Question

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

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

Answer 1

Answer: (D)

(A) Mn⁺=3d⁵4s¹;in presence of CO, effective configuration=3d⁶4s⁰.
Three lone pair for back bonding with vacant orbital of C in CO.
(B) $F{e}^0=3d^{6}4s^2$ ;in presence of CO, effective configuration $=3d^8$ .
Four lone pair for back bonding with CO.
(C) $C{r}^0=3d^{5}4s^1$ ;effective configuration $=3d^6$ .
Three lone pair for back bonding with CO.
(D) $V^{-}=3d^{4}4s^2$ ;effective configuration $=3d^6$ .
Three lone pair for back bonding with CO.
Maximum back bonding is present in $Fe(CO{)}_5$ therefore, $CO$ bond order is lowest here.