Question

The strongest \[{\text{ - CO}}\] bond is present in :
A) \[{{\text{[Cr(CO}}{{\text{)}}_{\text{6}}}{\text{]}}^{\text{ + }}}\]
B) \[[{\text{Fe(CO}}{{\text{)}}_{\text{5}}}{\text{]}}\]
C) \[{{\text{[V(CO}}{{\text{)}}_{\text{6}}}{\text{]}}^ - }\]
D) All have equal strength

Answer 1

Hint:Carbonyl is \[\pi \] acceptor ligand. It has a tendency to accept the electrons from the metal. The \[\pi \] back bonding tendency of metal depends on the electron cloud around the metal. Complex having lower metal to ligand \[\pi \] back bonding has the strongest \[{\text{ - CO}}\]bond.


Complete solution:
Carbonyl has a tendency to accept electrons from metal so it is known as \[\pi \] acceptor ligand. So in complexes having carbonyl ligands we observe \[{\text{M}} \to {\text{L}}\] charge transfer.
This electron transfer tendency of metal depends on the electron cloud around the metal. The greater the electron cloud around the metal the greater is \[{\text{M}} \to {\text{L}}\] charge transfer.
Metals having a higher oxidation state have lower electrons around it so it has a lower tendency of \[{\text{M}} \to {\text{L}}\] charge transfer. The strength of \[{\text{ - CO}}\]the bond increases with a decrease in \[\pi \] back bonding.
To determine which complex has the strongest \[{\text{ - CO}}\]bond we will determine the oxidation state of metal in each complex.
In all complexes, the only carbonyl is the ligand. We know that carbonyl is a neutral ligand so the oxidation state of the metal is nothing but the charge on the complex.
\[{{\text{[Cr(CO}}{{\text{)}}_{\text{6}}}{\text{]}}^{\text{ + }}}\]
Here, the charge on the complex is +1 so the oxidation state of \[{\text{Cr}}\] is +1.
\[[{\text{Fe(CO}}{{\text{)}}_{\text{5}}}{\text{]}}\]
Here, the charge on the complex is 0 so the oxidation state of \[{\text{Fe}}\] is 0.
\[{{\text{[V(CO}}{{\text{)}}_{\text{6}}}{\text{]}}^ - }\]
Here, the charge on the complex is -1 so the oxidation state of \[{\text{V}}\] is -1.
As \[{\text{Cr}}\] has a positive oxidation state we can say that it has lower electron density so it has a lower \[{\text{Cr}} \to {\text{ CO}}\] charge transfer tendency than \[{\text{Fe}} \to {\text{ CO}}\] and \[{\text{V}} \to {\text{ CO}}\].
The increased order of electron transfer tendency of given metal is as follows:
\[{\text{Cr}} \to {\text{ CO}} < {\text{Fe}} \to {\text{ CO}} < {\text{V}} \to {\text{ CO}}\]
So, the strongest \[{\text{ - CO}}\]bond is present in\[{{\text{[Cr(CO}}{{\text{)}}_{\text{6}}}{\text{]}}^{\text{ + }}}\] the complex.

Thus, the correct options are (A).

Note:\[{\text{M - CO}}\] bond strength increases with a decrease in \[\pi \] back bonding tendency. A \[\pi \] back bonding tendency of the metal decreases with an increase in the oxidation state. So carbonyl complexes having greater positive charge show stronger \[{\text{M - CO}}\] bond.