Bonding in metal carbonyls is represented by the end structures.
`overset(M-C-=O" "harr" "M=C=O)(I" "II)`
The contribution of I and II to the real structure depends upon the extent of `pi-` back bonding. Both `sigma` donation and `pi-` back bonding are synergitc. The filled orbitals CO overlap with vacant metal d orbitals which is called `sigma` donation. The filled metal d orbitals overlap with `pi` antibondig moleculat orbital of CO. which is called `pi-`back bonding .
Which one of the following has the highest C-O bond length?
A. `Ti(C_(2)H_(5))_(4)`
B. `Ni (CO)_(4)`
C. `Cr(CO)_(6)`
D. `[Fe(CN)_(6)]^(4-)`
