Question

Among the following metal carbonyls, the $C-O$ bond order is lowest in

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

Answer 1

The correct option is D $\left[V\right(CO{)}_6{]}^{-}$

The overlap of a lone pair on the C atom with the empty hybrid metal orbital forms a metal-to-carbon $\sigma$-bond. The transition metal atom in a metal carbonyl fills the non-bonding d-orbitals which are proper symmetry to overlap with the anti-bonding orbitals of CO. The electronic charge is transferred from the filled non-bonding orbitals of the metals to ${\pi }^{\star }$-orbitals of the ligand CO. This reduces the bond order of CO. The $\pi$ back-bonding strengthens the M-C bond order and weakens the C-O bond order.
CO is a strong ligand, it causes pairing of electrons.
$M{n}^{+}:(z=25)$
$\left[Ar\right]3d^{5}4s^1\Rightarrow \left[Ar\right]3d^{6}4s^0$
Three lone pairs are available for back-bonding with vacant orbital of C in CO.
$F{e}^0:(z=26)$
$\left[Ar\right]3d^{6}4s^2\Rightarrow \left[Ar\right]3d^8$
Four lone pairs are available for back-bonding with vacant orbotal of C in CO.
$C{r}^0:(z=24)$
$\left[Ar\right]3d^{5}4s^1\Rightarrow \left[Ar\right]3d^{6}4s^0$
Three loan pairs are available for back-bonding with vacant orbital of C in CO.
$V^{-}:(z=23)$
$\left[Ar\right]3d^{4}4s^2\Rightarrow \left[Ar\right]3d^{6}4s^0$
Three lone pairs are available for back-bonding with vacant orbital of C in CO.
Minimum back-bonding is possible in $\left[V\right(CO{)}_6{]}^{-1}$, so the M-C bond order is the lowest.