Correct Answer - A
The metal -carbon bond in carbonyls possisses both `sigma and pi` character-weak `sigma` bond being formed by donation of electron pair from carbon to metal `(M larr C-=O)` and a stronger `pi` bond being formed by back donation of electrons from filled `d_(xy)` orbital
of metal into empty antibonding `overset(**)pi` 2py orbital of carbon (`pi`M`rarr` C bond ) . The total bonding is thus M = C = O. Thus bond order of C-O bond is raduced from triple bond to double bond . As a rasult , C-O bond length of `1.128Ã…` in CO increases to about `1.15 Ã…` in many carbonyls.
