Question

The correct order of $M-C \,\pi$ bond and strength in given metal carbonyls is

• A. $\left[ Fe ( CO )_{4}\right]^{2-}>\left[ Co ( CO )_{4}\right]^{-}>\left[ Ni ( CO )_{4}\right]$
• B. $\left[ Ni ( CO )_{4}\right]>\left[ Co ( CO )_{4}\right]^{-}>\left[ Fe ( CO )_{4}\right]^{2-}$
• C. $\left[ Fe ( CO )_{4}\right]^{2-}>\left[ Ni \left( CO _{4}\right)\right]>\left[ Co ( CO )_{4}\right]^{-}$
• D. $\left[ Ni ( CO )_{4}\right]>\left[ Co ( CO )_{4}\right]^{-}=\left[ Fe ( CO )_{4}\right]^{2-}$

Answer 1

Answer: (A)

The greater the positive charge on the central metal atom, less readily the metal can donate electron density into the $\pi$ order of the carbon dioxide ligands to weaken the $C-O$ bonds. Hence, more the $-ve$ charge density on metal, more the strength of $M-C \pi$-bond.