Question

Which of the following complexes have the longest M-C bond length?

• A. $[\mathrm{Mn}{\left(\mathrm{CO}\right)}_6{]}^{+}$
• B. $\left[\mathrm{Cr}{\left(\mathrm{CO}\right)}_6\right]$
• C. $\left[\mathrm{Fe}{\left(\mathrm{CO}\right)}_5\right]$
• D. $[\mathrm{V}{\left(\mathrm{CO}\right)}_6{]}^{-}$

Answer 1

The correct option is A

$[\mathrm{Mn}{\left(\mathrm{CO}\right)}_6{]}^{+}$

Explanation for correct option:

A. $[\mathrm{Mn}{\left(\mathrm{CO}\right)}_6{]}^{+}$

Metal has a $+1$ charge and hence the electron density on metal is less. Due to the absence of back bonding $[\mathrm{Mn}{\left(\mathrm{CO}\right)}_6{]}^{+}$ has the longest M-C bond length.

Back bonding in metal carbonyls

• A metal carbonyl is a coordination compound surrounded by a carbonyl $\left(\mathrm{CO}\right)$ group as ligands.
• Carbonyl ligand consists of a single pair of electrons to donate to the metal atom as well as an empty anti-bonding molecular orbital to back bond with the d electrons of the metal atom.
• Back bonding will occur between atoms in a compound when one of the atoms has a lone pair of electrons while the other has a vacant orbital next to it.
• It brings a partial double-bond character and increases the bond order while reducing the bond length.

Explanation for incorrect options:

B. $\left[\mathrm{Cr}{\left(\mathrm{CO}\right)}_6\right]$

• Metal is in zero oxidation state and back bonding is present. Thus, it will have a short M-C bond length.

C. $\left[\mathrm{Fe}{\left(\mathrm{CO}\right)}_5\right]$

• Metal is in zero oxidation state and back bonding is present. Thus, it will have a short M-C bond length.

D. $[\mathrm{V}{\left(\mathrm{CO}\right)}_6{]}^{-}$

• Metal is in a negative oxidation state which indicates metal has a high electron density and hence back bonding is present. Thus, it will have the shortest M-C bond length due to high back bonding.

Hence, the correct option is A i.e. $[\mathrm{Mn}{\left(\mathrm{CO}\right)}_6{]}^{+}$has the longest M-C bond length.