Question

Which of the following has longest C–O bond length? (Free C–O bond length in CO is 1.128Å)
(a) Ni(CO)₄
(b) [Co(CO)₄]⁻
(c) [Fe(CO)₄]²⁻
(d) [Mn(CO)₆]⁺

Answer 1

Option (c) is correct .

Due to back bonding .

The metal ion having more electron density would be able to donate it in the pie antibonding orbital of carbon monoxide thus increasing the length of the C-O Bond .

Hope it clarifies the doubt.