Does Back-Donation Decrease Down the Column for Cr(CO)6, Fe(CO)5, and Ni(CO)4?

[plexus0208]

Cr(CO)₆
Fe(CO)₅
Ni(CO)₄

Is there more back-donation (from the CO) going up OR down the above column? I always get this mixed up.

 

[chemisttree]

Back donation is from the metal back to the ligand, not the other way as you have indicated ("from the CO").

These compounds are not isoelectronic and have different CN's. How do you want to compare the backbonding? Per ligand or per complex?

 

[quicknote]

The quick answer is that back-donation decreases going down the column. This is because the metal ions in Cr(CO)6, Fe(CO)5, and Ni(CO)4 have increasing atomic numbers going down the column, meaning they have more electrons available for back-donation. Back-donation occurs when the metal ion donates electron density to the CO ligands, forming metal-carbon bonds. As the metal ions have more electrons to donate, they are more likely to engage in back-donation, resulting in stronger metal-carbon bonds. Therefore, the strength of back-donation decreases going down the column.