Question

"electron-withdrawing substituents on carbon encourage back donation", then on the next page he says that "greater MCP character is favored by strong donor coligands, a net negative charge on a complex ion, and a low metal oxidation state". The way I see it, complexes with electropositive atoms (strong donors) favor back donation because they increase electron density on the metal, but that first sentence is confusing me. Wouldn't electron-withdrawing groups decrease back donation?

Answer 1

No, electron withdrawing groups increase back donation because they decrease the electron density on metal, they strongly hold up the electrons towards themselves and thus make metal strongly electropositive. Thus when the metal has a strong electropositive character and has empty pie orbital it will easily accept electrons from the ligand and form back bonding easily. So in this way electron withdrawing groups play role in increasing back bonding.

HOPE THIS HELPS BUDDY!!!