, M-C bond strength follows R > P. This is correct. B. Order of C - O bond order: P > Q: The order of C-O bond weakening is R > Q > P. Thus, the order of C-O bond order is P > Q > R. The statement P > Q is correct. C. Order of M - C bond length : P > R: As M-C bond strength is R > P, the M-C bond length is inversely related, meaning P > R. This is correct. D. Order of C - O bond length : Q > R: As C-O bond order is P > Q > R, the C-O bond length is inversely related, meaning R < Q < P. The statement Q > R is correct.