Question

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

(a) [Mn(CO)₆]⁺

(b) [Fe(CO)₅]

(c) [Cr(CO)₆]

(d) [V(CO)₆]^-

Answer 1

The correct option (b) [Fe(CO)₅]

Explanation:

(a) Mn⁺= 3d⁵ ,4s¹ . 

In the presence of CO effective configuration 

= 3d⁶ 4s⁰, 

Three lone pair for back bonding with vacant orbital of C in CO.

(b) Fe⁰ = 3d⁶, 4s². ln presence of CO effective configuration = 3d⁸, 

Four lone pair for back bonding with CO. 

(c) Cr⁰ = 3d⁵ ,4s¹,effective configuration = 3d⁶ Three lone pair for back bonding with CO. 

(d) V^- = 3d⁴ , 4s², effective configurat ion = 3d⁶ three lone pair for back bonding with CO. 

Maximum back bonding in Fe(CO)₅, therefore CO bond order is lowest here.