Question

The order of increasing CO stretching frequencies in $[Co(CO)_4]^-, [Cu(CO)_4]^+, [Fe(CO)_4]^{2-} and [Ni(CO_4)]$ is

• A. $[Cu(CO)_4]^+ < [Ni(CO)_4] < [Co(CO)_4]^- < [Fe(CO)_4]^{2-}$
• B. $[Fe(CO)_4]^{2-} < [Co(CO)_4]^- < [Ni(CO)_4] < [Cu(CO)_4]^+$
• C. $[Co(CO)_4]^- < [Fe(CO)_4]^{2-} < [Cu(CO)_4]^+ < [Ni(CO)_4]$
• D. $[Ni(CO)4] < [Cu(CO)_4]^+ < [Co(CO)_4]^- < [Fe(CO)_4]^{2-}$

Answer 1

The Correct Option is B

To determine the order of increasing CO stretching frequencies for the given metal carbonyl complexes, we need to consider the back-bonding effect and the oxidation states of the metals involved. The CO stretching frequency is influenced by \(\pi\)-back-donation from the transition metal to the anti-bonding orbitals of the CO ligand.

1. Back-bonding and Effect on CO Stretching Frequency:
   • Increased \(\pi\)-back-donation leads to a decrease in the CO bond order, thereby decreasing the CO stretching frequency.
   • Reduced \(\pi\)-back-donation means a higher CO bond order, and the CO stretching frequency increases.
2. Analyzing Each Complex:
   • \([Fe(CO)_4]^{2-}\): Iron is in a low oxidation state which generally allows for strong \(\pi\)-back-bonding, thus having a lower CO stretching frequency.
   • \([Co(CO)_4]^-\): Cobalt is also in a low oxidation state allowing significant \(\pi\)-back-bonding, slightly higher than iron.
   • \([Ni(CO)_4]\): Nickel, having a neutral oxidation state, also possesses efficient \(\pi\)-back-bonding, higher than cobalt.
   • \([Cu(CO)_4]^+\): Copper in this cationic state exhibits the least \(\pi\)-back-donation, resulting in the highest CO stretching frequency among the given complexes.
3. Order of CO Stretching Frequencies:

Given this understanding, the order of increasing CO stretching frequencies is:

\([Fe(CO)_4]^{2-} < [Co(CO)_4]^- < [Ni(CO)_4] < [Cu(CO)_4]^+\)

Therefore, the correct answer is:

$[Fe(CO)_4]^{2-} < [Co(CO)_4]^- < [Ni(CO)_4] < [Cu(CO)_4]^+$