Question

In metal carbonyls, there is synergic bonding interaction between metal and carbon monoxide.This leads to increase in strength of metal ligand bond and decrease in bond order of CO in carbonyl complex as compared to bond order in carbon monoxide.
Simple carbonyls are invariably spin-paired complexes except for vanadium metal.
In metal carbonyls, the increase in C-O bond length in CO as compared to carbon monoxide is due to :
A. The donation of lone pair of electrons on the carbon into a vacant orbital of the metal atom
B. The donation of a pair of electrons from a filled d-orbital of metal into the vacant antibonding `pi` orbital of carbon monoxide
C. (A) and (B) both
D. None

Answer 1

Correct Answer - B
Donation of electrons from a filled d-orbital of metal into the vacant antibonding `pi^**` bonding orbital of CO decreases the bond order three to two and half or slightly more thus increasing the bond length between C-O.
Bond order `prop 1/("Bond length")`