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) $Fe^{0}=3d^{6}4s^{2}$ ;in presence of CO, effective configuration $=3d^{8}$ .
Four lone pair for back bonding with CO.
(C) $Cr^{0}=3d^{5}4s^{1}$ ;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.